Namespaces
Variants
Views
Actions

std::is_placeholder

From cppreference.com
 
 
 
Function objects
Operator wrappers
Negators
Bind
bind(C++11)
is_bind_expression(C++11)
is_placeholder(C++11)
_1, _2, _3, ...(C++11)
Function wrappers
mem_fn(C++11)
function(C++11)
bad_function_call(C++11)
Reference wrappers
reference_wrapper(C++11)
ref
cref
(C++11)
(C++11)
Deprecated binders and adaptors
unary_function(deprecated)
binary_function(deprecated)
ptr_fun(deprecated)
pointer_to_unary_function(deprecated)
pointer_to_binary_function(deprecated)
mem_fun(deprecated)
mem_fun_t
mem_fun1_t
const_mem_fun_t
const_mem_fun1_t
(deprecated)
(deprecated)
(deprecated)
(deprecated)
mem_fun_ref(deprecated)
mem_fun_ref_t
mem_fun1_ref_t
const_mem_fun_ref_t
const_mem_fun1_ref_t
(deprecated)
(deprecated)
(deprecated)
(deprecated)
binder1st
binder2nd
(deprecated)
(deprecated)
bind1st
bind2nd
(deprecated)
(deprecated)
 
Defined in header <functional>
template< class T >
struct is_placeholder;
(since C++11)

If T is the type of a standard placeholder (_1, _2, _3, ...), then this template is derived from std::integral_constant<int,1>, std::integral_constant<int,2>, std::integral_constant<int,3>, respectively. If T is not a standard placeholder type, this template is derived from std::integral_constant<int,0>.

The template may be specialized for any user-defined type which should be treated by std::bind as if it was a placeholder for unbound arguments.

Contents

Inherited from std::integral_constant

Member constants

value
[static]
placeholder value or 0 for non-placeholder types
(public static member constant)

Member functions

operator int
converts the object to int, returns value
(public member function)

Member types

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

[edit] Example

#include <iostream>
#include <type_traits>
#include <functional>
 
struct My_2 {
} my_2;
 
namespace std {
    template<>
    struct is_placeholder<My_2> : public integral_constant<int, 2> {};
}
 
int f(int n1, int n2)
{
    return n1+n2;
}
 
int main()
{
    std::cout << "Standard placeholder _5 is for the argument number "
              << std::is_placeholder<decltype(std::placeholders::_5)>::value
              << '\n';
 
    auto b = std::bind(f, my_2, 2);
    std::cout << "Adding 2 to 11 selected with a custom placeholder gives " << b(10, 11) << '\n';
}

Output:

Standard placeholder _5 is for the argument number 5
Adding 2 to 11 selected with a custom placeholder gives 13

[edit] See also

(C++11)
binds one or more arguments to a function object
(function template) [edit]
placeholders for the unbound arguments in a std::bind expression
(constant) [edit]