Why do we want perfect forwarding?

[JUANDENT]

Hi,

why should we use this code:

do_print(std::forward<T>(arg));

instead of

do_print(arg);

I understand the first code will respect the lvalue or rvalue type of the arg (doing perfect forwarding), but why would we care the parameter remains lvalue or rvalue??


what is a real world example where we care the parameter's value category is important?

[Peter87]

This only matters if do_print might handle rvalue arguments differently (e.g. more efficiently) than lvalue arguments. If you know that do_print always takes its argument by const ref then there is no point in using std::forward here.

[Peter87]

Examples of where perfect forwarding is useful:

* the "emplace" functions on containers
https://en.cppreference.com/w/cpp/container/set/emplace
https://en.cppreference.com/w/cpp/container/vector/emplace_back

* the "make" functions for smart pointers
https://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique
https://en.cppreference.com/w/cpp/memory/shared_ptr/make_shared

These functions use perfect forwarding, to forward the arguments to the constructors.

For example, if the object that is being constructed is a std::vector<int> and you pass a temporary std::vector<int> object as argument to the function, then perfect forwarding allows the temporary object to be "moved" by the constructor.

std::vector<int> f()
{
	return {1, 2, 3};
}

std::set<std::vector<int>> s;

s.emplace(f());

Thanks to perfect forwarding, this calls the move constructor rather than the copy constructor, so there is no copying of the vector elements in the above example.

[JUANDENT]

Does this code demonstrates the advantage of using perfect forwarding?

void do_the_print(string n)
{}

void do_print(const string& s)
{
	do_the_print(s);
}
void do_print(string&& t)
{
	do_the_print(std::move(t));   /// THIS HERE IS THE ADVANTAGE
}
template<typename T>
void printer(T&& arg)
{
	do_print(arg);
	do_print(std::forward<T>(arg));
}
string get_string()
{
	return "Hey hello!"s;
}
void call_print()
{
	do_print(get_string());
	printer(get_string());
	string str{ "Goodbye" };
	printer(str);
}