Re: Partial classes

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Sun, 11 Mar 2007 11:03:30 -0400
Message-ID:
<jcudnQVGdbShh2nYnZ2dnUVZ_t6qnZ2d@comcast.com>
Adrian Hawryluk wrote:

Can one define a partial class (first part containing constants like
enums) and then define the rest of the class elsewhere?


No.

I ask this because I've had in the past needed to defined two classes
(call them A and B), each dependent upon the other. Class A requires
the enum type defined in Class B. An example follows:

<header file="A.h">
  #if !defined A_H
  # define A_H

  // Stub for forward referencing
  class A;

  # include ?B.h?

  class A
  {
    // Interface functions
    void f(B* b, B::b_e enums);
  ...
  };
  #endif
</header>

<header file="B.h">
  #if !defined B_H
  # define B_H

  // Stub for forward referencing
  class B;

  # include ?A.h?
  class B
  {
  public:
    enum b_e { enum1, enum2 };

    // Interface functions
  ...
  };
  #endif
</header>

The only way I see around this are:
1. Declare a public base class for B that contains the enums. It is
   not dependent on A so no problem occurs. It would be declared
   with the class B stub;


That's the best solution, IMO.

2. Declare the enums outside of class scope, removing the dependency
   lock. It would be declared with the class B stub;
3. Make a faux template class, making the class a stub. Haven't tried
   this.


No sure what you mean here.

Problems for each are:
1. Base classes would have a explicit size of 1, adding a small amount
   of overhead.


Not to the class inheriting from it.

class Base {};
class Derived : public Base { int a; };
#include <iostream>
int main() {
    std::cout << "sizeof(Base) = " << sizeof(Base) << std::endl;
    std::cout << "sizeof(Base in Derived) = "
              << sizeof(Derived) - sizeof(int) << std::endl;
}

2. Enum is not scoped to a class.
3. A bit overkill if it does work.

Any thoughts?


Base class!

V
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