std::function<R(Args...)>::function - cppreference.com (original) (raw)

function() noexcept; (1) (since C++11)
function( std::nullptr_t ) noexcept; (2) (since C++11)
function( const function& other ); (3) (since C++11)
(4)
function( function&& other ); (since C++11) (until C++20)
function( function&& other ) noexcept; (since C++20)
template< class F > function( F&& f ); (5) (since C++11)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc ) noexcept; (6) (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, std::nullptr_t ) noexcept; (7) (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, const function& other ); (8) (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, function&& other ); (9) (since C++11) (removed in C++17)
template< class F, class Alloc > function( std::allocator_arg_t, const Alloc& alloc, F f ); (10) (since C++11) (removed in C++17)

Constructs a std::function from a variety of sources.

1,2) Creates an empty std::function.

  1. Copies the target of other to the target of *this.

If other is empty, *this will be empty right after the call too.

  1. Moves the target of other to the target of *this.

If other is empty, *this will be empty right after the call too.

other is in a valid but unspecified state right after the call.

  1. Initializes the target with std::forward<F>(f). The target is of type std::decay<F>::type.

If f is a null pointer to function, a null pointer to member, or an empty value of some std::function specialization, *this will be empty right after the call.

This overload participates in overload resolution only if all following conditions are satisfied:

6-10) Same as (1-5) except that alloc is used to allocate memory for any internal data structures that the std::function might use.

When the target is a function pointer or a std::reference_wrapper, small object optimization is guaranteed, that is, these targets are always directly stored inside the std::function object, no dynamic allocation takes place. Other large objects may be constructed in dynamic allocated storage and accessed by the std::function object through a pointer.

[edit] Parameters

other - the function object used to initialize *this
f - a callable object used to initialize *this
alloc - an Allocator used for internal memory allocation
Type requirements
-Alloc must meet the requirements of Allocator.

[edit] Exceptions

3,8,9) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object.

4) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object. (until C++20)

5,10) Does not throw if f is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the copy constructor of the stored callable object.

[edit] Notes

std::function's allocator support was poorly specified and inconsistently implemented. Some implementations do not provide overloads (6-10) at all, some provide the overloads but ignore the supplied allocator argument, and some provide the overloads and use the supplied allocator for construction but not when the std::function is reassigned. As a result, allocator support was removed in C++17.

[edit] Example

#include #include #include   void print_num(int i) { std::cout << "print_num(" << i << ")\n"; }   int main() { std::function<void(int)> func1; // (1) empty constructor try { func1(333 << 1); } catch (const std::bad_function_call& ex) { std::cout << "1) " << ex.what() << '\n'; }   std::function<void(int)> func2{nullptr}; // (2) empty constructor try { func1(222 * 3); } catch (const std::bad_function_call& ex) { std::cout << "2) " << ex.what() << '\n'; }   func1 = print_num; // initializes func1 using assignment operator   std::function<void(int)> func3{func1}; // (3) copy constructor func3(33);   std::function<void(int)> func4{std::move(func3)}; // (4) move constructor, // func3 in unspecified state func4(44);   std::function<void(int)> func5{print_num}; // (5) constructor with function func5(55);   // (5) constructor with lambda std::function<void(int)> func6([](int i) { std::cout << "lambda(" << i << ")\n"; }); func6(66); }

Possible output:

  1. bad_function_call
  2. bad_function_call print_num(33) print_num(44) print_num(55) lambda(66)

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2132 C++11 overloads (5,10) might be ambiguous constrained
LWG 2774 C++11 (5,10) performed an additional move eliminated

[edit] See also

| | constructs a new std::move_only_function object (public member function of std::move_only_function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |