通用引用和重载

Question

Say I have two overloads of a method in a class, set_value which each take a universal reference. I'd like to have one function call the other to avoid code duplication, but I'm not sure which is the correct way to give the universal reference to the other function.

template <typename U>
void set_value(U&& value) {
    this->value_ = std::forward<U>(value);
}

template <typename U>
void set_value(U&& value, int overloaded) {
    // which is the correct way to use the other `set_value` overload?
    set_value(value); // just use value directly?
    set_value(std::forward<U>(value)); // or should I forward it?
}

Answer 1

If you want perfect forwarding of value , use std::forward

Even though value might be bound to an rvalue, when you use it by name inside set_value it is an lvalue. Therefore, if you do set_value(value); the universal reference in the one-parameter overload would be bound to an lvalue reference.

We can confirm this by adding doing something like this:

template<class T> class TD;  // Allows us to view type deduction results in
                             // the form of compilation errors.
                             // (From "Effective Modern C++")

...

template <typename U>
void set_value(U&& value) {
    TD<decltype(value)> td;
}

...

int i;
set_value(i, 0);
set_value(3, 0);

When forwarding is used, this will give you a compilation error that will reveal the type of value to be int& for set_value(i, 0); . If you comment out that statement you'll get a compilation error for set_value(3, 0); revealing the type of value to be int&& .
If you remove the forwarding you'll get int& in both cases.