Why a template alias specialization depends on the context in which it is referred?

Question

Consider this example code:

template <class T>
using pt_type = typename T::type;

template <class T>
class V {
  using type = int;
  public:
  using pt = pt_type<V>;
};

void g() {
  V<int>::pt a; // Does compile
  pt_type<V<int>> b; // Does not compile
}

V<int>::pt is an alias for pt_type<V<int>>. Nevertheless the fact it is defined depends on the context where it is referred.

Where is it explained in the C++ standard that the substitution of the template parameter by the template argument is performed in the context where is refered the alias specialization?

score 9 · Accepted Answer · edited Jan 25 '18 at 23:29

9

Nowhere. This is core issue 1554.

The interaction of alias templates and access control is not clear from the current wording of 14.5.7 [temp.alias]. For example:
template <class T> using foo = typename T::foo;

class B {
  typedef int foo;
  friend struct C;
};

struct C {
  foo<B> f;    // Well-formed?
};

edited Jan 25 '18 at 23:29

kmdreko

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answered Jan 25 '18 at 23:23

T.C.

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Caleb · Answer 2 · 2018-01-25T23:11:45.357

-1

using pt_type = typename T::type; can't access V::type becasue type is private.

The following works:

template <class T>
using pt_type = typename T::type;

template<class T>
class V
{
  public:
    using type = int;
    using pt = pt_type<V>;
};

void g()
{
    V<int>::pt a; //Do compile
    pt_type<V<int>> b; //Do not compile
}

edited Jan 25 '18 at 23:11

answered Jan 25 '18 at 23:09

Caleb

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Please read the code one more time. pt_type do access V::type in the definition of V::pt. – Oliv Jan 25 '18 at 23:11

LeDYoM · Answer 3 · 2018-01-25T23:14:30.323

-1

In V::pt you are accessing to your "own" type and you can do it, but the private makes it impossible in the second case. So V::pt crates an instanciation of pt_type passing your private type int. But in the second case you try directly and it does not work,

edited Jan 25 '18 at 23:14

answered Jan 25 '18 at 23:11

LeDYoM

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1
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OK This is second out of subject answer, I am waiting for a c++ standard quotation – Oliv Jan 25 '18 at 23:12
What standard quotation? That you cannot access private members? – LeDYoM Jan 25 '18 at 23:15
And you? Make it public and it works. As @Caleb already shown you. This is not context dependant because you are referring to them in different paths. Actually, with GCC 7.2 none of your 2 lines even work. As normal. – LeDYoM Jan 25 '18 at 23:35

Why a template alias specialization depends on the context in which it is referred?

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