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std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N

From cppreference.com
 
 
 
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is_bind_expression(C++11)
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_1, _2, _3, ...(C++11)
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function(C++11)
bad_function_call(C++11)
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unary_function(deprecated)
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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)
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mem_fun_ref_t
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(deprecated)
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binder1st
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Defined in header <functional>
extern /*unspecified*/ _1;

extern /*unspecified*/ _2;
.
.

extern /*unspecified*/ _N;

The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.

When used as an argument in a std::bind expression, the placeholder objects are stored in the generated function object, and when that function object is invoked with unbound arguments, each placeholder _N is replaced by the corresponding Nth unbound argument.

The types of the placeholder objects are DefaultConstructible and CopyConstructible, their default copy/move constructors do not throw exceptions, and for for any placeholder _N, the type std::is_placeholder<decltype(_N)> is defined and is derived from std::numeric_constant<int, N>.

[edit] Example

The following code shows the creation of function objects with a placeholder argument.

#include <functional>
#include <string>
#include <iostream>
 
void goodbye(const std::string& s)
{
    std::cout << "Goodbye " << s << '\n';
}
 
class Object {
public:
    void hello(const std::string& s)
    {
        std::cout << "Hello " << s << '\n';
    }
};
 
int main(int argc, char* argv[])
{
    typedef std::function<void(const std::string&)> ExampleFunction;
    Object instance;
    std::string str("World");
    ExampleFunction f = std::bind(&Object::hello, &instance, 
                                  std::placeholders::_1);
 
    // equivalent to instance.hello(str)
    f(str);
    f = std::bind(&goodbye, std::placeholders::_1);
 
    // equivalent to goodbye(str)
    f(str);    
    return 0;
}

Output:

Hello World
Goodbye World

[edit] See also

(C++11)
binds one or more arguments to a function object
(function template) [edit]
indicates that an object is a standard placeholder or can be used as one
(class template) [edit]