Re: Templates and inheritance

From:

Hendrik Schober <spamtrap@gmx.de>

Newsgroups:

comp.lang.c++

Date:

Sun, 09 Nov 2008 01:21:39 +0100

Message-ID:

<gf5afc$hdc$1@hoshi.visyn.net>

Jim West wrote:

Can someone please explain to me why the following compiles:

class A {
public:
    int d;
};

class B : public A {
public:
    int e;
    void f() { d = e; };
};

but if I make A and B templates it gives an error:

template <typename T>
class A {
public:
    T d;
};

template <typename T>
class B : public A<T> {
public:
    int e;
    void f() { d = e; }; // error: identifier "d" is undefined
};

  When the compiler encounters 'B' for the first time, it hasn't
  been instantiated, so it doesn't know what 'A' looks like.
  (Remember that 'A' could be specialized for some specific 'T'
  up to the point where 'B' is instantiated for the first time.)
  If it doesn't know 'A', it doesn't know 'A<T>::d'. If you spell
  'A<A>::d' out explicitly, it becomes a so called dependent type
  (which means it depends on some template parameter which isn't
  known when the template is parsed for the first time) and its
  lookup is deferred until 'B' is actually instantiated.

This occurs with GNU G++ and Intel ICC. Obviously I am missing a subtlety
of templates and/or inheritance. (I figured out that changing the line in
question to

void f() { A<T>::d = e; };

corrects the error, but I want understand why it is an error in the first
place.)

HTH,

Schobi