Recursive inheritance with variadic templates and inherited parameter problems

Question

If I use a chain of inheritance like the following example I could use vars from the deepest base without any problems:

    class A { public: int x; };
    class B : public A { };
    class C: public B { public: void Do() { cout << x << endl; } };

If I do the same with recursive variadic template classes I could not access my vars. Any idea how to access the vars and why I could not see my vars?

    template <class ...Parms>
    class Example;

    template <class Head, class ...Parms>
    class Example<Head, Parms...>: public Example<Parms...>
    {
    };

    template <>
    class Example<>
    {
        public:
        int x;
    };

    template <class ...Parms>
    class Last: public Example<Parms...>
    {
        void Do() { cout << x << endl; }
    };

Compile fails before any instance of the class is instantiated!

Angew is no longer proud of SO · Accepted Answer · 2012-11-15 09:37:42Z

8

x is a dependent name in this case, so you must access if as this->x (or bring it into scope by putting a using declaration in your class definition:

using Example<Params...>::x;

EDIT

The reason for this is discussed in [temp.res] in the standard. Basically: when the compiler is parsing your template, it sees just x. It has no way of knowing that x depends on the template's parameters (which it does in your case, because it comes from a base class which depends on them). Therefore, the compiler tries to resolve x using what it knows when parsing the template, and fails.

Writing this->x indicates that x refers to a member of the class; since a base class of your particular class depends on template parameters, the compiler knows it cannot resolve x when parsing the template, and will postpone the resolution until the template is instantiated. At that time, the template arguments are known.

The same holds true for using Example<Params...>::x;. That also tells the compiler that x depends on template parameters, and its resolution must be postponed to instanitation.

edited Nov 15, 2012 at 9:37

answered Nov 14, 2012 at 11:22

Angew is no longer proud of SO

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1 Comment

Klaus Over a year ago

Ok, this->x work, but I must say: I don't understand the difference between accessing x and this->x for data members. Can anyone explain why this happens?

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