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Perfect forwarding

12345678#include <utility> template<typename T, typename U> std::pair<T, U> make_pair_wrapper(T&& t, U&& u) { return std::make_pair(std::forward<T>(t), std::forward<U>(u)); }

This sample is licensed under the CC0 Public Domain Dedication.

Intent

Forward arguments of one function to another as though the wrapped function had been called directly.

Description

Perfect forwarding allows us to preserve an argument’s value category (lvalue/rvalue) and const/volatile modifiers. Perfect forwarding is performed in two steps: receive a forwarding reference (also known as universal reference), then forward it using std::forward.

In our example, the arguments t and u on line 4 are forwarding references because they are declared in the form X&& where X is a template parameter. We use std::forward on lines 6–7 to forward these arguments to std::make_pair, allowing them to be moved into the pair when the original argument was an rvalue expression.

Perfect forwarding is often used with variadic templates to wrap calls to functions with an arbitrary number of arguments. For example, std::make_unique and std::make_shared both use perfect forwarding to forward their arguments to the constructor of the wrapped type.

Contributors

  • Joseph Mansfield
  • Max Galkin

Last Updated

09 April 2015

Source

Fork this sample on GitHub

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