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## Extras

This section can be consulted selectively for specific topics of interest.

### Old Value Requirements (Templates)

Old values require to copy the expression passed to `BOOST_CONTRACT_OLDOF` thus the type of that expression needs to be copyable. More precisely, dereferencing an old value pointer of type `boost::contract::old_ptr``<T>` requires `boost::contract::is_old_value_copyable``<T>::value` to be `true`, otherwise this library will generate a compile-time error.

In some cases it might be acceptable, or even desirable, to cause a compile-time error when a program uses old value types that are not copyable (because it is not possible to fully check the correctness of the program as stated by the contract assertions that use these old values). In these cases, programmers can declare old values using `boost::contract::old_ptr` as seen so far.

However, in some other cases it might be desirable to simply skip assertions that use old values when the respective old value types are not copyable, without causing compile-time errors. Programmers can do this using `boost::contract::old_ptr_if_copyable` instead of `boost::contract::old_ptr` and checking if the old value pointer is not null before dereferencing it in postconditions. For example, consider the following function template that could in general be instantiated for types `T` that are not copy constructible (that is for which `boost::contract::is_old_value_copyable``<T>::value` is `false`, see `old_if_copyable.cpp`): [68]

    template<typename T> // T might or might not be copyable.
    void offset(T& x, int count) {
        // No compiler error if T has no copy constructor...
        boost::contract::old_ptr_if_copyable<T> old_x = BOOST_CONTRACT_OLDOF(x);
        boost::contract::check c = boost::contract::function()
            .postcondition([&] {
                // ...but old value null if T has no copy constructor.
                if(old_x) BOOST_CONTRACT_ASSERT(x == *old_x + count);
            })
        ;

        x += count;
    }

The old value pointer `old_x` is programmed using `boost::contract::old_ptr_if_copyable`. When `T` is copyable, `boost::contract::old_ptr_if_copyable``<T>` behaves like `boost::contract::old_ptr``<T>`. When `T` is not copyable instead, `boost::contract::old_ptr_if_copyable``<T>` will simply not copy `x` at run-time and leave `old_x` initialized to a null pointer. Therefore, `boost::contract::old_ptr_if_copyable` objects like `old_x` must be checked to be not null as in `if(old_x) ...` before they are dereferenced in postconditions and exception guarantees (to avoid run-time errors of dereferencing null pointers).

If the above example used `boost::contract::old_ptr` instead then this library would have generated a compile-time error when `offset` is instantiated with types `T` that are not copy constructible (but only if `old_x` is actually dereferenced somewhere, for example in the contract assertions using `*old_x ...` or `old_x->...`). [69]

When C++11 `auto` declarations are used with `BOOST_CONTRACT_OLDOF`, this library always defaults to using the `boost::contract::old_ptr` type (because its type requirements are more stringent than `boost::contract::old_ptr_if_copyable`). For example, the following statements are equivalent:

    auto old_x = BOOST_CONTRACT_OLDOF(x); // C++11 auto declarations always use `old_ptr` (never `old_ptr_if_copyable`).
    boost::contract::old_ptr<decltype(x)> old_x = BOOST_CONTRACT_OLDOF(x);

If programmers want to relax the copyable type requirement, they must do so explicitly by using the `boost::contract::old_ptr_if_copyable` type instead of using `auto` declarations.

##### Old Value Type Traits

This library uses `boost::contract::is_old_value_copyable` to determine if an old value type is copyable or not, and then `boost::contract::old_value_copy` to actually copy the old value.

By default, `boost::contract::is_old_value_copyable``<T>` is equivalent to `boost::is_copy_constructible<T>` and `boost::contract::old_value_copy``<T>` is implemented using `T`'s copy constructor. However, these type traits can be specialized by programmers for example to avoid making old value copies of types even when they have a copy constructor (maybe because these copy constructors are too expensive), or to make old value copies for types that do not have a copy constructor, or for any other specific need programmers might have for the types in question.

For example, the following specialization of `boost::contract::is_old_value_copyable` intentionally avoids making old value copies for all expressions of type `w` even if that type has a copy constructor (see `old_if_copyable.cpp`):

    // Copyable type but...
    class w {
    public:
        w(w const&) { /* Some very expensive copy operation here... */ }

        /* ... */

    // ...never copy old values for type `w` (because its copy is too expensive).
    namespace boost { namespace contract {
        template<>
        struct is_old_value_copyable<w> : boost::false_type {};
    } }

On the flip side, the following specializations of `boost::contract::is_old_value_copyable` and `boost::contract::old_value_copy` make old value copies of expressions of type `p` even if that type does not actually have a copy constructor (see `old_if_copyable.cpp`):

    // Non-copyable type but...
    class p : private boost::noncopyable {
        int* num_;

        friend struct boost::contract::old_value_copy<p>;

        /* ... */

    // ...still copy old values for type `p` (using a deep copy).
    namespace boost { namespace contract {
        template<>
        struct old_value_copy<p> {
            explicit old_value_copy(p const& old) {
                *old_.num_ = *old.num_; // Deep copy pointed value.
            }

            p const& old() const { return old_; }

        private:
            p old_;
        };

        template<>
        struct is_old_value_copyable<p> : boost::true_type {};
    } }

In general, `boost::is_copy_constructible` and therefore `boost::contract::is_old_value_copyable` require C++11 `decltype` and SFINAE to automatically detect if a given type is copyable or not. On non-C++11 compilers, it is possible to inherit the old value type from `boost::noncopyable`, or use `BOOST_MOVABLE_BUT_NOT_COPYABLE`, or specialize `boost::is_copy_constructible` (see `boost::is_copy_constructible` documentation for more information), or just specialize `boost::contract::is_old_value_copyable`. For example, for a non-copyable type `n` the following code will work also on non-C++11 compilers (see `old_if_copyable.cpp`):

    class n { // Do not want to use boost::noncopyable but...
        int num_;

    private:
        n(n const&); // ...unimplemented private copy constructor (so non-copyable).

        /* ... */

    // Specialize `boost::is_copy_constructible` (no need for this on C++11).
    namespace boost { namespace contract {
        template<>
        struct is_old_value_copyable<n> : boost::false_type {};
    } }

### Assertion Requirements (Templates)

In general, assertions can introduce a new set of requirements on the types used by the program. Some of these type requirements might be necessary only to check the assertions and they would not be required by the program otherwise.

In some cases it might be acceptable, or even desirable, to cause a compile-time error when a program uses types that do not provide all the operations needed to check contract assertions (because it is not possible to fully check the correctness of the program as specified by its contracts). In these cases, programmers can specify contract assertions as we have seen so far, compilation will fail if user types do not provide all operations necessary to check the contracts.

However, in some other cases it might be desirable to not augment the type requirements of a program just because of contract assertions and to simply skip assertions when user types do not provide all the operations necessary to check them, without causing compile-time errors. Programmers can do this using `if constexpr` on C++17 compilers, and `boost::contract::condition_if` (or `boost::contract::condition_if_c`) on non-C++17 compilers.

For example, let's consider the following `vector<T>` class template equivalent to `std::vector<T>`. C++ `std::vector<T>` does not require that its value type parameter `T` has an equality operator `==` (it only requires `T` to be copy constructible, see `std::vector` documentation). However, the contracts for the `vector<T>::push_back(value)` public function include a postcondition `back() == value` that introduces the new requirement that `T` must also have an equality operator `==`. Programmers can specify this postcondition as usual with `BOOST_CONTRACT_ASSERT(back() == value)` an let the program fail to compile when users instantiate `vector<T>` with a type `T` that does not provide an equality operator `==`. Otherwise, programmers can guard this postcondition using C++17 `if constexpr` to evaluate the asserted condition only for types `T` that have an equality operator `==` and skip it otherwise. [70] For example:

    template<typename T>
    class vector {
    public:
        void push_back(T const& value) {
            boost::contract::check c = boot::contract::public_function(this)
                .postcondition([&] {
                    // Guard with `if constexpr` for T without `==`.
                    if constexpr(boost::has_equal_to<T>::value)
                        BOOST_CONTRACT_ASSERT(back() == value);
                })
            ;

            vect_.push_back(value);
        }

        /* ... */

More in general, `if constexpr` can be used to guard a mix of both old value copies and contract assertions that introduce specific extra type requirements. For example, the following `swap` function can be called on any type `T` that is movable but its old value copies and postcondition assertions are evaluated only for types `T` that are also copyable and have an equality operator `==` (see `if_constexpr.cpp`):

    template<typename T>
    void swap(T& x, T& y) {
        constexpr bool b = boost::contract::is_old_value_copyable<T>::value &&
                boost::has_equal_to<T>::value;
        boost::contract::old_ptr<T> old_x, old_y;
        if constexpr(b) { // Contract requires copyable T...
            old_x = BOOST_CONTRACT_OLDOF(x);
            old_y = BOOST_CONTRACT_OLDOF(y);
        }
        boost::contract::check c = boost::contract::function()
            .postcondition([&] {
                if constexpr(b) { // ... and T with `==`...
                    BOOST_CONTRACT_ASSERT(x == *old_y);
                    BOOST_CONTRACT_ASSERT(y == *old_x);
                }
            })
        ;

        T t = std::move(x); // ...but body only requires movable T.
        x = std::move(y);
        y = std::move(t);
    }

##### No `if constexpr` (no C++17)

On non-C++17 compilers where `if constexpr` is not available, it is possible to use `boost::contract::condition_if` to skip assertions based on type requirements (even if this code is less readable and more verbose than using `if constexpr`). For example (see `condition_if.cpp`):

    template<typename T>
    class vector {
    public:
        void push_back(T const& value) {
            boost::contract::check c = boost::contract::public_function(this)
                .postcondition([&] {
                    // Instead of `ASSERT(back() == value)` for T without `==`.
                    BOOST_CONTRACT_ASSERT(
                        boost::contract::condition_if<boost::has_equal_to<T> >(
                            boost::bind(std::equal_to<T>(),
                                boost::cref(back()),
                                boost::cref(value)
                            )
                        )
                    );
                })
            ;

            vect_.push_back(value);
        }

        /* ... */

More in general, `boost::contract::condition_if` is used as follow:

    boost::contract::condition_if<Pred>(
        cond
    )

Where `Pred` is a nullary boolean meta-function and `cond` is a nullary boolean functor. If `Pred::value` is statically evaluated to be `true` at compile-time then `cond()` is called at run-time and its boolean result is returned by the enclosing `boost::contract::condition_if` call. Otherwise, if `Pred::value` is statically evaluated to be `false` at compile-time then `boost::contract::condition_if` trivially returns `true`. Therefore, if `cond` is a functor template instantiation (not just a functor) then its call that contains the assertion operations with the extra type requirements (e.g., the equality operator `==`) will not be actually instantiated and compiled unless the compiler determines at compile-time that `Pred::value` is `true` (when used this way, functor templates like `std::equal_to` and C++14 generic lambdas can be used to program `cond`, but C++11 lambdas cannot).

The `boost::contract::condition_if` function template is a special case of the more general `boost::contract::call_if`, specifically `boost::contract::condition_if``<Pred>(cond)` is logically equivalent to `boost::contract::call_if``<Pred>(cond).else_([] { return true; })`. [71] The `boost::contract::call_if` function template also accepts a number of optional else-if statements and one optional else statement:

    boost::contract::call_if<Pred1>(
        t1
    ).template else_if<Pred2>(          // Optional.
        t2
    )
    ...                                 // Optionally, other `else_if` statements.
    .else_(                             // Optional for `void` functors, otherwise required.
        e
    )

Where `Pred1`, `Pred2`, ... are nullary boolean meta-functions and `t1`, `t2`, ..., `e` are nullary functors. The return types of the functor calls `t1()`, `t2()`, ..., `e()` must either be all the same (including possibly all `void`) or be of types implicitly convertible into one another. At run-time `boost::contract::call_if` will call the functor `t1()`, or `t2()`, ..., or `e()` depending on which meta-function `Pred1::value`, `Pred2::value`, ... is statically evaluated to be `true` or `false` at compile-time, and it will return the value returned by the functor being called. If `t1`, `t2`, ..., `e` are functor template instantiations (not just functors) then their code will only be compiled if the compiler determines they need to be actually called at run-time (so only if the related `Pred1::value`, `Pred2::value`, ... are respectively evaluated to be `true` or `false` at compile-time). All the `else_if<...>(...)` statements are optional. The `else_(...)` statement is optional if the functor calls return `void`, otherwise it is required.

In general, `boost::contract::call_if` can be used to program contract assertions that compile and check different functor templates depending on related predicates being statically evaluated to be `true` or `false` at compile-time (but in most cases `boost::contract::condition_if` should be sufficient and less verbose to use). The `boost::contract::condition_if_c`, `boost::contract::call_if_c`, and `.else_if_c` function templates work similarly to their counterparts without the `..._c` postfix seen so far, but they take their predicate template parameters as static boolean values instead of nullary boolean meta-functions.

On compilers that support C++17 `if constexpr` there should be no need to use `boost::contract::condition_if` or `boost::contract::call_if` because `if constexpr` can be used instead (making the code more readable and easier to program). [72]

### Volatile Public Functions

This library allows to specify a different set of class invariants to check for volatile public functions. These volatile class invariants are programmed in a public `const volatile` function, named `invariant`, taking no argument, and returning `void` (see `BOOST_CONTRACT_INVARIANT_FUNC` to name the invariant function differently from `invariant` and Access Specifiers to not have to declare it `public`). Classes that do no have invariants for their volatile public functions, simply do not declare the `void invariant() const volatile` function.

In general, `const volatile` invariants work the same as `const` invariants (see Class Invariants) with the only difference that `volatile` and `const volatile` functions check `const volatile` invariants while non-`const` (i.e., neither `const` nor `volatile`) and `const` functions check `const` invariants. A given class can specify any combination of `static`, `const volatile`, and `const` invariant functions (see Class Invariants): [73]

- Constructors check `static` invariants at entry and exit (even if an exception is thrown), plus `const volatile` and `const` invariants in that order at exit but only if no exception is thrown.
- Destructors check `static` invariants at entry and exit (even if an exception is thrown), plus `const volatile` and `const` invariants in that order at entry (and at exit but only if an exception is thrown, even is destructors should in general never throw in C++).
- Both non-`const` and `const` public functions check `static` and `const` invariants at entry and at exit (even if an exception is thrown).
- Both `volatile` and `const volatile` public functions check `static` and `const volatile` invariants at entry and at exit (even if an exception is thrown).

These rules ensure that volatile class invariants are correctly checked (see Constructor Calls, Destructor Calls, and Public Function Calls). For example (see `volatile.cpp`):

    class u {
    public:
        static void static_invariant();     // Static invariants.
        void invariant() const volatile;    // Volatile invariants.
        void invariant() const;             // Const invariants.

        u() { // Check static, volatile, and const invariants.
            boost::contract::check c= boost::contract::constructor(this);
        }

        ~u() { // Check static, volatile, and const invariants.
            boost::contract::check c = boost::contract::destructor(this);
        }

        void nc() { // Check static and const invariants.
            boost::contract::check c = boost::contract::public_function(this);
        }

        void c() const { // Check static and const invariants.
            boost::contract::check c = boost::contract::public_function(this);
        }

        void v() volatile { // Check static and volatile invariants.
            boost::contract::check c = boost::contract::public_function(this);
        }

        void cv() const volatile { // Check static and volatile invariants.
            boost::contract::check c = boost::contract::public_function(this);
        }

        static void s() { // Check static invariants only.
            boost::contract::check c = boost::contract::public_function<u>();
        }
    };

This library does not automatically check `const volatile` invariants for non-`volatile` functions. However, if the contract specifications require it, programmers can explicitly call the `const volatile` invariant function from the `const` invariant function (preferably in that order to be consistent with the order `const volatile` and `const` invariants are checked for constructors and destructors). That way all public functions, `volatile` or not, will check `const volatile` invariants (while only `const` and non-`const` public functions will check only `const` invariants, correctly so because the `volatile` qualifier shall not be stripped away): [74]

    class u {
    public:
        void invariant() const volatile { ... }                 // Volatile invariants.

        void invariant() const {
            auto const volatile* cv = this; cv->invariant();    // Call `const volatile` invariant function above.
            ...                                                 // Other non-volatile invariants.
        }

        ...
    };

(As usual, private and protected functions do not check any invariant, not even when they are `volatile` or `const volatile`, see Private and Protected Functions).

### Move Operations

As with all public operations of a class, also public move operations should maintain class invariants (see [Stroustrup13], p. 520). Specifically, at a minimum C++ requires the following:

- The moved-from object can be copy assigned.
- The moved-from object can be move assigned.
- The moved-from object can be destroyed (if not for any other reason, this requires that class invariants are maintained by move operations because the destructor of the moved-from object requires class invariants to be satisfied at its entry, as always with destructors see Destructor Calls).

Therefore, both the move constructor and the move assignment operator need to maintain the class invariants of the moved-from object so their contracts can be programmed using `boost::contract::constructor` and `boost::contract::public_function` as usual. For example (see `move.cpp`):

    class circular_buffer :
            private boost::contract::constructor_precondition<circular_buffer> {
    public:
        void invariant() const {
            if(!moved()) { // Do not check (some) invariants for moved-from objects.
                BOOST_CONTRACT_ASSERT(index() < size());
            }
            // More invariants here that hold also for moved-from objects (e.g.,
            // all assertions necessary to successfully destroy moved-from objects).
        }

        // Move constructor.
        circular_buffer(circular_buffer&& other) :
            boost::contract::constructor_precondition<circular_buffer>([&] {
                BOOST_CONTRACT_ASSERT(!other.moved());
            })
        {
            boost::contract::check c = boost::contract::constructor(this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(!moved());
                    BOOST_CONTRACT_ASSERT(other.moved());
                })
            ;

            move(std::forward<circular_buffer>(other));
        }

        // Move assignment.
        circular_buffer& operator=(circular_buffer&& other) {
            // Moved-from can be (move) assigned (so no pre `!moved()` here).
            boost::contract::check c = boost::contract::public_function(this)
                .precondition([&] {
                    BOOST_CONTRACT_ASSERT(!other.moved());
                })
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(!moved());
                    BOOST_CONTRACT_ASSERT(other.moved());
                })
            ;

            return move(std::forward<circular_buffer>(other));
        }

        ~circular_buffer() {
            // Moved-from can always be destroyed (in fact no preconditions).
            boost::contract::check c = boost::contract::destructor(this);
        }

        bool moved() const {
            boost::contract::check c = boost::contract::public_function(this);
            return moved_;
        }

    private:
        bool moved_;

        /* ... */

This example assumes that it is possible to call the public function `moved()` on the moved-from object. [75]

| Note
The default move constructor and move assignment operator generated by C++ will not automatically check contracts. Therefore, unless the move operations are not public or they have no preconditions, no postconditions, and their class has no invariants, programmers should manually define them using `boost::contract::constructor`, `boost::contract::constructor_precondition`, and `boost::contract::public_function` instead of relying on their default implementations generated by C++. (Same as for all other operations automatically implemented by C++.)

As always, programmers can decide to not program contracts for a given type. Specifically, they might decide to not program contracts for a class that needs to be moved in order to avoid the run-time overhead of checking contract assertions and to maximize performance (see Benefits and Costs).

### Unions

A C++ `union` cannot have virtual functions, base classes, and cannot be used as a base class thus subcontracting (`boost::contract::virtual_`, `BOOST_CONTRACT_OVERRIDE`, etc.) do not apply to unions. Also a `union` cannot inherit from `boost::contract::constructor_precondition` (because it cannot have base classes), instead `boost::contract::constructor_precondition` is used to declare a local object that checks constructor preconditions (at the very beginning of the constructor before old value copies and other contracts, see declaration of `pre` in the example below). A part from that, this library is used as usual to program contracts for unions. For example (see `union.cpp`):

    union positive {
    public:
        static void static_invariant() { // Static class invariants (as usual).
            BOOST_CONTRACT_ASSERT(instances() >= 0);
        }

        void invariant() const { // Class invariants (as usual).
            BOOST_CONTRACT_ASSERT(i_ > 0);
            BOOST_CONTRACT_ASSERT(d_ > 0);
        }

        // Contracts for constructor, as usual but...
        explicit positive(int x) : d_(0) {
            // ...unions cannot have bases so constructor preconditions here.
            boost::contract::constructor_precondition<positive> pre([&] {
                BOOST_CONTRACT_ASSERT(x > 0);
            });
            boost::contract::old_ptr<int> old_instances =
                    BOOST_CONTRACT_OLDOF(instances());
            boost::contract::check c = boost::contract::constructor(this)
                .postcondition([&] {
                    { int y; get(y); BOOST_CONTRACT_ASSERT(y == x); }
                    BOOST_CONTRACT_ASSERT(instances() == *old_instances + 1);
                })
            ;

            i_ = x;
            ++instances_;
        }

        // Contracts for destructor (as usual).
        ~positive() {
            boost::contract::old_ptr<int> old_instances =
                    BOOST_CONTRACT_OLDOF(instances());
            boost::contract::check c = boost::contract::destructor(this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(instances() == *old_instances - 1);
                })
            ;

            --instances_;
        }

        // Contracts for public function (as usual, but no virtual or override).
        void get(int& x) const {
            boost::contract::check c = boost::contract::public_function(this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(x > 0);
                })
            ;

            x = i_;
        }

        // Contracts for static public function (as usual).
        static int instances() {
            boost::contract::check c = boost::contract::public_function<positive>();
            return instances_;
        }

    private:
        int i_;
        double d_;

        /* ... */

### Assertion Levels

This library provides three predefined assertion levels that can be used to selectively disable assertions depending on their computational complexity: [76]

- `BOOST_CONTRACT_ASSERT` is used to assert conditions that are not computationally expensive, at least compared to the cost of executing the function body. These assertions are the ones we have seen so far, they are always checked at run-time and they cannot be disabled.
- `BOOST_CONTRACT_ASSERT_AUDIT` is used to assert conditions that are computationally expensive compared to the cost of executing the function body. These assertions are not checked at run-time unless programmers explicitly define `BOOST_CONTRACT_AUDITS` (undefined by default), but the asserted conditions are always compiled and therefore validated syntactically (even when they are not actually evaluated and checked at run-time).
- `BOOST_CONTRACT_ASSERT_AXIOM` is used to assert conditions that are computationally prohibitive, at least compared to the cost of executing the function body. These assertions are never evaluated or checked at run-time, but the asserted conditions are always compiled and therefore validated syntactically (so these assertions can serve as formal comments to the code).

In addition, `BOOST_CONTRACT_CHECK_AUDIT` and `BOOST_CONTRACT_CHECK_AXIOM` are similar to `BOOST_CONTRACT_ASSERT_AUDIT` and `BOOST_CONTRACT_ASSERT_AXIOM` but they are used to program audit and axiom levels for implementation checks instead of assertions (see Implementation Checks).

For example, `BOOST_CONTRACT_ASSERT_AUDIT` can be used to program computationally expensive assertions (see `assertion_level.cpp`):

    template<typename RandomIter, typename T>
    RandomIter random_binary_search(RandomIter first, RandomIter last,
            T const& value) {
        RandomIter result;
        boost::contract::check c = boost::contract::function()
            .precondition([&] {
                BOOST_CONTRACT_ASSERT(first <= last); // Default, not expensive.
                // Expensive O(n) assertion (use AXIOM if prohibitive instead).
                BOOST_CONTRACT_ASSERT_AUDIT(std::is_sorted(first, last));
            })
            .postcondition([&] {
                if(result != last) BOOST_CONTRACT_ASSERT(*result == value);
            })
        ;

        /* ... */

Similarly, `BOOST_CONTRACT_AUDITS` can be used to disable expensive old value copies and related assertions that use them (see `assertion_level.cpp`):

    template<typename T>
    class vector {

    public:
        void swap(vector& other) {
            boost::contract::old_ptr<vector> old_me, old_other;
            #ifdef BOOST_CONTRACT_AUDITS
                old_me = BOOST_CONTRACT_OLDOF(*this);
                old_other = BOOST_CONTRACT_OLDOF(other);
            #endif // Else, skip old value copies...
            boost::contract::check c = boost::contract::public_function(this)
                .postcondition([&] {
                    // ...and also skip related assertions.
                    BOOST_CONTRACT_ASSERT_AUDIT(*this == *old_other);
                    BOOST_CONTRACT_ASSERT_AUDIT(other == *old_me);
                })
            ;

            vect_.swap(other.vect_);
        }

        /* ... */

    private:
        std::vector<T> vect_;
    };

The condition passed to `BOOST_CONTRACT_ASSERT_AXIOM` is compiled but not actually evaluated at run-time so this macro can be used to program computationally prohibitive assertions but also assertions that cannot actually be programmed in C++ using functions that are declared but left undefined. For example, (see `assertion_level.cpp`):

    // If valid iterator range (cannot implement in C++ but OK to use in AXIOM).
    template<typename Iter>
    bool valid(Iter first, Iter last); // Only declared, not actually defined.

    template<typename T>
    class vector {

    public:
        iterator insert(iterator where, T const& value) {
            iterator result;
            boost::contract::old_ptr<unsigned> old_capacity =
                    BOOST_CONTRACT_OLDOF(capacity());
            boost::contract::check c = boost::contract::public_function(this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(capacity() >= *old_capacity);
                    if(capacity() > *old_capacity) {
                        BOOST_CONTRACT_ASSERT_AXIOM(!valid(begin(), end()));
                    } else {
                        BOOST_CONTRACT_ASSERT_AXIOM(!valid(where, end()));
                    }
                })
            ;

            return result = vect_.insert(where, value);
        }

        /* ... */

    private:
        std::vector<T> vect_;
    };

In addition to these assertion levels that are predefined by this library, programmers are free to define their own. For example, the following macro could be used to program and selectively disable assertions that have exponential computational complexity `O(e^n)`:

    #ifdef EXPONENTIALLY_COMPLEX_ASSERTIONS
        // Following will compile and also evaluate `cond`.
        #define ASSERT_EXP(cond) BOOST_CONTRACT_ASSERT(cond)
    #else
        // Following will compile but never actually evaluate `cond`.
        #define ASSERT_EXP(cond) BOOST_CONTRACT_ASSERT(true || (cond))
    #endif

    ...

    ASSERT_EXP(some-exponentially-complex-boolean-condition);

### Disable Contract Checking

Checking contracts adds run-time overhead and can slow down program execution (see Benefits and Costs). Therefore, programmers can define any combination of the following macros (`-D` option in Clang and GCC, `/D` option in MSVC, etc.) to instruct this library to not check specific groups of contract conditions at run-time:

- Define `BOOST_CONTRACT_NO_PRECONDITIONS` to not check preconditions.
- Define `BOOST_CONTRACT_NO_POSTCONDITIONS` to not check postconditions.
- Define `BOOST_CONTRACT_NO_EXCEPTS` to not check exception guarantees.
- Define `BOOST_CONTRACT_NO_ENTRY_INVARIANTS` to not check class invariants at call entry.
- Define `BOOST_CONTRACT_NO_EXIT_INVARIANTS` to not check class invariants at call exit.
- Or, define `BOOST_CONTRACT_NO_INVARIANTS` to not check class invariants at both call entry and exit. (This is provided for convenience, it is equivalent to defining both `BOOST_CONTRACT_NO_ENTRY_INVARIANTS` and `BOOST_CONTRACT_NO_EXIT_INVARIANTS`.)
- Define `BOOST_CONTRACT_NO_CHECKS` to not evaluate implementation checks.

| Note
Old values can be used by both postconditions and exception guarantees so it is necessary to define both `BOOST_CONTRACT_NO_POSTCONDITIONS` and `BOOST_CONTRACT_NO_EXCEPTS` to disable old value copies.

By default, none of these macros are defined so this library checks all contracts. When these macros are defined by the user, the implementation code of this library is internally optimized to minimize as much as possible any run-time and compile-time overhead associated with checking and compiling contracts (see Disable Contract Compilation for techniques to completely remove any run-time and compile-time overheads associated with contract code).

For example, programmers could decide to check all contracts during early development builds, but later check only preconditions and maybe entry invariants for release builds by defining `BOOST_CONTRACT_NO_POSTCONDITIONS`, `BOOST_CONTRACT_NO_EXCEPTS`, `BOOST_CONTRACT_NO_EXIT_INVARIANTS`, and `BOOST_CONTRACT_NO_CHECKS`.

### Disable Contract Compilation (Macro Interface)

This library provides macros that can be used to completely disable compile-time and run-time overhead introduced by contracts but at the cost of manually programming `#ifndef BOOST_CONTRACT_NO_...` statements around contract code:

- This library defines `BOOST_CONTRACT_NO_CONSTRUCTORS` when contract checking is disabled for constructors.
- This library defines `BOOST_CONTRACT_NO_DESTRUCTORS` when contract checking is disabled for destructors.
- This library defines `BOOST_CONTRACT_NO_PUBLIC_FUNCTIONS` when contract checking is disabled for public functions.
- This library defines `BOOST_CONTRACT_NO_FUNCTIONS` when contract checking is disabled for (non-public and non-member) functions.
- This library defines `BOOST_CONTRACT_NO_OLDS` when old value copies are disabled.
- This library defines `BOOST_CONTRACT_NO_ALL` when all contracts above and also implementation checks (see `BOOST_CONTRACT_NO_CHECKS`) are disabled.

These macros are not configuration macros and they should not be defined directly by programmers (otherwise this library will generate compile-time errors). Instead, these macros are automatically defined by this library when programmers define `BOOST_CONTRACT_NO_PRECONDITIONS`, `BOOST_CONTRACT_NO_POSTCONDITIONS`, `BOOST_CONTRACT_NO_EXCEPTS`, `BOOST_CONTRACT_NO_INVARIANTS` (or `BOOST_CONTRACT_NO_ENTRY_INVARIANTS` and `BOOST_CONTRACT_NO_EXIT_INVARIANTS`), and `BOOST_CONTRACT_NO_CHECKS` (see Disable Contract Checking).

Alternatively, this library provides a macro-based interface defined in `boost/contract_macro.hpp` that can also be used to completely disable compile-time and run-time overheads introduced by contracts but without the burden of manually writing the `#ifndef BOOST_CONTRACT_NO_...` statements. For example, the following code shows how to use both the `boost/contract_macro.hpp` macro interface and the `#ifndef BOOST_CONTRACT_NO_...` statements to completely disable compile-time and run-time overheads for non-member function contracts (see `ifdef_macro.cpp` and `ifdef.cpp`):

Macro Interface | `#ifndef BOOST_CONTRACT_NO_...` Statements
--- | ---
    // Use macro interface to completely disable contract code compilation.
    #include <boost/contract_macro.hpp>

    int inc(int& x) {
        int result;
        BOOST_CONTRACT_OLD_PTR(int)(old_x, x);
        BOOST_CONTRACT_FUNCTION()
            BOOST_CONTRACT_PRECONDITION([&] {
                BOOST_CONTRACT_ASSERT(x < std::numeric_limits<int>::max());
            })
            BOOST_CONTRACT_POSTCONDITION([&] {
                BOOST_CONTRACT_ASSERT(x == *old_x + 1);
                BOOST_CONTRACT_ASSERT(result == *old_x);
            })
        ;

        return result = x++;
    }
    // Use #ifdef to completely disable contract code compilation.
    #include <boost/contract/core/config.hpp>
    #ifndef BOOST_CONTRACT_NO_ALL
        #include <boost/contract.hpp>
    #endif

    int inc(int& x) {
        int result;
        #ifndef BOOST_CONTRACT_NO_OLDS
            boost::contract::old_ptr<int> old_x = BOOST_CONTRACT_OLDOF(x);
        #endif
        #ifndef BOOST_CONTRACT_NO_FUNCTIONS
            boost::contract::check c = boost::contract::function()
                #ifndef BOOST_CONTRACT_NO_PRECONDITIONS
                    .precondition([&] {
                        BOOST_CONTRACT_ASSERT(x < std::numeric_limits<int>::max());
                    })
                #endif
                #ifndef BOOST_CONTRACT_NO_POSTCONDITIONS
                    .postcondition([&] {
                        BOOST_CONTRACT_ASSERT(x == *old_x + 1);
                        BOOST_CONTRACT_ASSERT(result == *old_x);
                    })
                #endif
            ;
        #endif

        return result = x++;
    }

The same can be done to disable contract code complication for private and protected functions. The `BOOST_CONTRACT_OLD_PTR_IF_COPYABLE` macro is provided to handle non-copyable old value types (similar to `boost::contract::old_ptr_if_copyable`).

For constructors, destructors, and public functions the `boost/contract_macro.hpp` macro interface and the `#ifndef BOOST_CONTRACT_NO_...` statements can be used as follow (see `ifdef_macro.cpp` and `ifdef.cpp`):

Macro Interface | `#ifndef BOOST_CONTRACT_NO_...` Statements
--- | ---
    class integers
        #define BASES public pushable<int>
        :
            // Left in code (almost no overhead).
            private boost::contract::constructor_precondition<integers>,
            BASES
    {
        // Followings left in code (almost no overhead).
        friend class boost::contract::access;

        typedef BOOST_CONTRACT_BASE_TYPES(BASES) base_types;
        #undef BASES

        BOOST_CONTRACT_INVARIANT({
            BOOST_CONTRACT_ASSERT(size() <= capacity());
        })

    public:
        integers(int from, int to) :
            BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION(integers)([&] {
                BOOST_CONTRACT_ASSERT(from <= to);
            }),
            vect_(to - from + 1)
        {
            BOOST_CONTRACT_CONSTRUCTOR(this)
                BOOST_CONTRACT_POSTCONDITION([&] {
                    BOOST_CONTRACT_ASSERT(int(size()) == (to - from + 1));
                })
            ;

            for(int x = from; x <= to; ++x) vect_.at(x - from) = x;
        }

        virtual ~integers() {
            BOOST_CONTRACT_DESTRUCTOR(this); // Check invariants.
        }

        virtual void push_back(
            int const& x,
            boost::contract::virtual_* v = 0 // Left in code (almost no overhead).
        ) /* override */ {
            BOOST_CONTRACT_OLD_PTR(unsigned)(old_size);
            BOOST_CONTRACT_PUBLIC_FUNCTION_OVERRIDE(override_push_back)(
                    v, &integers::push_back, this, x)
                BOOST_CONTRACT_PRECONDITION([&] {
                    BOOST_CONTRACT_ASSERT(size() < max_size());
                })
                BOOST_CONTRACT_OLD([&] {
                    old_size = BOOST_CONTRACT_OLDOF(v, size());
                })
                BOOST_CONTRACT_POSTCONDITION([&] {
                    BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
                })
                BOOST_CONTRACT_EXCEPT([&] {
                    BOOST_CONTRACT_ASSERT(size() == *old_size);
                })
            ;

            vect_.push_back(x);
        }

    private:
        BOOST_CONTRACT_OVERRIDE(push_back) // Left in code (almost no overhead).

        /* ... */
    class integers
        #define BASES public pushable<int>
        :
            #ifndef BOOST_CONTRACT_NO_PRECONDITIONS
                private boost::contract::constructor_precondition<integers>, BASES
            #else
                BASES
            #endif
    {
        #ifndef BOOST_CONTRACT_NO_ALL
            friend class boost::contract::access;
        #endif

        #ifndef BOOST_CONTRACT_NO_PUBLIC_FUNCTIONS
            typedef BOOST_CONTRACT_BASE_TYPES(BASES) base_types;
        #endif
        #undef BASES

        #ifndef BOOST_CONTRACT_NO_INVARIANTS
            void invariant() const {
                BOOST_CONTRACT_ASSERT(size() <= capacity());
            }
        #endif

    public:
        integers(int from, int to) :
            #ifndef BOOST_CONTRACT_NO_PRECONDITIONS
                boost::contract::constructor_precondition<integers>([&] {
                    BOOST_CONTRACT_ASSERT(from <= to);
                }),
            #endif
            vect_(to - from + 1)
        {
            #ifndef BOOST_CONTRACT_NO_CONSTRUCTORS
                boost::contract::check c = boost::contract::constructor(this)
                    #ifndef BOOST_CONTRACT_NO_POSTCONDITIONS
                        .postcondition([&] {
                            BOOST_CONTRACT_ASSERT(int(size()) == (to - from + 1));
                        })
                    #endif
                ;
            #endif

            for(int x = from; x <= to; ++x) vect_.at(x - from) = x;
        }

        virtual ~integers() {
            #ifndef BOOST_CONTRACT_NO_DESTRUCTORS
                // Check invariants.
                boost::contract::check c = boost::contract::destructor(this);
            #endif
        }

        virtual void push_back(
            int const& x
            #ifndef BOOST_CONTRACT_NO_PUBLIC_FUNCTIONS
                , boost::contract::virtual_* v = 0
            #endif
        ) /* override */ {
            #ifndef BOOST_CONTRACT_NO_OLDS
                boost::contract::old_ptr<unsigned> old_size;
            #endif
            #ifndef BOOST_CONTRACT_NO_PUBLIC_FUNCTIONS
                boost::contract::check c = boost::contract::public_function<
                        override_push_back>(v, &integers::push_back, this, x)
                    #ifndef BOOST_CONTRACT_NO_PRECONDITIONS
                        .precondition([&] {
                            BOOST_CONTRACT_ASSERT(size() < max_size());
                        })
                    #endif
                    #ifndef BOOST_CONTRACT_NO_OLDS
                        .old([&] {
                            old_size = BOOST_CONTRACT_OLDOF(v, size());
                        })
                    #endif
                    #ifndef BOOST_CONTRACT_NO_POSTCONDITIONS
                        .postcondition([&] {
                            BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
                        })
                    #endif
                    #ifndef BOOST_CONTRACT_NO_EXCEPTS
                        .except([&] {
                            BOOST_CONTRACT_ASSERT(size() == *old_size);
                        })
                    #endif
                ;
            #endif

            vect_.push_back(x);
        }

    private:
        #ifndef BOOST_CONTRACT_NO_PUBLIC_FUNCTIONS
            BOOST_CONTRACT_OVERRIDE(push_back)
        #endif

        /* ... */

Static and volatile class invariants can be programmed using `BOOST_CONTRACT_STATIC_INVARIANT` and `BOOST_CONTRACT_INVARIANT_VOLATILE` respectively (these macros expand code equivalent to the `static void BOOST_CONTRACT_STATIC_INVARIANT_FUNC()` and `void BOOST_CONTRACT_INVARIANT_FUNC() const volatile` functions).

The `boost/contract_macro.hpp` macro interface is usually preferred because more concise and easier to use than programming `#ifndef BOOST_CONTRACT_NO_...` statements by hand. However, C++ macros expand on a single line of code and that can make compiler errors less useful when using this macro interface plus all contract assertions within a given set of preconditions, postconditions, exception guarantees, and class invariants will list the same line number in error messages when assertions fail at run-time (but error messages still list the assertion code and that should still allow programmers to identify the specific assertion that failed). Finally, the macro interface leaves a bit of contract decorations in the code but that should add no measurable compile-time or run-time overhead (specifically, extra `boost::contract::virtual_``*` parameters, calls to `boost::contract::constructor_precondition` default constructor which does nothing, `BOOST_CONTRACT_BASE_TYPES` `typedef`s, and `boost::contract::access` friendships are left in user code even when contracts are disabled unless `#ifndef BOOST_CONTRACT_NO_...` statements are used).

Disabling contract as shown in Disable Contract Checking leaves the overhead of compiling contract code plus some small run-time overhead due to the initialization of old value pointers (even if those will be all null and no old value will be actually copied), the calls to the contract functions used to initialize `boost::contract::check` and `boost::contract::constructor_precondition` (even if those calls will be internally optimized by this library to essentially do nothing), etc. For truly performance critical code for which even such small run-time overhead might not be acceptable, the macro interface (or the `#ifndef BOOST_CONTRACT_NO_...` statements) can be used to completely disable compile-time and run-time overheads of contracts. However, for such performance critical code even the overhead of checking simple preconditions might be too much so it might be best to not program contracts at all.

Usually, if the overhead of checking preconditions and other assertions is already considered acceptable for an application then the compile-time overhead of contracts should not represent an issue and it should be sufficient to disable contract checking at run-time as indicated in Disable Contract Checking (without a real need to use the `boost/contract_macro.hpp` macro interface or the `#ifndef BOOST_CONTRACT_NO_...` statements in most cases).

### Separate Body Implementation

Contracts are part of the program specifications and not of its implementation (see Specifications vs. Implementation). However, this library uses function definitions to program contracts so contract code appears together with the function implementation code. This is not ideal (even if contracts programmed using this library will always appear at the very beginning of the function definition so programmers will easily be able to distinguish contract code from the rest of the function implementation code so this might not be real limitation in practise).

In some cases, it might be desirable to completely separate the contract code from the function implementation code. For example, this could be necessary for software that ships only header files and compiled object files to its users. If contracts are programmed in function definitions that are compiled in the object files, users will not be able to see the contract code to understand semantics and usage of the functions (again, this might not be a real problem in practice for example if contracts are already somehow extracted from the source code by some tool and presented as part of the documentation of the shipped software).

In any case, when it is truly important to separate contracts from function implementation code, function implementations can be programmed in extra body functions (here named `..._body`, but any other naming scheme could be used) that are compiled in object files. Function definitions that remain in header files instead will contain just contract code followed by calls to the extra body functions. This technique allows to keep the contract code in header files while separating the implementation code to source and object files. However, this adds the overhead of manually programming an extra function declaration for each body function (plus the limitation that constructor member initialization lists must be programmed in header files because that is where constructors need to be defined to list constructor contract code). [77]

For example, the following header file only contains function declarations, contract code, and constructor member initializations, but it does not contain the code implementing the function bodies (see `separate_body.hpp`):

    class iarray :
            private boost::contract::constructor_precondition<iarray> {
    public:
        void invariant() const {
            BOOST_CONTRACT_ASSERT(size() <= capacity());
        }

        explicit iarray(unsigned max, unsigned count = 0) :
            boost::contract::constructor_precondition<iarray>([&] {
                BOOST_CONTRACT_ASSERT(count <= max);
            }),
            // Still, member initializations must be here.
            values_(new int[max]),
            capacity_(max)
        {
            boost::contract::check c = boost::contract::constructor(this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(capacity() == max);
                    BOOST_CONTRACT_ASSERT(size() == count);
                })
            ;
            constructor_body(max, count); // Separate constructor body impl.
        }

        virtual ~iarray() {
            boost::contract::check c = boost::contract::destructor(this); // Inv.
            destructor_body(); // Separate destructor body implementation.
        }

        virtual void push_back(int value, boost::contract::virtual_* v = 0) {
            boost::contract::old_ptr<unsigned> old_size =
                    BOOST_CONTRACT_OLDOF(v, size());
            boost::contract::check c = boost::contract::public_function(v, this)
                .precondition([&] {
                    BOOST_CONTRACT_ASSERT(size() < capacity());
                })
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
                })
            ;
            push_back_body(value); // Separate member function body implementation.
        }

    private:
        // Contracts in class declaration (above), but body implementations are not.
        void constructor_body(unsigned max, unsigned count);
        void destructor_body();
        void push_back_body(int value);

        /* ... */

Instead, the function bodies are implemented in a separate source file (see `separate_body.cpp`):

    void iarray::constructor_body(unsigned max, unsigned count) {
        for(unsigned i = 0; i < count; ++i) values_[i] = int();
        size_ = count;
    }

    void iarray::destructor_body() { delete[] values_; }

    void iarray::push_back_body(int value) { values_[size_++] = value; }

    /* ... */

The same technique can be used for non-member, private, and protected functions, etc.

| Note
When contracts are programmed only in `.cpp` files and also all this library headers are `#include`d only from `.cpp` files, then these `.cpp` files can be compiled disabling specific contract checking (for example, `BOOST_CONTRACT_NO_POSTCONDITIONS`, `BOOST_CONTRACT_NO_EXCEPTS`, and `BOOST_CONTRACT_NO_EXIT_INVARIANTS`, see Disable Contract Checking). Then the code in these `.cpp` files will always have such contract checking disabled even when linked to some other user code that might have been compiled with a different set of contracts disabled (i.e., a different set of `BOOST_CONTRACT_NO_...` macros defined). This technique might be useful to ship compiled object files (e.g., for a library) that will never check some contracts (e.g., postconditions, exception guarantees, and exit invariants) regardless of the definition of the `BOOST_CONTRACT_NO_...` macros used to compile code that links against the shipped object files. On the flip side, if contracts are programmed only in header files (e.g., using extra `..._body` functions as shown in this section) and this library headers are `#include`d only in these header files that are being shipped, then end users can enable or disables contract checking of the shipped code by defining the `BOOST_CONTRACT_NO_...` macros when they compile the shipped header files as part of their code. This technique might be useful in other situations when programmers that ship code want to leave it up the their end users to decide which contracts of the shipped code should be checked at run-time.

### No Lambda Functions (No C++11)

This section shows how to use this library without C++11 lambda functions. This has some advantages:

- It allows to use this library on compilers that do not support C++11 lambda functions (essentially most C++03 compilers with adequate support for SFINAE can be used in that case, see No Macros to also avoid using variadic macros). [78]
- Contract functions (i.e., the `..._precondition`, `..._old`, and `..._postcondition` functions in the example below) can be programmed to fully enforce constant-correctness and other contract requirements at compile-time (see Constant-Correctness). [79]
- Code of the contract functions is separated from function body implementations (see Separate Body Implementation).

However, not using C++11 lambda functions comes at the significant cost of having to manually program the extra contract functions and related boiler-plate code. For example, the header file (see `no_lambdas.hpp`):

    class iarray :
            private boost::contract::constructor_precondition<iarray> {
    public:
        static void static_invariant() {
            BOOST_CONTRACT_ASSERT(instances() >= 0);
        }

        void invariant() const {
            BOOST_CONTRACT_ASSERT(size() <= capacity());
        }

        explicit iarray(unsigned max, unsigned count = 0);
            static void constructor_precondition(unsigned const max,
                    unsigned const count) {
                BOOST_CONTRACT_ASSERT(count <= max);
            }
            static void constructor_old(boost::contract::old_ptr<int>&
                    old_instances) {
                old_instances = BOOST_CONTRACT_OLDOF(instances());
            }
            void constructor_postcondition(unsigned const max, unsigned const count,
                    boost::contract::old_ptr<int> const old_instances) const {
                BOOST_CONTRACT_ASSERT(capacity() == max);
                BOOST_CONTRACT_ASSERT(size() == count);
                BOOST_CONTRACT_ASSERT(instances() == *old_instances + 1);
            }

        virtual ~iarray();
            void destructor_old(boost::contract::old_ptr<int>& old_instances)
                    const {
                old_instances = BOOST_CONTRACT_OLDOF(instances());
            }
            static void destructor_postcondition(boost::contract::old_ptr<int> const
                    old_instances) {
                BOOST_CONTRACT_ASSERT(instances() == *old_instances - 1);
            }

        virtual void push_back(int value, boost::contract::virtual_* v = 0);
            void push_back_precondition() const {
                BOOST_CONTRACT_ASSERT(size() < capacity());
            }
            void push_back_old(boost::contract::virtual_* v,
                    boost::contract::old_ptr<unsigned>& old_size) const {
                old_size = BOOST_CONTRACT_OLDOF(v, size());
            }
            void push_back_postcondition(
                    boost::contract::old_ptr<unsigned> const old_size) const {
                BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
            }

        unsigned capacity() const;
        unsigned size() const;

        static int instances();

    private:
        int* values_;
        unsigned capacity_;
        unsigned size_;
        static int instances_;
    };

And, the source file (see `no_lambdas.cpp`):

    iarray::iarray(unsigned max, unsigned count) :
        boost::contract::constructor_precondition<iarray>(boost::bind(
                &iarray::constructor_precondition, max, count)),
        values_(new int[max]), // Member initializations can be here.
        capacity_(max)
    {
        boost::contract::old_ptr<int> old_instances;
        boost::contract::check c = boost::contract::constructor(this)
            .old(boost::bind(&iarray::constructor_old, boost::ref(old_instances)))
            .postcondition(boost::bind(
                &iarray::constructor_postcondition,
                this,
                boost::cref(max),
                boost::cref(count),
                boost::cref(old_instances)
            ))
        ;

        for(unsigned i = 0; i < count; ++i) values_[i] = int();
        size_ = count;
        ++instances_;
    }

    iarray::~iarray() {
        boost::contract::old_ptr<int> old_instances;
        boost::contract::check c = boost::contract::destructor(this)
            .old(boost::bind(&iarray::destructor_old, this,
                    boost::ref(old_instances)))
            .postcondition(boost::bind(&iarray::destructor_postcondition,
                    boost::cref(old_instances)))
        ;

        delete[] values_;
        --instances_;
    }

    void iarray::push_back(int value, boost::contract::virtual_* v) {
        boost::contract::old_ptr<unsigned> old_size;
        boost::contract::check c = boost::contract::public_function(v, this)
            .precondition(boost::bind(&iarray::push_back_precondition, this))
            .old(boost::bind(&iarray::push_back_old, this, boost::cref(v),
                    boost::ref(old_size)))
            .postcondition(boost::bind(&iarray::push_back_postcondition, this,
                    boost::cref(old_size)))
        ;

        values_[size_++] = value;
    }

    unsigned iarray::capacity() const {
        // Check invariants.
        boost::contract::check c = boost::contract::public_function(this);
        return capacity_;
    }

    unsigned iarray::size() const {
        // Check invariants.
        boost::contract::check c = boost::contract::public_function(this);
        return size_;
    }

    int iarray::instances() {
        // Check static invariants.
        boost::contract::check c = boost::contract::public_function<iarray>();
        return instances_;
    }

    int iarray::instances_ = 0;

If programmers also want to fully enforce all contract programming constant-correctness requirements at compile-time, they should follow these rules when programming the contract functions (see Constant-Correctness):

- Precondition functions (i.e., the `..._precondition` functions in the example above) can take their arguments either by `const` value or by `const&`, and when they are member functions they should be either `static` or `const` functions.
- Postcondition functions (i.e., the `..._postcondition` functions in the example above) should take their arguments by `const&`, and when they are member functions they should be either `static` or `const` functions.
- Similarly, exception guarantee functions (not shown in the example above) should take their arguments by `const&`, and when they are member functions they should be either `static` or `const` functions.
- Old value functions (i.e., the `..._old` functions in the example above) should take their arguments by `const&` a part from old value pointers that should be taken by `&` (so only old value pointers can be modified), and when they are member functions they should be either `static` or `const` functions.
- For constructors: Precondition, old value, and exception guarantee functions should be `static` (because there is no valid object `this` if the constructor body does not run successfully, see Constructor Calls).
- For destructors: Postcondition functions should be `static` (because there is no valid object `this` after the destructor body runs successfully, but exception guarantee functions do not have to be `static` since the object `this` is still valid because the destructor body did not run successfully, see Destructor Calls).

Note that the extra contract functions also allow to keep the contract code in the header file while all function bodies are implemented in a separate source file (including the constructor member initialization list, that could not be done with the techniques shown in Separate Body Implementation). [80] Also note that the contract functions can always be declared `private` if programmers need to exactly control the public members of the class (this was not done in this example only for brevity).

The authors think this library is most useful when used together with C++11 lambda functions (because of the large amount of boiler-plate code required when C++11 lambdas are not used as also shown by the example above).

### No Macros (and No Variadic Macros)

It is possible to specify contracts without using most of the macros provided by this library and programming the related code manually instead (the only macros that cannot be programmed manually are `BOOST_CONTRACT_OVERRIDE`, `BOOST_CONTRACT_OVERRIDES`, and `BOOST_CONTRACT_NAMED_OVERRIDE`).

| Note
Some of this library macros are variadic macros, others are not (see below). Variadic macros were officially added to the language in C++11 but most compilers have been supporting them as an extension for a long time, plus all compilers that support C++11 lambda functions should also support C++11 variadic macros (and this library might rarely be used without the convenience of C++11 lambda functions, see No Lambda Functions). [81] Therefore, the rest of this section can be considered mainly a curiosity because programmers should seldom, if ever, need to use this library without using its macros.

##### Overrides

As shown in Public Function Overrides and Named Overrides, this library provides the `BOOST_CONTRACT_OVERRIDE` and `BOOST_CONTRACT_NAMED_OVERRIDE` macros to program contracts for overriding public functions (see `BOOST_CONTRACT_MAX_ARGS` for compilers that do not support variadic templates). [82] These macro cannot be programmed manually but they are not variadic macros (so programmers should be able to use them on any C++ compiler with a sound support for SFINAE). [83] The `BOOST_CONTRACT_OVERRIDES` macro is a variadic macro instead but programmes can manually repeat the non-variadic macro `BOOST_CONTRACT_OVERRIDE` for each overriding public function name on compilers that do not support variadic macros.

##### Assertions (Not Variadic)

As shown in Preconditions, Postconditions, Exception Guarantees, Class Invariants, etc. this library provides the `BOOST_CONTRACT_ASSERT` macro to assert contract conditions. This is not a variadic macro and programmers should be able to use it on all C++ compilers. In any case, the invocation `BOOST_CONTRACT_ASSERT(``cond``)` simply expands to code equivalent to the following: [84]

    if(!(cond)) {
        throw boost::contract::assertion_failure(__FILE__, __LINE__,
                BOOST_PP_STRINGIZE(cond));
    }

In fact, this library considers any exception thrown from within preconditions, postconditions, exception guarantees, and class invariants as a contract failure and reports it calling the related contract failure handler (`boost::contract::precondition_failure`, etc.). If there is a need for it, programmers can always program contract assertions that throw specific user-defined exceptions as follow (see Throw on Failures):

    if(!cond) throw exception-object;

However, using `BOOST_CONTRACT_ASSERT` is convenient because it always allows this library to show an informative message in case of assertion failure containing the assertion code, file name, line number, etc.

As shown in Assertion Levels, this library pre-defines `BOOST_CONTRACT_ASSERT_AUDIT` and `BOOST_CONTRACT_ASSERT_AXIOM` assertion levels. These macros are not variadic macros and programmers should be able to use them on all C++ compilers. In any case, their implementations are equivalent to the following:

    #ifdef BOOST_CONTRACT_AUDITS
        #define BOOST_CONTRACT_ASSERT_AUDIT(cond) \
            BOOST_CONTRACT_ASSERT(cond)
    #else
        #define BOOST_CONTRACT_ASSERT_AUDIT(cond) \
            BOOST_CONTRACT_ASSERT(true || (cond))
    #endif

    #define BOOST_CONTRACT_ASSERT_AXIOM(cond) \
        BOOST_CONTRACT_ASSERT(true || (cond))

##### Base Types (Variadic)

As shown in Base Classes, this library provides the `BOOST_CONTRACT_BASE_TYPES` variadic macro to declare the `base_types` member type that will expand to the list of all public bases for a derived class. Programmers can also declare `base_types` without using `BOOST_CONTRACT_BASE_TYPES` at the cost of writing a bit more code and increase maintenance efforts. For example (see `base_types_no_macro.cpp`):

    #include <boost/mpl/vector.hpp>

    class chars :
        private boost::contract::constructor_precondition<chars>,
        public unique_chars,
        public virtual pushable<char>,
        virtual protected has_size,
        private has_empty
    {
    public:
        // Program `base_types` without macros (list only public bases).
        typedef boost::mpl::vector<unique_chars, pushable<char> > base_types;

        /* ... */

The `base_types` member type must be a `boost::mpl::vector` which must list all and only `public` base classes (because only public bases subcontract, see Function Calls), and in the same order these public base classes appear in the derived class inheritance list. If the `BOOST_CONTRACT_BASE_TYPES` macro is not used, it is the responsibility of the programmers to maintain the correct list of bases in the `boost::mpl::vector` each time the derived class inheritance list changes (this might significantly complicate maintenance).

In general, it is recommended to use the `BOOST_CONTRACT_BASE_TYPES` macro whenever possible.

##### Old Values (Variadic)

As shown in Old Values, this library provides the `BOOST_CONTRACT_OLDOF` variadic macro to assign old value copies. Programmers can also assign old values without using `BOOST_CONTRACT_OLDOF` at the cost of writing a bit more code manually. For example (see `old_no_macro.cpp`):

    template<typename T>
    class vector {
    public:
        virtual void push_back(T const& value, boost::contract::virtual_* v = 0) {
            // Program old value instead of using `OLD(size())` macro.
            boost::contract::old_ptr<unsigned> old_size =
                boost::contract::make_old(v, boost::contract::copy_old(v) ?
                        size() : boost::contract::null_old())
            ;

            boost::contract::check c = boost::contract::public_function(v, this)
                .postcondition([&] {
                    BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
                })
            ;

            vect_.push_back(value);
        }

        /* ... */

The ternary operator `boost::contract::copy_old(v) ? size() : boost::contract::null_old()` must be used here to avoid evaluating and copying the old value expression `size()` when `boost::contract::copy_old` returns `false` (because old values are not being copied when postcondition and exception guarantee checking is disabled at run-time, an overridden virtual function call is not checking postconditions or exception guarantees yet, etc.). The enclosing `boost::contract::make_old` copies the old value expression and creates an old value pointer. Otherwise, `boost::contract::null_old` indicates that a null old value pointer should be created.

The `boost::contract::make_old` and `boost::contract::copy_old` functions are used exactly as shown above but without the extra `v` parameter when they are called from within non-virtual functions (see Public Function Overrides). The old value pointer returned by `boost::contract::make_old` can be assigned to either `boost::contract::old_ptr` or `boost::contract::old_ptr_if_copyable` (see Old Value Requirements).

In general, it is recommended to use the `BOOST_CONTRACT_OLDOF` macro whenever possible.

##### Macro Interface (Variadic)

Almost all macros defined in `boost/contract_macro.hpp` are variadic macros. On compilers that do not support variadic macros, programmers can manually disable contract code compilation using `#ifndef BOOST_CONTRACT_NO_...` statements as shown in Disable Contract Compilation.

[68] Rationale: [N1962] and other proposals to add contracts to C++ do not provide a mechanism to selectively disable copies only for old value types that are not copy constructible. However, this library provides such a mechanism to allow to program contracts for template code without necessarily adding extra copy constructible type requirements that would not be present if it were not for copying old values (so compiling the code with and without contracts will not necessarily alter the type requirements of the program). Something similar could be achieved combing C++17 `if constexpr` with [N1962] or [P0380] so that old value expressions within template code can be guarded by `if constexpr` statements checking if the old value types are copyable or not. For example, assuming old values are added to [P0380] (using some kind of `oldof(...)` syntax) and that C++17 `if constexpr` can be used within [P0380] contracts:

    template<typename T>
    void offset(T& x, int count)
        [[ensures: if constexpr(std::is_copy_constructible<T>::value) x == oldof(x) + count]]
    ...

[69] Technically, on C++17 it is possible to use `boost::contract::old_ptr` together with `if constexpr` instead of using `boost::contract::old_ptr_if_copyable`, for example:

    template<typename T>
    void offset(T& x, int count) {
        boost::contract::old_ptr<T> old_x;
        if constexpr(boost::contract::is_old_value_copyable<T>::value) old_x = BOOST_CONTRACT_OLDOF(x);
        boost::contract::check c = boost::contract::function()
            .postcondition([&] {
                if constexpr(boost::contract::is_old_value_copyable<T>::value) BOOST_CONTRACT_ASSERT(x == *old_x + count);
            })
        ;

        x += count;
    }

However, the authors find this code less readable and more verbose than its equivalent that uses `boost::contract::old_ptr_if_copyable`. Guarding old value copies and related assertions with `if constexpr` is useful instead when the guard condition checks type requirements more complex than just `boost::contract::is_old_value_copyable` (as shown later in this documentation).

[70] Rationale: [N1962] and other proposals to add contracts to C++ do not provide a mechanism to selectively disable assertions based on their type requirements. However, this library provides such a mechanism to allow to program contracts for template code without necessarily adding extra type requirements that would not be present if it was not for the contracts (so compiling the code with and without contracts will not alter the type requirements of the program). Something similar could be achieved combing C++17 `if constexpr` with [N1962] or [P0380] so that contract assertions within template code could be guarded by `if constexpr` statements checking the related type requirements ([N1962] already allows of `if` statements in contracts under the name of select assertions, [P0380] does not so probably `if` statements should be added to [P0380] as well). For example, assuming C++17 `if constexpr` can be used within [P0380] contracts:

    template<typename T>
    class vector {
    public:
        void push_back(T const& value)
            [[ensures: if constexpr(boost::has_equal_to<T>::value) back() == value]]
        ...
    };

[71] The internal implementation of `boost::contract::condition_if` is optimized and it does not actually use `boost::contract::call_if`.

[72]

A part from its use within contracts, `boost::contract::call_if` can be used together with C++14 generic lambdas to emulate C++17 `if constexpr` (`boost::hana::if_` and probably other approaches can also be used together with generic lambdas to emulate C++17 `if constexpr` on C++14 compilers). For example, the following implementation of `myadvance` will compile since C++14 and it is more concise, easier to read and maintain than the usual implementation of `std::advance` that uses tag dispatching (see `call_if_cxx14.cpp`):

    template<typename Iter, typename Dist>
    void myadvance(Iter& i, Dist n) {
        Iter* p = &i; // So captures change actual pointed iterator value.
        boost::contract::call_if<is_random_access_iterator<Iter> >(
            std::bind([] (auto p, auto n) { // C++14 generic lambda.
                *p += n;
            }, p, n)
        ).template else_if<is_bidirectional_iterator<Iter> >(
            std::bind([] (auto p, auto n) {
                if(n >= 0) while(n--) ++*p;
                else while(n++) --*p;
            }, p, n)
        ).template else_if<is_input_iterator<Iter> >(
            std::bind([] (auto p, auto n) {
                while(n--) ++*p;
            }, p, n)
        ).else_(
            std::bind([] (auto false_) {
                static_assert(false_, "requires at least input iterator");
            }, std::false_type()) // Use constexpr value.
        );
    }

Of course, since C++17 the implementation that uses `if constexpr` is even more readable and concise:

    template<typename Iter, typename Dist>
    void myadvance(Iter& i, Dist n) {
        if constexpr(is_random_access_iterator<Iter>::value) {
            i += n;
        } else if constexpr(is_bidirectional_iterator<Iter>::value) {
            if(n >= 0) while(n--) ++i;
            else while(n++) --i;
        } else if constexpr(is_input_iterator<Iter>::value) {
            while(n--) ++i;
        } else {
            static_assert(false, "requires at least input iterator");
        }
    }

[73] Rationale: Constructors and destructors check `const volatile` and `const` invariants in that order because the qualifier that can be applied to more calls is checked first (note that `const volatile` calls can be made on any object while `const` calls cannot be made on `volatile` objects, in that sense the `const volatile` qualifier can be applied to more calls than `const` alone can). This is consistent with `static` class invariants that are checked even before `const volatile` invariants (the `static` classifier can be applied to even more calls than `const volatile`, in fact an object is not even needed to make static calls).

[74] Rationale: Note that while all public functions can be made to check `const volatile` invariants, it is never possible to make volatile public functions check `const` non-volatile invariants. That is because both `const` and `volatile` can always be added but never stripped in C++ (a part from forcefully via `const_cast`) but `const` is always automatically added by this library in order to enforce contract constant-correctness (see Constant-Correctness). That said, it would be too stringent for this library to also automatically add `volatile` and require all functions to check `const volatile` (not just `const`) invariants because only `volatile` members can be accessed from `const volatile` invariants so there could be many `const` (but not `const volatile`) members that are accessible from `const` invariants but not from `const volatile` invariants. To avoid this confusion, this library has chosen to draw a clear dichotomy between `const` and `const volatile` invariants so that only volatile public functions check `const volatile` invariants and only non-volatile public functions check `const` (but not `const volatile`) invariants. This is a clear distinction and it should serve most cases. If programmers need non-volatile public functions to also check `const volatile` invariants, they can explicitly do so by calling the `const volatile` invariant function from the `const` invariant function as shown in this documentation.

[75] In this example, the `moved()` function is simple enough that programmers could decide to not even call `boost::contract::public_function` from it for optimization reasons. However, calling `boost::contract::public_function` from `moved()` has no negative impact, a part from run-time overhead, because this library automatically disables contract checking while checking other contracts (so this call will not cause infinite recursion).

[76] The assertion levels predefined by this library are similar to the default, audit, and axiom levels from [P0380].

[77] When used as default parameter values, lambda functions allow to program code statements within function declarations. However, these lambadas cannot be effectively used to program contracts in function declarations instead of definitions. That is because the C++11 standard does not allow lambdas in function declarations to capture any variable (for the good reason that it is not at all obvious how to correctly define the semantics of such captures). For example, the following code is not valid C++ and it does not compile:

    // Specifications (in declaration).
    int inc(int& x,
        // Error: Lambdas in default parameters cannot capture `this`, `x`, or any other variable.
        std::function<void ()> pre = [&] {
            BOOST_CONTRACT_ASSERT(x < std::numeric_limits<int>::max());
        },
        std::function<void (int const&, boost::contract::old_ptr<int> const&)> post
            = [&] (int const& result, boost::contract::old_ptr<int> const& old_x)
        {
            BOOST_CONTRACT_ASSERT(x == *old_x + 1);
            BOOST_CONTRACT_ASSERT(result == *old_x);
        }
    );

    // Implementation (in definition).
    int inc(int& x,
        std::function<void ()> pre,
        std::function<void (int const&, boost::contract::old_ptr<int> const&)> post
    ) {
        int result;
        boost::contract::old_ptr<int> old_x = BOOST_CONTRACT_OLDOF(x);
        boost::contract::check c = boost::contract::function()
            .precondition(pre)
            .postcondition(std::bind(post, std::cref(result), std::cref(old_x)))
        ;

        return result = x++; // Function body.
    }

In any case, even if the above code compiled, it would require significant boiler-plate code to bind return and old values.

[78] Alternatively, on compilers that do not support C++11 lambda functions, Boost.LocalFunction could be used to program the contract functors still within the function definitions (for example, see `no_lambda_local_func.cpp`). In general, such a code is less verbose than the example shown in this section that uses contract functions programmed outside of the original function definitions (about 30% less lines of code) but the contract code is hard to read. Other libraries could also be used to program the contract functors without C++11 lambda functions (Boost.Lambda, Boost.Fusion, etc.) but again all these techniques will result in contract code either more verbose, or harder to read and maintain than the code that uses C++11 lambda functions.

[79] If C++ allowed lambda functions to capture variables by constant reference (for example allowing a syntax like this `[const&] { ... }` and `[const&` `variable-name``] { ... }`, see https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/0UKQw9eo3N0) also lambdas could be used to program contract functors that fully enforce Constant-Correctness at compile-time. Note that C++11 lambdas allow to capture variables by value (using `[=] { ... }` and `[``variable-name``] { ... }`) and these value captures are `const` (unless the lambda is explicitly declared `mutable`) but they are not suitable to program postconditions and exception guarantees using this library (because those require capturing by reference, see Postconditions and Exception Guarantees), plus they introduce a copy of the captured value that might be too expensive in general and therefore not suitable for preconditions either.

[80] In this example, `bind` was used to generate nullary functors from the contract functions. As always with `bind`, `cref` and `ref` must be used to bind arguments by `const&` and `&` respectively, plus it might be necessary to explicitly `static_cast` the function pointer passed to `bind` for overloaded functions.

[81] Compilation times of this library were measured to be comparable between compilers that support variadic macros and compilers that do not.

[82] Rationale: The `BOOST_CONTRACT_MAX_ARGS` macro is named after `BOOST_FUNCTION_MAX_ARGS`.

[83] Rationale: These macros expand to SFINAE-based introspection template code that are too complex to be programmed manually by users (that remains the case even if C++14 generic lambdas were to be used here). On a related note, in theory using C++14 generic lambdas, the `BOOST_CONTRACT_OVERRIDE` macro could be re-implemented in a way that can be expanded at function scope, instead of class scope (but there is not really a need to do that).

[84] Rationale: There is no need for the code expanded by `BOOST_CONTRACT_ASSERT` to also use C++11 `__func__`. That is because `__func__` will always expand to the name `operator()` of the functor used to program the contract assertions (e.g., the internal name the compiler assigns to lambda functions) and it will not expand to the name of the actual function enclosing the contract declaration.