Is it possible to define an alias for a template-template parameter?

Question

I'm experimenting with template-template for fun. I have the following class:

template<template<class> class T, typename R> class Unit
{    
    using FullType = T<R>;
    using Ratio = R;
    //using Type = T;

    ...
};

I have define type R and T<R> as member-types Ratio and FullType. Is it possible to alias T as Type ?

The commented line above give me the following errors on g++ 4.7:

expected nested-name-specifier before 'Type'
using-declaration for non-member at class scope
expected ';' before '=' token
expected unqualified-id before '=' token

I tried some more or less random syntaxes, but none of them compiled.

Thanks !

score 4 · Answer 1 · edited May 23 '17 at 11:43

4

Since T isn't a type, the question as asked doesn't make sense. However, you can make an alias for T, like:

template <template <typename> class T, typename R> class Unit
{
    template <typename U> using MyTemplate = T<U>;
    // ...

    // use e.g. MyTemplate<int> to get T<int>
};

Pre-C++11 you would need something more notationally involved as outlined in this answer of mine (and used, for example, in the standard library by the standard allocator's rebind mechanic.)

edited May 23 '17 at 11:43

Community

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answered Dec 15 '12 at 21:48

Kerrek SB

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Thanks, that's exactly what I wanted.I'll edit the question in order for it to make sense – arnoo Dec 15 '12 at 21:54

score 3 · Accepted Answer · answered Dec 16 '12 at 13:25

You cannot make an alias for T. The following was discussed in the committee to make an alias for T (because a very late C++11 draft contained notes that stated that it is an alias for T which a Defect Report cleaned up).

// Courtesy of @KerrekSB
template <template <typename> class T, typename R> class Unit
{
    template <typename U> using MyTemplate = T<U>;
    // ...

    // use e.g. MyTemplate<int> to get T<int>
};

Note while MyTemplate<int> is the same type as T<int>, that MyTemplate is not the same as T. The wording at http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1286 was supposed to change that, but in the last meeting it was considered to be a very special machinery that doesn't really fit what alias templates turned out to be (own templates), and it was pushed back to review. To get that effect, a using MyTemplate = T; in future may fit the bill (if proposed and accepted, of course).

Is it possible to define an alias for a template-template parameter?

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