Simple Example of Universal Reference

universal_ref.cpp

#include <iostream>
#include <type_traits>  // std::is_same

class Foo {
 public:
  Foo() = default;
  Foo(const Foo &other) { std::cout << "copy!" << std::endl; }
  Foo &operator=(const Foo &other) {
    std::cout << "copy = !" << std::endl;
    return *this;
  }
  Foo(Foo &&other) { std::cout << "move!" << std::endl; }
  Foo &operator=(Foo &&other) {
    std::cout << "move = !" << std::endl;
    return *this;
  }
  ~Foo() = default;
};

template <typename T>
void foo(T &&value) {
  // IMPORTANT: `value` is a universal reference
  std::cout << std::boolalpha << std::is_same<decltype(value), T &>()
            << std::endl;
}

int main(int argc, char const *argv[]) {
  Foo f;
  foo(f);      // true, `value` is lvalue-reference to `Foo`
  foo(Foo());  // false, `value` is rvalue-reference
  return 0;
}