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std::bind_front, std::bind_back

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Old binders and adaptors
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Defined in header <functional>
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)
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:

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 (1,2) 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.

[edit] 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).

[edit] 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.

[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.

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)

[edit] 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)...);
        };
}

[edit] 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
}

[edit] 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]

[edit] See also

(C++11)
binds one or more arguments to a function object
(function template) [edit]
(C++11)
creates a function object out of a pointer to a member
(function template) [edit]