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 {};
} }

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]

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]

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).

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:

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).

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_;

    /* ... */

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

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);

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:

	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.

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:

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):

// 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):

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 typedefs, 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).

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 #included 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 #included 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.

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

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):

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).

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.