# std::bind_front

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
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>> && ...)

### 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`
Type requirements
- `std::decay_t<F> and each type in std::decay_t<Args>...` must meet the requirements of MoveConstructible.

### 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

- `Ns` is an integer pack `0, 1, ..., (sizeof...(Args) - 1)`
- `g` is 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)

### Exceptions

Only throws if construction of stored function object or any of the bound arguments throws.

### 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.

### Example

| This section is incomplete Reason: no example

### See also

bind (C++11) | binds one or more arguments to a function object (function template)
mem_fn (C++11) | creates a function object out of a pointer to a member (function template)

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