std::bind_front, std::bind_back

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Old binders and adaptors
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Defined in header <functional>
std::bind_front
template< class F, class... Args >
constexpr /* unspecified */ bind_front( F&& f, Args&&... args );
 (1) (since C++20)
template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_front( Args&&... args );
 (2) (since C++26)
std::bind_back
template< class F, class... Args >
constexpr /* unspecified */ bind_back( F&& f, Args&&... args );
 (3) (since C++23)
template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_back( Args&&... args );
 (4) (since C++26)

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.

1,3) The call wrapper holds a copy of the target callable object f.
2,4) The call wrapper does not hold a callable target (it is statically determined).
1) std::bind_front(f, bound_args...)(call_args...) is expression-equivalent to
std::invoke(f, bound_args..., call_args...).
2) std::bind_front<ConstFn>(bound_args...)(call_args...) is expression-equivalent to
std::invoke(ConstFn, bound_args..., call_args...).
3) std::bind_back(f, bound_args...)(call_args...) is expression-equivalent to
std::invoke(f, call_args..., bound_args...).
4) std::bind_back<ConstFn>(bound_args...)(call_args...) is expression-equivalent to
std::invoke(ConstFn, call_args..., bound_args...).

The following conditions must be true, otherwise the program is ill-formed:

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
Type requirements
-
std::decay_t<F> must meet the requirements of MoveConstructible.
-
std::decay_t<Args>... must meet the requirements of MoveConstructible.
-
decltype(ConstFn) must meet the requirements of Callable.

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.

Let bind-partial be either std::bind_front or std::bind_back.

The returned object has the following properties:

bind-partial return type

Member objects

The returned object behaves as if it holds:

1,3) A member object fd of type std::decay_t<F> direct-non-list-initialized from std::forward<F>(f), and
1-4) 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 (1,3) bind-partial(f, args...) or (2,4) bind-partial<ConstFn>(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:

1) std::invoke(g.fd, std::get<Ns>(g.tup)..., call_args...), when bind-partial is std::bind_front,
2) std::invoke(ConstFn, std::get<Ns>(g.tup)..., call_args...), when bind-partial is std::bind_front,
3) std::invoke(g.fd, call_args..., std::get<Ns>(g.tup)...), when bind-partial is std::bind_back,
4) std::invoke(ConstFn, call_args..., std::get<Ns>(g.tup)...), when bind-partial is std::bind_back,

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 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).

Exceptions

1,3) Throw any exception thrown by calling the constructor of the stored function object.
1-4) Throw any exception thrown by calling the constructor of any of the bound arguments.

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).

Feature-test macro Value Std Feature
__cpp_lib_bind_front 201907L (C++20) std::bind_front, (1)
202306L (C++26) Allow passing callable objects as non-type template arguments to std::bind_front, (2)
__cpp_lib_bind_back 202202L (C++23) std::bind_back, (3)
202306L (C++26) Allow passing callable objects as non-type template arguments to std::bind_back, (4)

Possible implementation

(2) bind_front 
namespace detail
{
    template<class T, class U>
    struct copy_const
        : std::conditional<std::is_const_v<T>, U const, U> {};
 
    template<class T, class U,
             class X = typename copy_const<std::remove_reference_t<T>, U>::type>
    struct copy_value_category
        : std::conditional<std::is_lvalue_reference_v<T&&>, X&, X&&> {};
 
    template <class T, class U>
    struct type_forward_like
        : copy_value_category<T, std::remove_reference_t<U>> {};
 
    template <class T, class U>
    using type_forward_like_t = typename type_forward_like<T, U>::type;
}
 
template<auto ConstFn, class... Args>
constexpr auto bind_front(Args&&... args)
{
    using F = decltype(ConstFn);
 
    if constexpr (std::is_pointer_v<F> or std::is_member_pointer_v<F>)
        static_assert(ConstFn != nullptr);
 
    return
        [... bound_args(std::forward<Args>(args))]<class Self, class... T>
        (
            this Self&&, T&&... call_args
        )
        noexcept
        (
            std::is_nothrow_invocable_v<F,
                detail::type_forward_like_t<Self, std::decay_t<Args>>..., T...>
        )
        -> std::invoke_result_t<F,
                detail::type_forward_like_t<Self, std::decay_t<Args>>..., T...>
        {
            return std::invoke(ConstFn, std::forward_like<Self>(bound_args)...,
                               std::forward<T>(call_args)...);
        };
}
(4) bind_back 
namespace detail { /* is the same as above */ }
 
template<auto ConstFn, class... Args>
constexpr auto bind_back(Args&&... args)
{
    using F = decltype(ConstFn);
 
    if constexpr (std::is_pointer_v<F> or std::is_member_pointer_v<F>)
        static_assert(ConstFn != nullptr);
 
    return
        [... bound_args(std::forward<Args>(args))]<class Self, class... T>
        (
            this Self&&, T&&... call_args
        )
        noexcept
        (
            std::is_nothrow_invocable_v<F,
                detail::type_forward_like_t<Self, T..., std::decay_t<Args>>...>
        )
        -> std::invoke_result_t<F,
                detail::type_forward_like_t<Self, T..., std::decay_t<Args>>...>
        {
            return std::invoke(ConstFn, std::forward<T>(call_args)...,
                               std::forward_like<Self>(bound_args)...);
        };
}

Example

Run this code
#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
}

References

  • C++26 standard (ISO/IEC 14882:2026):
  • TBD Function templates bind_front and bind_back [func.bind.partial]
  • C++23 standard (ISO/IEC 14882:2024):
  • 22.10.14 Function templates bind_front and bind_back [func.bind.partial]
  • C++20 standard (ISO/IEC 14882:2020):
  • 20.14.14 Function template bind_front [func.bind.front]

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)
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