std::bind_front, std::bind_back
| Defined in header <functional>
|
||
| template <class F, class... Args> constexpr /*unspecified*/ bind_front( F&& f, Args&&... args ); |
(1) | (since C++20) |
| template <class F, class... Args> constexpr /*unspecified*/ bind_back( F&& f, Args&&... args ); |
(2) | (since C++23) |
Function templates bind_front and bind_back generate a forwarding call wrapper for f. Calling this wrapper is equivalent to invoking f with its (1) first or (2) last sizeof...(Args) parameters bound to args.
In other words:
- std::bind_front(f, bound_args...)(call_args...) is equivalent to std::invoke(f, bound_args..., call_args...).
- std::bind_back(f, bound_args...)(call_args...) is equivalent to std::invoke(f, call_args..., bound_args...).
The program is ill-formed if any of the following is false:
- std::is_constructible_v<std::decay_t<F>, F>
- std::is_move_constructible_v<std::decay_t<F>>
- (std::is_constructible_v<std::decay_t<Args>, Args> && ...)
- (std::is_move_constructible_v<std::decay_t<Args>> && ...)
Contents |
[edit] Parameters
| f | - | Callable object (function object, pointer to function, reference to function, pointer to member function, or pointer to data member) that will be bound to some arguments |
| args | - | list of the arguments to bind to the (1) first or (2) last sizeof...(Args) parameters of f
|
| Type requirements | ||
-std::decay_t<F> and each type in std::decay_t<Args>... must meet the requirements of MoveConstructible.
| ||
[edit] Return value
A function object of type T that is unspecified, except that the types of objects returned by two calls to std::bind_front or std::bind_back with the same arguments are the same.
The returned object (call wrapper) has the following properties:
bind-partial return type
Let bind-partial be either std::bind_front or std::bind_back.
Member objects
The returned object behaves as if it holds a member object fd of type std::decay_t<F> direct-non-list-initialized from std::forward<F>(f), and an std::tuple object tup constructed with std::tuple<std::decay_t<Args>...>(std::forward<Args>(args)...), except that the returned object's assignment behavior is unspecified and the names are for exposition only.
Constructors
The return type of bind-partial behaves as if its copy/move constructors perform a memberwise copy/move. It is CopyConstructible if all of its member objects (specified above) are CopyConstructible, and is MoveConstructible otherwise.
Member function operator()
Given an object G obtained from an earlier call to bind-partial(f, args...), when a glvalue g designating G is invoked in a function call expression g(call_args...), an invocation of the stored object takes place, as if by
- std::invoke(g.fd, std::get<Ns>(g.tup)..., call_args...), when
bind-partialisstd::bind_front, or by - std::invoke(g.fd, call_args..., std::get<Ns>(g.tup)...), when
bind-partialisstd::bind_back, where
-
Nsis an integer pack0, 1, ..., (sizeof...(Args) - 1) -
gis an lvalue in the std::invoke expression if it is an lvalue in the call expression, and is an rvalue otherwise. Thus std::move(g)(call_args...) can move the bound arguments into the call, where g(call_args...) would copy.
-
The program is ill-formed if g has volatile-qualified type.
The member operator() is noexcept if the std::invoke expression it calls is noexcept (in other words, it preserves the exception specification of the underlying call operator).
[edit] Exceptions
Only throws if construction of stored function object or any of the bound arguments throws.
[edit] Notes
These function templates are intended to replace std::bind. Unlike std::bind, they do not support arbitrary argument rearrangement and have no special treatment for nested bind-expressions or std::reference_wrappers. On the other hand, they pay attention to the value category of the call wrapper object and propagate exception specification of the underlying call operator.
As described in std::invoke, when invoking a pointer to non-static member function or pointer to non-static data member, the first argument has to be a reference or pointer (including, possibly, smart pointer such as std::shared_ptr and std::unique_ptr) to an object whose member will be accessed.
The arguments to std::bind_front or std::bind_back are copied or moved, and are never passed by reference unless wrapped in std::ref or std::cref.
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__cpp_lib_bind_front |
201907L | (C++20) | std::bind_front
|
__cpp_lib_bind_back |
202202L | (C++23) | std::bind_back
|
[edit] Example
#include <functional> #include <iostream> int minus(int a, int b) { return a - b; } struct S { int val; int minus(int arg) const noexcept { return val - arg; } }; int main() { auto fifty_minus = std::bind_front(minus, 50); std::cout << fifty_minus (3) << '\n'; // equivalent to `minus(50, 3)` auto member_minus = std::bind_front(&S::minus, S{50}); std::cout << member_minus (3) << '\n'; // equivalent to `S tmp{50}; tmp.minus(3)` // noexcept-specification is preserved! static_assert(! noexcept(fifty_minus (3))); static_assert(noexcept(member_minus (3))); // binding of a lambda auto plus = [](int a, int b) { return a + b; }; auto forty_plus = std::bind_front(plus, 40); std::cout << forty_plus(7) << '\n'; // equivalent to `plus(40, 7)` #ifdef __cpp_lib_bind_back auto madd = [](int a, int b, int c) { return a * b + c; }; auto mul_plus_seven = std::bind_back(madd, 7); std::cout << mul_plus_seven(4, 10) << '\n'; // equivalent to `madd(4, 10, 7)` #endif }
Possible output:
47 47 47 47
[edit] See also
| (C++11) |
binds one or more arguments to a function object (function template) |
| (C++11) |
creates a function object out of a pointer to a member (function template) |
