CODEWITHSUNDEEP
c++templatesinheritancescopeoverriding
gameboo
gameboo

Reputation: 128

C++ "was not declared in this scope" when overriding a pure virtual method in a derived class

here is what I need to do :

#include <iostream>                                                                                                                                                                                              

using namespace std;

class A
{
    public :
    virtual void fa() = 0;
};

template <typename type>
class B : public A
{
    protected :
    int v_b;
};

template <typename type>
class C : public B<type>
{
    public :
    void fa()
    {
        // whatever action that try to access v_b
        cout << "v_b = " << v_b << endl;
    }
};

int main()
{
    C<int> toto;
    toto.fa();
    return 0;
}

and here is g++ output :

test.cpp: In member function ‘void C<type>::fa()’:
test.cpp:25:29: error: ‘v_b’ was not declared in this scope

In my understanding, v_b is a protected member of B and is therefore accessible in C. A and B are both abstract classes, and I need to override the f_a() method of A in class C in order to instanciate it. Since the compiler is telling me this

test.cpp: In member function ‘void C<type>::fa()’:

and NOT

‘void A::fa()’:

i don't get why the v_b variable is not present in the scope. Is this a problem in the way I use templates?

Can anyone help me out on that one ?

thx

edit :

I tryed to use this->v_b or B<type>::v_b as suggested here and it worked fine ! thx for your help

Upvotes: 3

Views: 2911

Answers (1)

David Rodr&#237;guez - dribeas
David Rodr&#237;guez - dribeas

Reputation: 208363

In the expression:

    cout << "v_b = " << v_b << endl;

v_b is a non-dependent expression (i.e. it does not look like it depends on the template arguments. For a non-dependent expression the first phase lookup must resolve the symbol, and it will do so by looking only in non-dependent contexts. This does not include a templated base (as it does depend on the type argument). The simple fix is to qualify the call with this:

    cout << "v_b = " << this->v_b << endl;

Now it is a dependent expression (this which clearly depends on the instantiating type), and lookup is delayed to the second phase where the type is substituted and the bases can be checked.

Upvotes: 3

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