C++ Forwarding references

Forwarding references (also called universal references) use the syntax T&& where T is a deduced template parameter, allowing a function to accept both lvalues and rvalues and preserve their value category. With function template argument deduction and reference collapsing rules, T&& binds to both lvalue and rvalue arguments.

Use forwarding references with std::forward to write generic functions that preserve argument categories and avoid unnecessary copies.

This example shows forwarding references accepting both lvalues and rvalues with their original categories preserved.

// compile: g++ -std=c++17 -o forward forward.cpp
// run: ./forward
// description: forwarding references accept and preserve lvalue/rvalue distinction
 
#include <iostream>
#include <utility>
 
class Widget {
public:
    Widget() { std::cout << "Widget constructed\n"; }
    Widget(const Widget&) { std::cout << "Widget copied\n"; }
    Widget(Widget&&) { std::cout << "Widget moved\n"; }
};
 
template <typename T>
void process(T&& arg) {
    std::cout << "Received ";
    Widget w = std::forward<T>(arg);  // preserves lvalue/rvalue
}
 
int main() {
    Widget w;
    process(w);              // lvalue: calls copy
    process(Widget());       // rvalue: calls move
 
    return 0;
}