CODEWITHSUNDEEP
c++templatestemplate-templates
Helloer
Helloer

Reputation: 467

Passing a template's template member type as a template template argument

A class WithTTMember has a template member type named TT.

struct WithTTMember {
    template<typename> using TT = void;
};

Another class ExpectTT takes a template template parameter:

template< template<typename> typename TT >
struct ExpectTT {};

ExpectTT<WithTTMember::TT> can be successfully instantiated.

A third class ExpectTWithTT expects a template parameter with a template member type named TT, and instantiates ExpectTT using it:

template<typename T>
struct ExpectTWithTT {
    using X = ExpectTT<typename T::TT>;  // this doesn't compile
};

I expect ExpectTWithTT<WithTTMember>::X to be the same type as ExpectTT<WithTTMember::TT>. However the code above is fails to compile.

I tried injecting the faulty line with a combination of template and typename keywords following compiler messages and my instinct, but I couldn't get it to work.

How can I express what I want?

Any C++ version is fine.

Upvotes: 4

Views: 475

Answers (1)

songyuanyao
songyuanyao

Reputation: 172924

You should use template keyword to tell that T::TT is a template.

template<typename T>
struct ExpectTWithTT {
    using X = ExpectTT<T::template TT>;
    //                    ^^^^^^^^
};

Upvotes: 5

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