using type = smth. and templated types

Question

I have some trouble with a piece of C++ code.

First, if I do this, it works quite well :

struct A
{
    using my_type1 = double;
    using my_type2 = int;
};

struct B
{
    using size_type = A::my_type2;
};

However, I want to be able to choose my_type1, so I went the template way :

template <typename T>
struct A
{
    using my_type1 = T;
    using my_type2 = int;
};

template <typename T>
struct B
{
    using size_type = A<T>::my_type2;
};

Here, gcc fails with : "expected type specifier" on the line

using size_type = A<T>::my_type2;

I could just put my_type2 in template too but this is a type that shouldn't change much.

So why doesn't my method work ? Thanks!

Templates won't let you choose types at runtime anyway. – n. m. could be an AI Oct 30 '13 at 15:46 — n. m. could be an AI, Oct 30 '13 at 15:46
@n.m. : yes, that's not what I meant, sorry. – Jean-Michaël Celerier Oct 30 '13 at 16:23 — Jean-Michaël Celerier, Oct 30 '13 at 16:23

score 1 · Accepted Answer · answered Oct 30 '13 at 15:43

1

You have to add typename:

using size_type = typename A<T>::my_type2;

answered Oct 30 '13 at 15:43

awesoon

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1

Okay, works perfectly! Where did you get to know this ? – Jean-Michaël Celerier Oct 30 '13 at 16:22

using type = smth. and templated types

1 Answers1