Compiling problem

[Barrie]

I'm having a quite complicate situation, and it doesn't want to
compile.
The situation is as follows:
I have three classes, let's call them A, B and C. They each have a .h
and a .cpp file.
All headers use the following construct to make sure the headers are
not included multiple times:
#ifndef _A_H_
#define _A_H_
class A {
...
};
#endif
A has a static member of class B,
B has a member of class C and works with objects of class A in some
methods,
and C has a member of class A.

When i try to compile this, i get errors saying that there is no class
A when C is compiling, and so on. How could i fix this?

[Alf P. Steinbach]

* Barrie:

I'm having a quite complicate situation, and it doesn't want to
compile.
The situation is as follows:
I have three classes, let's call them A, B and C. They each have a .h
and a .cpp file.
All headers use the following construct to make sure the headers are
not included multiple times:
#ifndef _A_H_
Names starting with an underscore followed by uppercase are reserved for
the implementation; you have Undefined Behavior already.
#define _A_H_
class A {
...
};
#endif
A has a static member of class B,
B has a member of class C and works with objects of class A in some
methods,
and C has a member of class A.

When i try to compile this, i get errors saying that there is no class
A when C is compiling, and so on. How could i fix this?

The best way: fix your design, which is hopeless, + read up on C++ as a
language, where you need to learn such basic things as naming rules (see
above).

Or technically, which is the worst you can do, include the proper
headers at the right places and use forward declarations to make your
current bad/evil design compile; see the FAQ for example(s). If you
choose to do this, then there is a good chance that after it compiles,
and after fixing the name issue mentioned above, you'll still have UB
due to static initialization order problems. And there is an even
better chance that you won't notice until something else blows up.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Bob Hairgrove]

On 18 Mar 2006 02:58:29 -0800, "Barrie" <ba********@gmail.com> wrote:

I'm having a quite complicate situation, and it doesn't want to
compile.
The situation is as follows:
I have three classes, let's call them A, B and C. They each have a .h
and a .cpp file.
All headers use the following construct to make sure the headers are
not included multiple times:
#ifndef _A_H_
#define _A_H_
class A {
...
};
#endif
A has a static member of class B,
B has a member of class C and works with objects of class A in some
methods,
and C has a member of class A.

When i try to compile this, i get errors saying that there is no class
A when C is compiling, and so on. How could i fix this?
You will always have circular dependency issues if you have these
objects as members. As Alf P. Steinbach has pointed out, a design like
this will never work.

When a class declaration is compiled, the compiler must know how large
the object will be by the time the semicolon after the closing brace
is reached. That means that for all class members, the size of each
member must be known. If you have a pointer or reference member, the
compiler can accept it without including that type's header because
the size of a pointer is known without knowing the type (except for
pointer-to-member under certain circumstances). All one must do is to
write a forward-declaration of the class' name, e.g.:

class A;

class B {
// B can use the name, but not the type:
A* pa;
};

For member variables of class type, each header will have to include
the headers for those members. IOW,
A has a static member of class B, ....means that header for A must include header for B;
B has a member of class C and works with objects of class A in some
methods, ....means that B's header must include C's header, but not necessarily
A's header because it isn't needed until the definitions of the
methods (which you should put in a separate .cpp file);
and C has a member of class A.

....means that C's header must include A's header. But wait a minute
.... A needs B, which needs C, which needs A ... you see where this is
going? It is a circular dependency.

One possible solution, if you cannot avoid the dependencies, is to use
pointer or reference members instead of objects, and forward-declare
the class instead of including the class header. You will eventually
need to include the headers, but you can include them in the .cpp
file.

--
Bob Hairgrove
No**********@Home.com