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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../utility/functional/placeholders.html below:

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

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;.

Implementations are encouraged to declare the placeholders as if by inline constexpr /*unspecified*/ _1;, although declaring them by 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>.

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

#include <functional>
#include <iostream>
#include <string>
 
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

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