Contracts for non-member functions are programmed using boost::contract::function. For example (see non_member.cpp):

#include <boost/contract.hpp>

// Contract for a non-member function.
int inc(int& x) {
    int result;
    boost::contract::old_ptr<int> old_x = BOOST_CONTRACT_OLDOF(x);
    boost::contract::check c = boost::contract::function()
        .precondition([&] {
            BOOST_CONTRACT_ASSERT(x < std::numeric_limits<int>::max());
        })
        .postcondition([&] {
            BOOST_CONTRACT_ASSERT(x == *old_x + 1);
            BOOST_CONTRACT_ASSERT(result == *old_x);
        })
        .except([&] {
            BOOST_CONTRACT_ASSERT(x == *old_x);
        })
    ;

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

All necessary header files of this library are included by #include <boost/contract.hpp>. Alternatively, programmers can selectively include only the header files they actually need among boost/contract/*.hpp (see Getting Started).

It is possible to specify preconditions, postconditions, and exception guarantees for non-member functions (see Preconditions, Postconditions, and Exception Guarantees).

The boost::contract::function function returns an RAII object that must always be assigned to a local variable of type boost::contract::check (otherwise this library will generate a run-time error, see BOOST_CONTRACT_ON_MISSING_CHECK_DECL). ^[19] Furthermore, C++11 auto declarations cannot be used here and the boost::contract::check type must be explicitly specified (otherwise this library will generate a compile-time error prior C++17 and a run-time error post C++17). ^[20] The function body is programmed right after the declaration of this RAII object.

	Note
	In some cases, it might be necessary to program some code before the contract. For example for acquiring resources that will be used while checking the contract like old values, but also to lock mutexes (or other synchronization mechanisms) in multi-threaded programs.

At construction, the boost::contract::check RAII object for non-member functions does the following (enclosing function entry):

At destruction instead (enclosing function exit):

This ensures that non-member function contracts are correctly checked at run-time (see Function Calls). (Also note that functions will correctly check their contracts even when they are called via function pointers, function objects, etc.)

	Note
	A non-member function can avoid calling boost::contract::function for efficiency but only when it has no preconditions, no postconditions, and no exception guarantees.

When preconditions are specified, they are programmed using a functor r passed to .precondition(r) that can be called with no parameters as in r(). Contracts that do not have preconditions simply do not call .precondition(...). Preconditions must appear before postconditions and exception guarantees when these are all present (see Postconditions and Exception Guarantees).

C++11 lambda functions are convenient to program preconditions, but any other nullary functor can be used (see No Lambda Functions). ^[21] For example, for boost::contract::function (similarly for public functions, instead destructors do not have preconditions and constructors use boost::contract::constructor_precondition, see Public Functions, Destructors, and Constructors):

void f(...) {
    boost::contract::check c = boost::contract::function()  // Same for all other contracts.
        .precondition([&] {                                 // Capture by reference or value...
            BOOST_CONTRACT_ASSERT(...);                     // ...and should not modify captures.
            ...
        })
        ...
    ;

    ...
}

The precondition functor should capture all the variables that it needs to assert the preconditions. These variables can be captured by value when the overhead of copying such variables is acceptable. ^[22] In any case, programmers should not write precondition assertions that modify the value of the captured variables, even when those are captured by reference (see Constant-Correctness).

Any code can be programmed in the precondition functor, but it is recommended to keep this code simple using mainly assertions and if-statements (to avoid programming complex preconditions that might be buggy and also slow to check at run-time). It is also recommended to use BOOST_CONTRACT_ASSERT to program precondition assertions because that enables this library to print informative error messages when the asserted conditions are evaluated to be false (note that this is not a variadic macro, see No Macros):

BOOST_CONTRACT_ASSERT(boolean-condition)
// Or, if `boolean-condition` contains commas `,` not already within parenthesis `()`...
BOOST_CONTRACT_ASSERT((boolean-condition)) // ...use extra parenthesis (not a variadic macro).

This library will automatically call the failure handler boost::contract::precondition_failure if any of the BOOST_CONTRACT_ASSERT conditions are false or, more in general, if calling the functor specified via .precondition(...) throws any exception. By default, this failure handler prints an error message to std::cerr and terminates the program calling std::terminate (see Throw on Failures to change the failure handler to throw exceptions, exit the program with an error code, etc.).

Note

Contracts are most useful when their assertions only use public members that are accessible to the caller so the caller can properly check and use the contract. In particular, preconditions of a public function or constructor that use non-public members are essentially incorrect because they cannot be fully checked by the caller (in fact, Eiffel generates a compile-time error in this case). However, this library does not enforce such a constraint and it leaves it up to programmers to only use public members when programming contracts, especially when asserting preconditions (see Specifications vs. Implementation).

When postconditions are specified, they are programmed using a functor s passed to .postcondition(s) that can be called with no parameters as in s(). Contracts that do not have postconditions simply do not call .postcondition(...). Postconditions must appear after preconditions but before exception guarantees when these are all present (see Preconditions and Exception Guarantees).

C++11 lambda functions are convenient to program postconditions, but any other nullary functor can be used (see No Lambda Functions). For example, for boost::contract::function (similarly for all other contracts):

void f(...) {
    boost::contract::check c = boost::contract::function()  // Same for all other contracts.
        ...
        .postcondition([&] {                                // Capture by reference...
            BOOST_CONTRACT_ASSERT(...);                     // ...but should not modify captures.
            ...
        })
        ...
    ;

    ...
}

The postcondition functor should capture all the variables that it needs to assert the postconditions. In general, these variables should be captured by reference and not by value (because postconditions need to access the value that these variables will have at function exit, and not the value these variables had when the postcondition functor was first declared). Postconditions can also capture return and old values (see Return Values and Old Values). In any case, programmers should not write postcondition assertions that modify the value of the captured variables, even when those are captured by reference (see Constant-Correctness).

Any code can be programmed in the postcondition functor, but it is recommended to keep this code simple using mainly assertions and if-statements (to avoid programming complex postconditions that might be buggy and slow to check at run-time). It is also recommended to use BOOST_CONTRACT_ASSERT to program postcondition assertions because that enables this library to print informative error messages when the asserted conditions are evaluated to be false (note that this is not a variadic macro, see No Macros):

BOOST_CONTRACT_ASSERT(boolean-condition)
// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...
BOOST_CONTRACT_ASSERT((boolean-condition)) // ...use extra parenthesis (not a variadic macro).

This library will automatically call the failure handler boost::contract::postcondition_failure if any of the BOOST_CONTRACT_ASSERT conditions are false or, more in general, if calling the functor specified via .postcondition(...) throws any exception. By default, this failure handler prints an error message to std::cerr and terminates the program calling std::terminate (see Throw on Failures to change the failure handler to throw exceptions, exit the program with an error code, etc.).

For non-void virtual public functions and non-void public function overrides, the functor s passed to .postcondition(s) is not a nullary functor, instead it is a unary functor taking a variable holding the return value as its one parameter s(result) (this is to properly support subcontracting, see Virtual Public Functions and Public Function Overrides).

In non-void functions, postconditions might need to access the function return value to program assertions. In these cases, programmers are responsible to declare a local variable before the contract and to assign it to the return value at function exit (when the function does not throw an exception). ^[23] For example, for boost::contract::function (similarly for all other contracts):

return_type f(...) {
    return_type result;                                     // Must be later assigned to return value.
    boost::contract::check c = boost::contract::function()  // Same for all other contracts.
        ...
        .postcondition([&] {                                // Also capture `result` reference...
            BOOST_CONTRACT_ASSERT(result == ...);           // ...but should not modify captures.
            ...
        })
        ...
    ;

    ...
    return result = ...;                                    // Assign `result` at each return.
}

At any point where the enclosing function returns, programmers are responsible to assign the result variable to the expression being returned. This can be done ensuring that all return statements in the function are of the form:

return-type result;
...
return result = return-expression;                           // Assign `result` at each return.

The functor used to program postconditions should capture the result variable by reference and not by value (because postconditions must access the value the result variable will have at function exit, and not the value the result variable had when the postcondition functor was first declared). The return value should never be used in preconditions, old value copies, or exception guarantees (because the return value is not yet correctly evaluated and set when preconditions are checked, old values are copied, or if the function throws an exception). In any case, programmers should not modify the result variable in the contract assertions (see Constant-Correctness).

It is also possible to declared the result variable using boost::optional when the function return type does not have a default constructor, or if the default constructor is too expensive or undesirable to execute when first declaring the result variable (see Optional Return Values).

Non-void virtual public functions and non-void public function overrides must always declare and use a result variable even when postconditions do not directly use the function return value (this is to properly support subcontracting, see Virtual Public Functions and Public Function Overrides).

When old values are used in postconditions or in exception guarantees, programmes are responsible to declare local variables before the contract and to assign them to related old value expressions using BOOST_CONTRACT_OLDOF. ^[24] For example, for boost::contract::function (similarly for all other contracts):

void f(...) {
    boost::contract::old_ptr<old_type> old_var = BOOST_CONTRACT_OLDOF(old_expr);
    ...                                                     // More old value declarations here if needed.
    boost::contract::check c = boost::contract::function()  // Same for all other contracts.
        ...                                                 // Preconditions shall not use old values.
        .postcondition([&] {                                // Capture by reference...
            BOOST_CONTRACT_ASSERT(*old_var == ...);         // ...but should not modify captures.
            ...
        })
        .except([&] {                                       // Capture by reference...
            BOOST_CONTRACT_ASSERT(old_var->...);            // ...but should not modify captures.
            ...
        })
    ;

    ...
}

Old values are handled by this library using the smart pointer class template boost::contract::old_ptr (so programmers do not directly manage allocation and deallocation of the pointed memory). ^[25] The pointed old value type is automatically qualified as const (so old values cannot be mistakenly changed by contract assertions, see Constant-Correctness). This library ensures that old value pointers are always not null by the time postconditions and exception guarantees are checked (so programmers can safely dereference and use these pointers in postcondition and exception guarantee assertions using operator* and operator-> without having to check if old value pointers are not null first).

Old values should not be used in preconditions and this library does not guarantee that old value pointers are always not null when preconditions are checked. ^[26] See Old Values Copied at Body for delaying the copy of old values until after class invariants (for constructors, destructors, and public functions) and preconditions are checked (when necessary, this allows to program old value expressions under the simplifying assumption that class invariant and precondition assertions are satisfied already).

BOOST_CONTRACT_OLDOF is a variadic macro and it takes an extra parameter when used in virtual public functions or public function overrides (see Virtual Public Functions and Public Function Overrides). C++11 auto declarations can be used with BOOST_CONTRACT_OLDOF for brevity auto old_variable-name = BOOST_CONTRACT_OLDOF(expression) (but see also Old Value Requirements). See No Macros to program old values without using BOOST_CONTRACT_OLDOF (e.g., on compilers that do not support variadic macros).

Note

This library ensures that old values are copied only once. This library also ensures that old values are never copied when postconditions and exception guarantees are disabled defining both BOOST_CONTRACT_NO_POSTCONDITIONS and BOOST_CONTRACT_NO_EXCEPTS (note that both these two macros must be defined, defining only BOOST_CONTRACT_NO_POSTCONDITIONS or only BOOST_CONTRACT_NO_EXCEPTS is not sufficient to prevent the run-time cost of old value copies).

When exception guarantees are specified, they are programmed using a functor e passed to .except(e) that can be called with no parameters as in e(). Contracts that do not have exception guarantees simply do not call .except(...). Exception guarantees must appear after both preconditions and postconditions when these are all present (see Preconditions and Postconditions).

C++11 lambda functions are convenient to program exception guarantees, but any other nullary functor can be used (see No Lambda Functions). For example, for boost::contract::function (similarly for all other contracts):

void f(...) {
    boost::contract::check c = boost::contract::function()  // Same for all other contracts.
        ...
        .except([&] {                                       // Capture by reference...
            BOOST_CONTRACT_ASSERT(...);                     // ...but should not modify captures.
            ...
        })
    ;

    ...
}

The exception guarantee functor should capture all the variables that it needs to assert the exception guarantees. In general, these variables should be captured by reference and not by value (because exception guarantees need to access the value that these variables will have when the function throws, and not the value these variables had when the exception guarantee functor was first declared). Exception guarantees can also capture old values (see Old Values) but they should not access the function return value instead (because the return value is not be properly set when the function throws an exception). In any case, programmers should not write exception guarantee assertions that modify the value of the captured variables, even when those are captured by reference (see Constant-Correctness).

Note

In real production code, it might be difficult to program meaningful exception guarantees without resorting to expensive old value copies that will slow down execution. Therefore, the authors recognize that exception guarantees, even if supported by this library, might not be used often in practice (and they are not used in most of the examples listed in the rest of this documentation). In any case, these performance considerations are ultimately left to programmers and their specific application domains.

Any code can be programmed in the exception guarantee functor, but it is recommended to keep this code simple using mainly assertions and if-statements (to avoid programming complex exception guarantees that might be buggy and slow to check at run-time). It is also recommended to use BOOST_CONTRACT_ASSERT to program exception guarantee assertions because that enables this library to print informative error messages when the asserted conditions are evaluated to be false (note that this is not a variadic macro, see No Macros):

BOOST_CONTRACT_ASSERT(boolean-condition)
// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...
BOOST_CONTRACT_ASSERT((boolean-condition)) // ...use extra parenthesis (not a variadic macro).

This library will automatically call the failure handler boost::contract::except_failure if any of the BOOST_CONTRACT_ASSERT conditions are false or, more in general, if calling the functor specified via .except(...) throws any exception. By default, this failure handler prints an error message to std::cerr and terminates the program calling std::terminate (see Throw on Failures to change the failure handler to exit the program with an error code or to take some other custom action).

Note

While it is technically possible for programmers to specify an exception guarantee handler that throws an exception in case of an exception guarantee failure, this will force C++ to terminate the program. That is because the handler will throw an exception while there is already an active exception on the stack (the exception thrown by the function body that caused the exception guarantees to be checked in the first place). Therefore, programmers should not change the exception guarantee failure handler to throw exceptions.

Public member functions, constructors, and destructors can be programmed to also check class invariants. When class invariants are specified, they are programmed in a public const function named invariant taking no argument and returning void. Classes that do not have invariants, simply do not declare the invariant function. ^[27] For example:

class u {
public:                                 // Must be public.
    void invariant() const {            // Must be const.
        BOOST_CONTRACT_ASSERT(...);
        ...
    }

    ...
};

This member function must be const because contracts should not modify the object state (see Constant-Correctness). This library will generate a compile-time error if the const qualifier is missing (unless BOOST_CONTRACT_PERMISSIVE is defined).

Any code can be programmed in the invariant function, but it is recommended to keep this code simple using mainly assertions and if-statements (to avoid programming complex invariants that might be buggy and slow to check at run-time). It is also recommended to use BOOST_CONTRACT_ASSERT to program class invariant assertions because that enables this library to print informative error messages when the asserted conditions are evaluated to be false (note that this is not a variadic macro, see No Macros):

BOOST_CONTRACT_ASSERT(boolean-condition)
// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...
BOOST_CONTRACT_ASSERT((boolean-condition)) // ...use extra parenthesis (not a variadic macro).

This library will automatically call failure handlers boost::contract::entry_invariant_failure or boost::contract::exit_invariant_failure if any of the BOOST_CONTRACT_ASSERT conditions are false or, more in general, if the invariant function throws an exception when invariants are checked at function entry or exit respectively. By default, these handlers print an error message to std::cerr and terminate the program calling std::terminate (see Throw on Failures to change these failure handlers to throw exceptions, exit the program with an error code, etc.).

See Access Specifiers to avoid making the invariant member function public. ^[28] See BOOST_CONTRACT_INVARIANT_FUNC to use a name different from invariant (e.g., because invariant clashes with other names in user-defined classes).

Note

Contract assertions are not checked (not even class invariants) when data members are accessed directly (this is different from Eiffel where even accessing public data members checks class invariants). Therefore, it might be best for both classes and structs (and also unions, see Unions) that have invariants to have no mutable public data members and to access data members publicly only via appropriate public functions (e.g., setters and getters) that can be programmed to check the class invariants using this library.

See Volatile Public Functions to program invariants for classes with volatile public functions.

Static Class Invariants

Static public functions can be programmed to check static class invariants. When static class invariants are specified, they are programmed in a public static function named static_invariant taking no argument and returning void. Classes that do not have static class invariants, simply do not declare the static_invariant function. ^[29] For example:

class u {
public:                                 // Must be public.
    static void static_invariant() {    // Must be static.
        BOOST_CONTRACT_ASSERT(...);
        ...
    }

    ...
};

This member function must be static (and it correctly cannot access the object this). This library will generate a compile-time error if the static classifier is missing (unless the BOOST_CONTRACT_PERMISSIVE macro is defined).

Any code can be programmed in the static_invariant function, but it is recommended to keep this code simple using mainly assertions and if-statements (to avoid programming complex static invariants that might be buggy and slow to check at run-time). It is also recommended to use BOOST_CONTRACT_ASSERT to program the assertions because that enables this library to print informative error messages when the asserted conditions are evaluated to be false (note that this is not a variadic macro, see No Macros):

BOOST_CONTRACT_ASSERT(boolean-condition)
// Or, if `boolean-condition` has commas `,` not already within parenthesis `()`...
BOOST_CONTRACT_ASSERT((boolean-condition)) // ...use extra parenthesis (not a variadic macro).

This library will automatically call failure handlers boost::contract::entry_invariant_failure or boost::contract::exit_invariant_failure if any of the BOOST_CONTRACT_ASSERT conditions are false or, more in general, if the static_invariant function throws an exception when invariants are checked at function entry or exit respectively. By default, these handlers print an error message to std::cerr and terminate the program calling std::terminate (see Throw on Failures to change these failure handlers to throw exceptions, exit the program with an error code, etc.).

See Access Specifiers to avoid making static_invariant member function public. ^[30] See BOOST_CONTRACT_STATIC_INVARIANT_FUNC to use a name different from static_invariant (e.g., because static_invariant clashes with other names in user-defined classes). ^[31]

Contracts for constructors are programmed using the boost::contract::constructor function and the boost::contract::constructor_precondition base class. For example (see public.cpp):

class unique_identifiers :
    private boost::contract::constructor_precondition<unique_identifiers>
{
public:
    void invariant() const {
        BOOST_CONTRACT_ASSERT(size() >= 0);
    }

public:
    // Contract for a constructor.
    unique_identifiers(int from, int to) :
        boost::contract::constructor_precondition<unique_identifiers>([&] {
            BOOST_CONTRACT_ASSERT(from >= 0);
            BOOST_CONTRACT_ASSERT(to >= from);
        })
    {
        boost::contract::check c = boost::contract::constructor(this)
            .postcondition([&] {
                BOOST_CONTRACT_ASSERT(size() == (to - from + 1));
            })
        ;

        // Constructor body.
        for(int id = from; id <= to; ++id) vect_.push_back(id);
    }

    /* ... */
};

It is not possible to specify preconditions using .precondition(...) for constructors (this library will generate a compile-time error if .precondition(...) is used on the object returned by boost::contract::constructor). Constructor preconditions are specified using the boost::contract::constructor_precondition base class instead (same considerations as the ones made in Preconditions apply also to the precondition functor passed to boost::contract::constructor_precondition). Programmes should not access the object *this from constructor preconditions (because the object does not exists yet before the constructor body is executed). ^[32] Constructors without preconditions simply do not explicitly initialize the boost::contract::constructor_precondition base (because boost::contract::constructor_precondition default constructor checks no contract). When the boost::contract::constructor_precondition base class is used: ^[33]

	Note
	A class can avoid inheriting from boost::contract::constructor_precondition for efficiency but only when all its constructors have no preconditions.

It is possible to specify postconditions for constructors (see Postconditions), but programmers should not access the old value of the object *this in constructor postconditions (because the object did not exist yet before the constructor body was executed). ^[35] It is also possible to specify exceptions guarantees for constructors (see Exception Guarantees), but programmers should not access the object *this or its old value in constructor exception guarantees (because the object did not exist before executing the constructor body and it was not properly constructed given the constructor body threw an exception). ^[36] The boost::contract::constructor function takes this as a parameter (because constructors check class invariants, see Class Invariants).

The boost::contract::constructor function returns an RAII object that must always be assigned to a local variable of type boost::contract::check (otherwise this library will generate a run-time error, see BOOST_CONTRACT_ON_MISSING_CHECK_DECL). Furthermore, C++11 auto declarations cannot be used here and the boost::contract::check type must be explicitly specified (otherwise this library will generate a compile-time error prior C++17 and a run-time error post C++17). The constructor body is programmed right after the declaration of this RAII object.

At construction, the boost::contract::check RAII object for constructors does the following (enclosing constructor entry):

At destruction instead (enclosing constructor exit):

This together with C++ object construction mechanism of base classes and the use of boost::contract::constructor_precondition ensures that the constructor contracts are correctly checked at run-time (see Constructor Calls).

Note

A constructor can avoid calling boost::contract::constructor for efficiency but only when it has no postconditions, no exception guarantees, and its class has no invariants (even if boost::contract::constructor is not used by a derived class, contracts of base class constructors will still be correctly checked by C++ object construction mechanism). The default constructor and copy constructor automatically generated by C++ will not check contracts. Therefore, unless these constructors are not public or they have no preconditions, no postconditions, no exception guarantees, and their class has no invariants, programmers should manually define them using boost::contract::constructor and boost::contract::constructor_precondition. Similar considerations apply to all other constructors automatically generated by C++ (e.g., the move constructor).

Private and Protected Constructors

Private and protected constructors can omit boost::contract::constructor (because they are not part of the public interface of the class so they are not required to check class invariants, see Constructor Calls). They could still use boost::contract::constructor_precondition to check preconditions before member initializations, and even use boost::contract::function (but not boost::contract::constructor) to only check postconditions and exception guarantees without checking class invariants and without calling .precondition(...) (see Private and Protected Functions). For example:

class u : private boost::contract::constructor_precondition<u> {
protected:
    // Contract for a protected constructor (same for private constructors).
    u() : // Still use this base class to check constructor preconditions.
        boost::contract::constructor_precondition<u>([&] {
            BOOST_CONTRACT_ASSERT(...);
            ...
        })
    {
        // Following will correctly not check class invariants.
        boost::contract::check c = boost::contract::function()
            // Do not use `.precondition(...)` here.
            .postcondition([&] {
                BOOST_CONTRACT_ASSERT(...);
                ...
            })
            .except([&] {
                BOOST_CONTRACT_ASSERT(...);
                ...
            })
        ;

        ... // Constructor body.
    }

    ...
};

Contracts for destructors are programmed using boost::contract::destructor. For example (see public.cpp):

class unique_identifiers :
    private boost::contract::constructor_precondition<unique_identifiers>
{
public:
    void invariant() const {
        BOOST_CONTRACT_ASSERT(size() >= 0);
    }

public:
    // Contract for a destructor.
    virtual ~unique_identifiers() {
        // Following contract checks class invariants.
        boost::contract::check c = boost::contract::destructor(this);

        // Destructor body here... (do nothing in this example).
    }

    /* ... */
};

It is not possible to specify preconditions for destructors (this library will generate a compile-time error if .precondition(...) is used here and that is because destructors can be called at any time after construction so they have no precondition). It is possible to specify postconditions for destructors (see Postconditions, and also Static Public Functions for an example), but programmers should not access the object *this in destructor postconditions (because the object no longer exists after the destructor body has been executed). ^[37] It is also possible to specify exceptions guarantees for destructors (see Exception Guarantees, even if destructors should usually be programmed to not throw exceptions in C++, in fact destructors are implicitly declared noexcept since C++11). ^[38] The boost::contract::destructor function takes this as a parameter (because destructors check class invariants, see Class Invariants).

The boost::contract::destructor function returns an RAII object that must always be assigned to a local variable of type boost::contract::check (otherwise this library will generate a run-time error, see BOOST_CONTRACT_ON_MISSING_CHECK_DECL). Furthermore, C++11 auto declarations cannot be used here and the boost::contract::check type must be explicitly specified (otherwise this library will generate a compile-time error prior C++17 and a run-time error post C++17). The destructor body is programmed right after the declaration of this RAII object.

At construction, the boost::contract::check RAII object for destructors does the following (enclosing destructor entry):

At destruction instead (enclosing destructor exit):

This together with C++ object destruction mechanism of base classes ensures that destructor contracts are correctly checked at run-time (see Destructor Calls).

Note

A destructor can avoid calling boost::contract::destructor for efficiency but only when it has no postconditions, no exception guarantees, and its class has no invariants (even if boost::contract::destructor is not used by a derived class, contracts of base class destructors will still be correctly checked by C++ object destruction mechanism). The default destructor automatically generated by C++ will not check contracts. Therefore, unless the destructor is not public or it has no postconditions, no exception guarantees, and its class has no invariants, programmers should manually define it using boost::contract::destructor.

Private and Protected Destructors

Private and protected destructors can omit boost::contract::destructor (because they are not part of the public interface of the class so they are not required to check class invariants, see Destructor Calls). They could use boost::contract::function (but not boost::contract::destructor) to only check postconditions and exception guarantees without checking class invariants and without calling .precondition(...) (see Private and Protected Functions). For example:

class u {
protected:
    // Contract for a protected destructor (same for private destructors).
    virtual ~u() {
        // Following will correctly not check class invariants.
        boost::contract::check c = boost::contract::function()
            // Do not use `.precondition(...)` here.
            .postcondition([&] {
                BOOST_CONTRACT_ASSERT(...);
                ...
            })
            // Could use `.except(...)` here in rare cases of destructors declared to throw.
        ;

        ... // Destructor body.
    }

    ...
};

Contracts for public functions are programmed using boost::contract::public_function. In this section, let's consider public functions that are not static, not virtual, and do not override any function from base classes. For example (see public.cpp):

class unique_identifiers :
    private boost::contract::constructor_precondition<unique_identifiers>
{
public:
    void invariant() const {
        BOOST_CONTRACT_ASSERT(size() >= 0);
    }

public:
    // Contract for a public function (but no static, virtual, or override).
    bool find(int id) const {
        bool result;
        boost::contract::check c = boost::contract::public_function(this)
            .precondition([&] {
                BOOST_CONTRACT_ASSERT(id >= 0);
            })
            .postcondition([&] {
                if(size() == 0) BOOST_CONTRACT_ASSERT(!result);
            })
        ;

        // Function body.
        return result = std::find(vect_.begin(), vect_.end(), id) !=
                vect_.end();
    }

    /* ... */
};

It is possible to specify preconditions, postconditions, and exception guarantees for public functions (see Preconditions, Postconditions, and Exception Guarantees). When called from non-static public functions, the boost::contract::public_function function takes this as a parameter (because public functions check class invariants, see Class Invariants).

The boost::contract::public_function function returns an RAII object that must always be assigned to a local variable of type boost::contract::check (otherwise this library will generate a run-time error, see BOOST_CONTRACT_ON_MISSING_CHECK_DECL). Furthermore, C++11 auto declarations cannot be used here and the boost::contract::check type must be explicitly specified (otherwise this library will generate a compile-time error prior C++17 and a run-time error post C++17). The public function body is programmed right after the declaration of this RAII object.

At construction, the boost::contract::check RAII object for public functions does the following (enclosing public function entry):

At destruction instead (enclosing public function exit):

This ensures that public function contracts are correctly checked at run-time (see Public Function Calls).

Note

A public function can avoid calling boost::contract::public_function for efficiency but only when it has no preconditions, no postconditions, no exception guarantees, it is not virtual, it does not override any virtual function, and its class has no invariants. The default copy assignment operator automatically generated by C++ will not check contracts. Therefore, unless this operator is not public or it has no preconditions, no postconditions, no exception guarantees, and its class has no invariants, programmers should manually define it using boost::contract::public_function. Similar considerations apply to all other operators automatically generated by C++ (e.g., the move operator).

Contracts for public functions are programmed using boost::contract::public_function. In this section, let's consider public functions that are virtual but that do not override any function from base classes. For example (see public.cpp):

class unique_identifiers :
    private boost::contract::constructor_precondition<unique_identifiers>
{
public:
    void invariant() const {
        BOOST_CONTRACT_ASSERT(size() >= 0);
    }

public:
    // Contract for a public virtual function (but no override).
    virtual int push_back(int id, boost::contract::virtual_* v = 0) { // Extra `v`.
        int result;
        boost::contract::old_ptr<bool> old_find =
                BOOST_CONTRACT_OLDOF(v, find(id)); // Pass `v`.
        boost::contract::old_ptr<int> old_size =
                BOOST_CONTRACT_OLDOF(v, size()); // Pass `v`.
        boost::contract::check c = boost::contract::public_function(
                v, result, this) // Pass `v` and `result`.
            .precondition([&] {
                BOOST_CONTRACT_ASSERT(id >= 0);
                BOOST_CONTRACT_ASSERT(!find(id)); // ID cannot be already present.
            })
            .postcondition([&] (int const result) {
                if(!*old_find) {
                    BOOST_CONTRACT_ASSERT(find(id));
                    BOOST_CONTRACT_ASSERT(size() == *old_size + 1);
                }
                BOOST_CONTRACT_ASSERT(result == id);
            })
        ;

        // Function body.
        vect_.push_back(id);
        return result = id;
    }

    /* ... */
};

Virtual public functions must declare an extra trailing parameter of type boost::contract::virtual_* with default value 0 (i.e., nullptr). ^[39] This extra parameter is the last parameter and it has a default value so it does not alter the calling interface of the virtual function (callers will rarely, if ever, have to explicitly deal with this extra parameter a part from when manipulating the virtual function type directly for function pointer type-casting, etc.). Programmers must pass the extra virtual parameter as the very first argument to all BOOST_CONTRACT_OLDOF and boost::contract::public_function calls in the virtual public function definition. ^[40]

When called from virtual public functions, the boost::contract::public_function function takes this as a parameter (because public functions check class invariants, see Class Invariants). For virtual public functions returning void:

class u {
public:
    // A void virtual public function (that does not override).
    virtual void f(t_1 a_1, ..., t_n a_n, boost::contract::virtual_* v = 0) {
        boost::contract::check c = boost::contract::public_function(
                v, this)                                    // No result parameter...
            .precondition([&] { ... })
            .postcondition([&] { ... })                     // ...so nullary functor.
            .except([&] { ... })
        ;

        ...
    }

    ...
}

For virtual public functions not returning void, programmers must also pass a reference to the function return value as the second argument to boost::contract::public_function. In this case, the library will pass this return value reference to the postcondition functor that must therefore take one single argument matching the return type, otherwise this library will generate a compile-time error (the functor parameter can be a constant reference const& to avoid extra copies of the return value): ^[41]

class u {
public:
    // A void virtual public function (that does not override).
    virtual t f(t_1 a_1, ..., t_n a_n, boost::contract::virtual_* v = 0) {
        t result;
        boost::contract::check c = boost::contract::public_function(
                v, result, this)                            // Result parameter...
            .precondition([&] { ... })
            .postcondition([&] (t const& result) { ... })   // ...so unary functor.
            .except([&] { ... })
        ;

        ...                                                 // Assign `result` at each return.
    }

    ...
}

Important

It is the responsibility of the programmers to pass the extra virtual parameter v to all BOOST_CONTRACT_OLDOF and boost::contract::public_function calls within virtual public functions, and also to pass the return value reference after v to boost::contract::public_function for non-void virtual public functions. This library cannot automatically generate compile-time errors if programmers fail to do so (but in general this will prevent the library from correctly checking contracts at run-time). [42] Mnemonics: When v is present, always pass it as the first argument to boost::contract::public_function and BOOST_CONTRACT_OLDOF. Always pass result to boost::contract::public_function right after v for non-void functions.

For the rest, considerations made in Public Functions apply to virtual public functions as well.

	Note
	A virtual public function should always call boost::contract::public_function (even if it has no preconditions, no postconditions, no exception guarantees, and its class has no invariants), otherwise this library will not be able to correctly use it for subcontracting.

Contracts for public functions are programmed using boost::contract::public_function. In this section, let's consider public functions (virtual or not) that override virtual public functions from one or more of their public base classes. For example (see public.cpp): ^[43]

class identifiers
    #define BASES public unique_identifiers
    : BASES
{
public:
    typedef BOOST_CONTRACT_BASE_TYPES(BASES) base_types; // Bases typedef.
    #undef BASES

    void invariant() const { // Check in AND with bases.
        BOOST_CONTRACT_ASSERT(empty() == (size() == 0));
    }

public:
    // Contract for a public function override.
    int push_back(int id, boost::contract::virtual_* v = 0) /* override */ {
        int result;
        boost::contract::old_ptr<bool> old_find =
                BOOST_CONTRACT_OLDOF(v, find(id));
        boost::contract::old_ptr<int> old_size =
                BOOST_CONTRACT_OLDOF(v, size());
        boost::contract::check c = boost::contract::public_function<
            override_push_back // Pass override type plus below function pointer...
        >(v, result, &identifiers::push_back, this, id) // ...and arguments.
            .precondition([&] { // Check in OR with bases.
                BOOST_CONTRACT_ASSERT(id >= 0);
                BOOST_CONTRACT_ASSERT(find(id)); // ID can be already present.
            })
            .postcondition([&] (int const result) { // Check in AND with bases.
                if(*old_find) BOOST_CONTRACT_ASSERT(size() == *old_size);
            })
        ;

        // Function body.
        if(!find(id)) unique_identifiers::push_back(id); // Else, do nothing.
        return result = id;
    }
    BOOST_CONTRACT_OVERRIDE(push_back) // Define `override_push_back`.

    /* ... */
};

The extra typedef declared using BOOST_CONTRACT_BASE_TYPES is required by this library for derived classes and it is internally used to detect base classes for subcontracting (see Base Classes). This library will generate a compile-time error if there is no suitable virtual function to override in any of the public base classes for subcontracting. ^[44]

When called from public function overrides, the boost::contract::public_function function template takes an explicit template argument override_function-name that must be defined using BOOST_CONTRACT_OVERRIDE:

BOOST_CONTRACT_OVERRIDE(function-name)

This can be declared at any point in the public section of the enclosing class (see Access Specifiers to use BOOST_CONTRACT_OVERRIDE also in a non-public section of the class). BOOST_CONTRACT_OVERRIDE is used only once in a class for a given function name and overloaded functions can reuse the same override_function-name definition (see Function Overloads). BOOST_CONTRACT_NAMED_OVERRIDE can be used to generate a name different than override_function-name (e.g., to avoid generating C++ reserved names containing double underscores "__" for function names that already start with an underscore "_", see Named Overrides). For convenience BOOST_CONTRACT_OVERRIDES can be used with multiple function names instead of repeating BOOST_CONTRACT_OVERRIDE for each function name (on compilers that support variadic macros). For example, for three functions named f, g, and h (but same for any other number of functions), the following:

BOOST_CONTRACT_OVERRIDES(f, g, h)

Is equivalent to: ^[45]

BOOST_CONTRACT_OVERRIDE(f)
BOOST_CONTRACT_OVERRIDE(g)
BOOST_CONTRACT_OVERRIDE(h)

Public function overrides must always list the extra trailing parameter of type boost::contract::virtual_* with default value 0 (i.e., nullptr), even when they are not declared virtual, if this parameter is present in the signature of the virtual function being overridden from base classes. Programmers must pass the extra virtual parameter as the very first argument to all BOOST_CONTRACT_OLDOF and boost::contract::public_function calls in the public function override definition (see Virtual Public Functions).

When called from public function overrides, the boost::contract::public_function function takes a pointer to the enclosing function, the object *this (because public function overrides check class invariants, see Class Invariants), and references to each function argument in the order they appear in the function declaration. ^[46] For public function overrides returning void:

class u {
public:
    // A void public function override.
    void f(t_1 a_1, ..., t_n a_n, boost::contract::virtual_* v = 0) /* override */ {
        boost::contract::check c = boost::contract::public_function<override_f>(
                v, &u::f, this, a_1, ..., a_n)              // No result parameter...
            .precondition([&] { ... })
            .postcondition([&] { ... })                     // ...so nullary functor.
            .except([&] { ... })
        ;

        ...
    }
    BOOST_CONTRACT_OVERRIDE(f)

    ...
}

For public function overrides not returning void, programmers must also pass a reference to the function return value as the second argument to boost::contract::public_function (this library will generate a compile-time error otherwise). ^[47] In this case, the library will pass this return value reference to the postcondition functor that must therefore take one single argument matching the return type, otherwise this library will generate a compile-time error (the functor parameter can be a constant reference const& to avoid extra copies of the return value, similarly to non-overriding non-void Virtual Public Functions):

class u {
public:
    // A non-void public function override.
    t f(t_1 a_1, ..., t_n a_n, boost::contract::virtual_* v = 0) /* override */ {
        t result;
        boost::contract::check c = boost::contract::public_function<override_f>(
                v, result, &u::f, this, a_1, ..., a_n)      // Result parameter...
            .precondition([&] { ... })
            .postcondition([&] (t const& result) { ... })   // ...so unary functor.
            .except([&] { ... })
        ;

        ...                                                 // Assign `result` at each return.
    }
    BOOST_CONTRACT_OVERRIDE(f)

    ...
}

This library will throw boost::contract::bad_virtual_result_cast if programmers specify return values for public function overrides in derived classes that are not consistent with the return types of the virtual public functions being overridden in the base classes. ^[48]

Important

It is the responsibility of the programmers to pass the extra virtual parameter v to all BOOST_CONTRACT_OLDOF and boost::contract::public_function calls within public function overrides, and also to pass the return value reference after v to boost::contract::public_function for non-void public function overrides. This library cannot always generate compile-time errors if programmers fail to do so (but in general this will prevent the library from correctly checking contracts at run-time). Mnemonics: When override_... is present, always pass it as template parameter to boost::contract::public_function. When v is present, always pass it as the first argument to boost::contract::public_function and BOOST_CONTRACT_OLDOF. Always pass result to boost::contract::public_function right after v for non-void functions.

At construction, the boost::contract::check RAII object for public function overrides does the following (enclosing public function override entry):

At destruction instead (enclosing public function override exit):

This ensures that contracts and subcontracts of public function overrides are correctly checked at run-time in accordance with the substitution principle (see Public Function Calls).

For the rest, considerations made in Virtual Public Functions apply to public function overrides as well.

	Note
	A public function override should always call boost::contract::public_function (even if it has no preconditions, no postconditions, no exception guarantees, and its class has no invariants), otherwise this library will not be able to correctly use it for subcontracting.

In order for this library to support subcontracting, programmers must specify the bases of a derived class declaring a public member type named base_types via a typedef using BOOST_CONTRACT_BASE_TYPES. For example (see base_types.cpp):

class chars
    #define BASES /* local macro (for convenience) */ \
        private boost::contract::constructor_precondition<chars>, \
        public unique_chars, \
        public virtual pushable<char>, \
        virtual protected has_size, \
        private has_empty
    : BASES // Bases of this class.
{
public:
    typedef BOOST_CONTRACT_BASE_TYPES(BASES) base_types; // Bases typedef.
    #undef BASES // Undefine local macro.

    /* ... */

For convenience, a local macro named BASES can be used to avoid repeating the base list twice (first in the derived class declaration class class-name : base-list and then again when invoking BOOST_CONTRACT_BASE_TYPES(base-list)). Being a local macro, BASES must be undefined using #undef BASES after it is used to declare the base_types typedef (to avoid name clashes and macro redefinition errors). ^[49]

BOOST_CONTRACT_BASE_TYPES is a variadic macro and accepts a list of bases separated by commas (see No Macros to program base_types without using macros). As already noted in Constructors, when the extra base boost::contract::constructor_precondition is used to program constructor preconditions, its inheritance access level must always be private and it must be specified as the very first base.

Important

Each base passed to BOOST_CONTRACT_BASE_TYPES must explicitly specify its inheritance access level public, protected, or private (but virtual is optional and can be specified either before or after the access level as usual in C++). This library will generate a compile-time error if the first base is missing its inheritance access level, but this library will not be able to always generate an error if the access level is missing for bases after the first one. [50] It is the responsibility of the programmers to make sure that all bases passed to BOOST_CONTRACT_BASE_TYPES explicitly specify their inheritance access level (inheritance access levels are instead optional in C++ because private is implicitly assumed for class types and public for struct types). Mnemonics: Always explicitly specify the inheritance access level public, protected, or private for base classes passed to BOOST_CONTRACT_BASE_TYPES.

See Access Specifiers to avoid making the base_types member type public. ^[51] See BOOST_CONTRACT_BASES_TYPEDEF to use a name different from base_types (e.g., because base_types clashes with other names in user-defined classes).

Contracts for public functions are programmed using boost::contract::public_function. In this section, let's consider static public functions. For example (see static_public.cpp):

template<class C>
class make {
public:
    static void static_invariant() { // Static class invariants.
        BOOST_CONTRACT_ASSERT(instances() >= 0);
    }

    // Contract for a static public function.
    static int instances() {
        // Explicit template parameter `make` (check static invariants).
        boost::contract::check c = boost::contract::public_function<make>();

        return instances_; // Function body.
    }

    /* ... */

It is possible to specify preconditions, postconditions, and exception guarantees for static public functions (see Preconditions, Postconditions, and Exception Guarantees). When called from static public functions, boost::contract::public_function cannot take the object this as a parameter (because there is no object this in static member functions) so the enclosing class type is specified via an explicit template parameter as in boost::contract::public_function<class-type> (the class type is required to check static class invariants, see Class Invariants):

class u {
public:
    // A static public function.
    static void f() {
        boost::contract::check c = boost::contract::public_function<u>() // Class type `u` as explicit template parameter.
            .precondition([&] { ... })
            .postcondition([&] { ... })
            .except([&] { ... })
        ;

        ...
    }

    ...
};

The boost::contract::public_function function returns an RAII object that must be assigned to a local variable of type boost::contract::check (otherwise this library will generate a run-time error, see BOOST_CONTRACT_ON_MISSING_CHECK_DECL). Furthermore, C++11 auto declarations cannot be used here and the boost::contract::check type must be explicitly specified (otherwise this library will generate a compile-time error prior C++17 and a run-time error post C++17). The static public functions body is programmed right after the declaration of this RAII object.

At construction, the boost::contract::check RAII object for static public functions does the following (enclosing static public function entry):

At destruction instead (enclosing static public function exit):

This ensures that static public function contracts are correctly checked at run-time (static public functions do not subcontract because they have no object this and therefore there is no inheritance, see Public Function Calls).

	Note
	A static public function can avoid calling boost::contract::public_function for efficiency but only when it has no preconditions, no postconditions, no exception guarantees, and its class has no static invariants (the class can still have non-static invariants or base classes instead).