problem with template inheritance

Question

I'm trying to understand whay i get an error on this code: (the error is under g++ unix compiler. VS is compiling OK)

template<class T> class A {
public:
    T t;
public:
    A(const T& t1) : t(t1) {}
    virtual void Print() const { cout<<*this<<endl;}
    friend ostream& operator<<(ostream& out, const A<T>& a) {
            out<<"I'm "<<typeid(a).name()<<endl;
            out<<"I hold "<<typeid(a.t).name()<<endl;
            out<<"The inner value is: "<<a.t<<endl;
            return out;
    }
};

template<class T> class B : public A<T> {
public:
    B(const T& t1) : A<T>(t1) {}
    const T& get() const { return t; }
};

int main() {
    A<int> a(9);
    a.Print();
    B<A<int> > b(a); 
    b.Print();
    (b.get()).Print();  
    return 0;
}

This code is giving the following error:

main.cpp: In member function 'const T& B::get() const':
main.cpp:23: error: 't' was not declared in this scope

It did compiled when i changed the code of B to this:

template<class T> class B : public A<T> {
public:
    B(const T& t1) : A<T>(t1) {}
    const T& get() const { return A<T>::t; }
};

I just cant understand what is the problem with the first code...
It doesn't make sense that i really need to write "A::" every time...

James McNellis · Accepted Answer · 2010-06-06 00:42:54Z

7

You can also use this->t to access the base class template member.

In B::get(), the name t is not dependent on the template parameter T, so it is not a dependent name. Base class A<T> is obviously dependent on the template parameter T and is thus a dependent base class. Nondependent names are not looked up in dependent base classes. A detailed description of why this is the case can be found in the C++ FAQ Lite.

edited Jun 6, 2010 at 0:42

answered Jun 6, 2010 at 0:37

James McNellis

357k76 gold badges934 silver badges983 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Johannes Schaub - litb Over a year ago

The FAQ is wrong on the reason why "this->" works. Compare it with the Standard (emphasize by me). FAQ: "Since this is always implicitly dependent in a template, this->f is dependent and the lookup is therefore deferred until the template is actually instantiated, at which point all base classes are considered ", Standard: "... the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.". The FAQ says it's found because it's dependent, but that's wrong.

Johannes Schaub - litb Over a year ago

GCC also implements this wrong interpretation, and as a result has bug reports open

Collectives™ on Stack Overflow

problem with template inheritance

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related