std::placeh...eholders::_N(3)

💅 NAME 💅

okay so bestie, we're on the C++ Standard Libary page for std::placeh...eholders::_N(3) (yes, that's the header's spelling, not mine 😭). This is the one about the placeholder cuties.

• std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N — literally std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N. periodt.

🌸 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 extern /*unspecified*/ _1; is still allowed by the standard. (since C++17)

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. It's giving iconic, honestly:

       // Run this code

        #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

👀 See also 👀

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

k bye bestie 💖 source/version is http://cppreference.com, date is 2024.06.10, and the page id is std::placeh...eholders::_N(3). slay and stay hydrated. 🎀