CODEWITHSUNDEEP
c++
anon
anon

Reputation:

What kind of class template definition is this?

template<class T> struct is_ref_wrapper<std::reference_wrapper<T>> : std::true_type {};

It appears in the possible implementation of std::experimental::make_array. The reason it looks odd to me is that the class name contains a template parameter list <std::reference_wrapper<T>> mentioning only one parameter, but that parameter is inside the template argument of the class (i.e. something has been elided).

Upvotes: 1

Views: 63

Answers (2)

Quentin
Quentin

Reputation: 63154

That lines goes in tandem with the one above it:

template<class> struct is_ref_wrapper : std::false_type {};
template<class T> struct is_ref_wrapper<std::reference_wrapper<T>> : std::true_type {};

The first line declares the (primary) template is_ref_wrapper. The second line declares a specialization, which will be chosen instead of the primary template if and only if the parameter matches std::reference_wrapper<T> for some T. Which is exactly the required behaviour.

Upvotes: 2

R Sahu
R Sahu

Reputation: 206737

The other related line from the page you linked to is:

template<class> struct is_ref_wrapper : std::false_type {};

This defines the generic template. The next line is a specialization.

template<class T> struct is_ref_wrapper<std::reference_wrapper<T>> : std::true_type {};

When is_ref_wrapper is instantiated with a std::reference_wrapper<T> as the template parameter, the specialization is used.

Upvotes: 2

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