refer to a non-dependent name without specifying template parameter

Question

Consider the following:

template<typename T> struct Foo {
     typedef void                  NonDependent;
     typedef typename T::Something Dependent;
}

I would like to refer to NonDependent without specifying any template parameter, as in Foo::NonDependent.

I know I can always use a dummy parameter:

Foo<WhateverSuits>::NonDependent bla;

But that is ugly, and since NonDependent is invariant with respect to T, I would like to refer to it without relying on the dummy. Is it possible?

Thanks

@Mohit I assume in real code it aliases some other type with a long and ugly name, not `void`. — HolyBlackCat, Jul 19 '18 at 10:07
I doubt it's possible, but you could add a default value for `T` and do `Foo<>::NonDependent` instead. — HolyBlackCat, Jul 19 '18 at 10:09

score 6 · Accepted Answer · answered Jul 19 '18 at 10:17

You can not refer to NonDependent without specifying template parameter because it may vary or completely absent depending on template parameter. For example:

template<> struct Foo<int>
{
   typedef float NonDependent;
};
template<> struct Foo<std::string>
{
   typedef typename std::string::value_type Dependent;
};

You may need to move NonDependent declaration into base (non-template) struct and refer to it instead:

struct FooBase{ typedef void NonDependent; };

template<typename T> struct Foo: public FooBase
{
    typedef typename T::Something Dependent;
};
template<> struct Foo<int>: public FooBase
{
   typedef float NonDependent;
};

FooBase::NonDependent bla;

Oh, I see... Explicit specializations might change the definition of the same name or even remove it. I like the non-template base solution. Thanks! — L. Bruce, Jul 19 '18 at 17:02

score 0 · Answer 2 · answered Jul 19 '18 at 13:41

As previously stated this can not be done. Mainly because Foo is not a type available at run-time, rather a template available at compile time, so things like Foo::NonDependent are invalid.

In orther to have something available at run time you have to instantiate the template, providing the minimum number of arguments in order to produce a class, of which subtypes can be referred to at run time.

refer to a non-dependent name without specifying template parameter

2 Answers2