Thread: wtf...? deriving from template

Code:

template <class T> class B {
protected:
  T *p;
  // even if replacing T with int doesnt help
};

template <class T> class D : public B<T> {
public:
  D()
  {
    p = NULL; // error ?!?!?!?!?!
  }
};

why does it say p is undefined?

(using gcc)
is it a syntatic problem?

Works for me using MS.Net 2003.

yes, that code works for me too on ms-vc6.0
but it fails to compile with g++ >_<
error:
test.cpp:18: error: `p' undeclared (first use this function)
test.cpp:18: error: (Each undeclared identifier is reported only once for each function it appears in.)

[edit]
so my first guess would be that its a syntatic problem and the ms compilers allow that syntax while the c++ standard does not... i might be wrong though

[edit2]
the funny thing is that when replacing B<T> with e.g. B<int> in line
template <class T> class D : public B<T>
it compiles...

oh well...

i found this "fix"
but would still be nice if anyone could tell me what the problem is,
btw: gcc - v yields
gcc version 3.4.3 20050227 (Red Hat 3.4.3-22.fc3)

Code:

template <class T>
class D : public B<T> {
public:
  using B<T>::p; // !!!

  D()
  {
    p = NULL;
  }
};

methods yield the same error.

so its either a bug, or in some documents it states that it has to be done that way... strange

well i could try a newer version of gcc (even though there are only 2-3 newer versions)

>why does it say p is undefined?
Because it is undefined. p doesn't depend on the template parameter T for class D, but class B does. The standard says that dependent base classes aren't searched when looking up non-dependent names. The solution is either to qualify p with B<T>:

Code:

template <class T> class B {
protected:
  T *p;
  // even if replacing T with int doesnt help
};

template <class T> class D : public B<T> {
public:
  D()
  {
    B<T>::p = NULL; // error ?!?!?!?!?!
  }
};

Or this:

Code:

template <class T> class B {
protected:
  T *p;
  // even if replacing T with int doesnt help
};

template <class T> class D : public B<T> {
public:
  D()
  {
    this->p = NULL; // error ?!?!?!?!?!
  }
};

Assuming you don't include a header that defines NULL, you also have an error there.

hehe, so
adding
using B<T>::p (posted earlier in the thread)
to my D class was just the right thing to do

thxalot! well my other guess was also right: there is some standard which states that this shouldnt compile
and it also shows that m$ doesnt follow it...

>there is some standard which states that this shouldnt compile
Yes

>and it also shows that m$ doesnt follow it...
Despite your apparent attempt to bash Microsoft, you won't find a compiler that completely conforms to the C++ standard, but Microsoft's is one of the few that come close.

now that i have fedora core 3 running, let me have the fun bashing ms
and ms needs to bashed for many reasons (like threatening asian governments who use linux)
and i don't wanna know what happens at the vote this wednesday when they decide about software patents in europe >_<

edit:
oops, this wednesday is tomorrow, i meant next wednesday

that code to a programmer isn't ambiguous, but if the standard says so, then we'll have to carry the book around whenever we want to use more complex syntax...

it's funny to see that the website for c/c++ standard, doesn't use standard html

What does the C++ standard have to do with the HTML standard?

Just remember that it's not a good idea to initialize base members in the derived constructor.