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

Question

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

possible duplicate of [Accessing public members of base class fails](http://stackoverflow.com/questions/7281072/accessing-public-members-of-base-class-fails) — CB Bailey, Jun 29 '12 at 22:37
Or http://stackoverflow.com/questions/1624564/access-protected-member-of-a-class-in-a-derived-class — CB Bailey, Jun 29 '12 at 22:39
`this->v_b` will fix the code. The links from @CharlesBailey explain why. — Mike Steinert, Jun 29 '12 at 22:47
Are you using an older version of g++? http://ideone.com/fBXNK — bames53, Jun 29 '12 at 23:04
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) (sry if you answered before I suppressed the part where it didn't work with the "second technique", it was another unrelated mistake) thx for the links — gameboo, Jun 29 '12 at 23:07

score 3 · Accepted Answer · answered Jun 29 '12 at 22:48

3

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.

answered Jun 29 '12 at 22:48

David Rodríguez - dribeas

204,818
23
294
489

thanks for the fast answer, even faster than my edit ! I tried to look in that direction but I still have issues that I don't understand. Would you please have a look at what happened ? thanks a lot for your help – gameboo Jun 29 '12 at 22:58
no you're right ! when trying to test different solutions, as desperate as it may look, I left a "template class C : public B {" when i made my edit ! Thx for pointing this out ! I can sleep now ;) – gameboo Jun 30 '12 at 12:38

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

1 Answers1