std::bind_front, std::bind_back

Header: <functional>

Function templates std::bind_front and std::bind_back generate a perfect forwarding call wrapper which allows to invoke the callable target with its (1,2) first or (3,4) last sizeof…(Args) parameters bound to args.

# Declarations

std::bind_front

template< class F, class... Args >
constexpr /* unspecified */ bind_front( F&& f, Args&&... args );

(since C++20)

template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_front( Args&&... args );

(since C++26)

std::bind_back

template< class F, class... Args >
constexpr /* unspecified */ bind_back( F&& f, Args&&... args );

(since C++23)

template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_back( Args&&... args );

(since C++26)

# 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 (1,2) first or (3,4) last sizeof…(Args) parameters of the callable target

# Return value

A function object (the call wrapper) 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.

# 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.

Typically, binding arguments to a function or a member function using (1) std::bind_front and (3) std::bind_back requires storing a function pointer along with the arguments, even though the language knows precisely which function to call without a need to dereference the pointer. To guarantee “zero cost” in those cases, C++26 introduces the versions (2,4) (that accept the callable object as an argument for non-type template parameter).

# Example

#include <cassert>
#include <functional>
 
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);
    assert(fifty_minus(3) == 47); // equivalent to: minus(50, 3) == 47
 
    auto member_minus = std::bind_front(&S::minus, S{50});
    assert(member_minus(3) == 47); //: S tmp{50}; tmp.minus(3) == 47
 
    // 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);
    assert(forty_plus(7) == 47); // equivalent to: plus(40, 7) == 47
 
#if __cpp_lib_bind_front >= 202306L
    auto fifty_minus_cpp26 = std::bind_front<minus>(50);
    assert(fifty_minus_cpp26(3) == 47);
 
    auto member_minus_cpp26 = std::bind_front<&S::minus>(S{50});
    assert(member_minus_cpp26(3) == 47);
 
    auto forty_plus_cpp26 = std::bind_front<plus>(40);
    assert(forty_plus(7) == 47);
#endif
 
#if __cpp_lib_bind_back >= 202202L
    auto madd = [](int a, int b, int c) { return a * b + c; };
    auto mul_plus_seven = std::bind_back(madd, 7);
    assert(mul_plus_seven(4, 10) == 47); //: madd(4, 10, 7) == 47
#endif
 
#if __cpp_lib_bind_back >= 202306L
    auto mul_plus_seven_cpp26 = std::bind_back<madd>(7);
    assert(mul_plus_seven_cpp26(4, 10) == 47);
#endif
}

# See also

• bind
• mem_fn