How to inherit a tamplate and accessing its parameters in C++

Question

I am trying to inherit a template in C++. This template is using a namespace NSA, the Child Class has also a namespace called NSB. I want to access the protected variables of the template inside a third Class. But I don't know how.

Those are headers:

NSA{
    template <typename T> class A{
    protected:
       unsigned int my_var;
    }
}

NSB{
    class B{ #Don't know how to inherit template A
    ...
    }
}

Inside the cpp file of class C (all header files are included)

using namespace NSA;
NSB{
    unsigned int x = my_var #Get an error. Unidentified
    ...
}

Thank you.

Where does `class C` come from? You're missing `namespace` before `NSA` and `NSB`. — YSC, Jan 02 '19 at 15:10
Might be related/dup : https://stackoverflow.com/q/495021/5470596 — YSC, Jan 02 '19 at 15:11
Class C is just a third class where I include the headers of Class A and Template B. — Anna Noukou, Jan 02 '19 at 15:12
Possible duplicate of [Inheriting from a template class in c++](https://stackoverflow.com/questions/8810224/inheriting-from-a-template-class-in-c) — Fantastic Mr Fox, Jan 02 '19 at 15:12
@YSC, use the galactic dupe hammer and close this one. That feels like exactly the right dupe. — Fantastic Mr Fox, Jan 02 '19 at 15:13
Try to edit the code in question in a way that it compiles instead of explaining what you meant in comments. — Öö Tiib, Jan 02 '19 at 15:14

score 1 · Answer 1 · answered Jan 02 '19 at 15:18

You can't derived from a template; you can only derive from a class. So class B has to be derived from an instantiation of A:

class B : public A<int> {
};

If you want B to be a template you have to say so:

template <class Ty>
class B : public A<Ty> {
};

In either case, accessing members of the base class is a bit tricker than with an ordinary class, because a template can be specialized, and members declared in the template itself might not exist in a particular specialization. So you have to say that you're talking about a member:

class B : public A<int> {
    unsigned f() { return A<int>::my_var; }
};

Here's another way to say the same thing:

class B : public A<int> {
    unsigned f() { return this->my_var; }
};

score 0 · Answer 2 · answered Jan 02 '19 at 15:20

Skipping the 'C' mumbo jumbo, (which you didn't explain clearly)

I believe this is what you are looking for :

namespace NSA{
    template <typename T> class A{
    protected:
       unsigned int my_var;
    };
}

namespace NSB{
    class B : public NSA::A<int> {
                        //  ^^^ some type or make class B as template 
    //...

    unsigned int x = my_var;
    };
}

How to inherit a tamplate and accessing its parameters in C++

2 Answers2