Inheritance with templates?

[saneman]

In the below non-template example a subclass BOB returns a protected
field from a super class POP:


class POP {
protected:
int blop;
};



class BOB : public POP {
public:
int size () {return blop;}
};


If I declared them as templates I get an error:

template <class A>
class POP {
protected:
int blop;
};

template <typename A, typename B = POP<A> >
class BOB : public B {
public:
int size () {return blop;}
};


error: ‘blop’ was not declared in this scope

It only works if it change the size() function to:

int size () {return B::blop;}

But I can also change it to:

int size () {return A::blop;}

even though I don't make sense, but the compiler does not complain.

When is it necessary to qualify inherited fields when using templates?

 

[saneman]

..... I meant WHY its necessary to qualify inherited fields?

 

[ltcmelo]

In the below non-template example a subclass BOB returns a protected
field from a super class POP:

class POP {
protected:
int blop;

};

class BOB : public POP {
public:
int size () {return blop;}

};

If I declared them as templates I get an error:

template <class A>
class POP {
protected:
int blop;

};

template <typename A, typename B = POP<A> >
class BOB : public B {
public:
int size () {return blop;}

};

error: 'blop' was not declared in this scope

It only works if it change the size() function to:

int size () {return B::blop;}

But I can also change it to:

int size () {return A::blop;}

even though I don't make sense, but the compiler does not complain.

When is it necessary to qualify inherited fields when using templates?

Basically because C++ rules say that non-dependent names should not be
looked up in dependent base classes (which is your case). You could
also use "this->blop" or "using B::blop" to avoid the compilation
error. Please, take a look at this faq: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.19