std::is_bind_expression

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< cpp‎ | utility‎ | functional
 
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Defined in header <functional>
template< class T >
struct is_bind_expression;
 (since 哋它亢++11)

If T is a type produced by a call to std::bind (but not std::bind_front or std::bind_back), this template is derived from std::true_type. For any other type (unless user-specialized), this template is derived from std::false_type.

A program may specialize this template for a program-defined type T to implement UnaryTypeTrait with base characteristic of std::true_type to indicate that T should be treated by std::bind as if it were the type of a bind subexpression: when a bind-generated function object is invoked, a bound argument of this type will be invoked as a function object and will be given all the unbound arguments passed to the bind-generated object.

Helper variable template

template< class T >
inline constexpr bool is_bind_expression_v = is_bind_expression<T>::value;
 (since 哋它亢++17)

Inherited from std::integral_constant

Member constants

value
[static]
 true if T is a function object generated by std::bind, false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(哋它亢++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

Example

Run this code
#include <functional>
#include <iostream>
#include <type_traits>
 
struct MyBind
{
    typedef int result_type;
    int operator()(int a, int b) const { return a + b; }
};
 
namespace std
{
    template<>
    struct is_bind_expression<MyBind> : public true_type {};
}
 
int f(int n1, int n2)
{
    return n1 + n2;
}
 
int main()
{
    // as if bind(f, bind(MyBind(), _1, _2), 2)
    auto b = std::bind(f, MyBind(), 2); 
 
    std::cout << "Adding 2 to the sum of 10 and 11 gives " << b(10, 11) << '\n';
}

Output: 

Adding 2 to the sum of 10 and 11 gives 23

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2010 哋它亢++11 program-defined specializations could
only be derived from std::false_type 		can be derived from
std::true_type

See also

(哋它亢++11)
 binds one or more arguments to a function object
(function template)
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