samedi 26 novembre 2016

Is this a forwarding reference?

The distinction between rvalue references and forwarding references was made clear enough in this example by Scott Meyers:

Widget&& var1 = someWidget;     // here, “&&” means rvalue reference (1)

auto&& var2 = var1;             // here, “&&” does not mean rvalue reference (2)

template<typename T>
void f(std::vector<T>&& param); // here, “&&” means rvalue reference (3)

template<typename T>
void f(T&& param);              // here, “&&”does not mean rvalue reference (4)

Essentially the distinction happens when we have a deducible context, hence case (3) explicitly states that we have a vector<...>&& whereas the T in case (4) is to be deduced and (after applying reference collapsing rules) categorized in terms of "value category".

But what happens with a bit more complex pattern matching? Take the following case for example :

template <template <class...> class Tuple, class... Ts>
void f(Tuple<Ts...>&& arg)
{

}

What does && mean here ?

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