NAME¶

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

Synopsis¶

Defined in header <functional>
/*see below*/ _1;

/*see below*/ _2;
.
.

/*see below*/ _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.

Each placeholder is declared as if by extern /*unspecified*/ _1; (until C++17)
Implementations are encouraged to declare the placeholders as if by
inline constexpr /*unspecified*/ _1;, although declaring them by (since C++17)
extern /*unspecified*/ _1; is still allowed by the standard.

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

Example¶

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

// Run this code

#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()
{
using namespace std::placeholders;

using ExampleFunction = std::function<void(const std::string&)>;
Object instance;
std::string str("World");

ExampleFunction f = std::bind(&Object::hello, &instance, _1);
f(str); // equivalent to instance.hello(str)

f = std::bind(&goodbye, std::placeholders::_1);
f(str); // equivalent to goodbye(str)

auto lambda = [](std::string pre, char o, int rep, std::string post) {
std::cout << pre;
while (rep-- > 0) std::cout << o;
std::cout << post << '\n';
};

// binding the lambda:
std::function<void(std::string, char, int, std::string)> g =
std::bind(&decltype(lambda)::operator(), &lambda, _1, _2, _3, _4);
g("G", 'o', 'o'-'g', "gol");
}

Output:¶

Hello World
Goodbye World
Goooooooogol

See also¶

bind binds one or more arguments to a function object
(C++11) (function template)
is_placeholder indicates that an object is a standard placeholder or can be used as
(C++11) one
(class template)
ignore placeholder to skip an element when unpacking a tuple using tie
(C++11) (constant)