Re: Cannot access type in base class

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Thu, 04 Mar 2010 21:25:07 +0100

Message-ID:

<hmp4v8$h3o$1@news.eternal-september.org>

* none:

I have the following base class:

================== Base.h ====================================

#ifndef BASE_H
#define BASE_H

#include <vector>
#include "MyType.h"

class Base {
public:

  // Types
  // We use pointers for polymorphic support.
  typedef std::vector<MyType*> ContainerType;

  // Pure virtual
  virtual ContainerType createJobs() = 0;

protected:
  ContainerType jobs;

};

#endif // BASE_H

================== Sub.h ====================================

#ifndef SUB_H
#define SUB_H

#include "Base.h"

class Sub : public Base {
public:
  // Types
  //typedef typename Base::ContainerType ContainerType;

  // Type from base class
  ContainerType createJobs();

};

#endif // SUB_H

================== Sub.cpp ====================================
#include "Sub.h"

  ContainerType Sub::createJobs() {

Make that

Base::ContainerType Sub::createJobs() {

or

Sub::ContainerType Sub::createJobs() {

     // Create some jobs and add to parent container
    return this->jobs;

  }

================== End ====================================

The above does not compile:

    Sub.cpp:12: error: ?ContainerType? does not name a type

I have tried to typedef the type in the Sub.h:

  typedef typename Base::ContainerType ContainerType;

but that does not help. Why is it not possible to use a type defined in
the parent class?

It's possible, but there's a gotcha for function result specifications: at that
point the compiler doesn't yet (by the rules of the language) know that it's
dealing with a member function definition.

However, when it gets to the argument list it knows.

I've raised the issue a few times e.g. in this group, that it would be more
consistent if result types were treated like argument types, but changing the
language could possibly break a lot of existing code, namely where the result
type name is coincidentally also defined in the relevant class.

Cheers & hth.,

- Alf