std::bind_front
| Defined in header <functional>
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||
| template <class F, class... Args> /*unspecified*/ bind_front( F&& f, Args&&... args ); |
(since C++20) | |
The function template bind_front generates a forwarding call wrapper for f. Calling this wrapper is equivalent to invoking f with its first 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....).
The program is ill-formed if any of the following is not true:
- 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 first sizeof...(Args) parameters of f
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| 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 with the same arguments are the same.
The returned object (call wrapper) has the following properties:
std::bind_front return type
Member objects
The returned object behaves as if it holds a member object fd of type std::decay_t<F> constructed from std::forward<F>(f), and an std::tuple object tup constructed with std::make_tuple(std::forward<Args>(args)...) (this unwraps any std::reference_wrappers), except that the returned object's assignment behavior is unspecified and the names are for exposition only.
Constructors
The return type of std::bind_front 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_front(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...)
, 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 behavior is undefined 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
This function is intended to replace std::bind. Unlike std::bind, it does not support arbitrary argument rearrangement and has no special treatment for nested bind-expressions. On the other hand, it pays attention to the value category of the call wrapper object and propagates 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 are copied or moved, and are never passed by reference unless wrapped in std::ref or std::cref.
[edit] Example
| This section is incomplete Reason: no example |
[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) |
