[SOLVED] C++ interdependant classes

MTK358 · 05-28-2011, 02:47 PM

I have two C++ classes that depend on each other. Here's a simple example:

Code:

class A
{
public:
	B foo() {}
};

class B
{
public:
	A bar() {}
};

int main()
{
	return 0;
}

It fails to compile:

Code:

$ g++ test.cpp 
test.cpp:4:2: error: ‘B’ does not name a type

Is it possible to make this work?

paulsm4 · 05-28-2011, 02:49 PM

Sure:

Code:

class B;

class A
{
public:
	B foo() {}
};

class B
{
public:
	A bar() {}
};

int main()
{
	return 0;
}

johnsfine · 05-28-2011, 03:43 PM

I guess there could be cases simple enough that works (just predeclaring B).

Quote:

Originally Posted by paulsm4

Code:

class B;

class A
{
public:
    B foo() {}
};

class B
{
public:
    A bar() {}
};

int main()
{
    return 0;
}

But in most real world examples, you need to go a step further:

Code:

class B;

class A
{
public:
    B foo();
};

class B
{
public:
    A bar() {}
};

B A::foo() {}

int main()
{
    return 0;
}

In addition to predeclaring B before defining A, I also only declared (not defined) foo inside the definition of A. Then I defined A::foo after defining B.

(In all these examples, I am assuming the {} stands for actual code rather than literally {})

You (MTK358) need to understand the relationship between declaring and defining:

The code in the first post declares and defines A together as one step, similarly for foo, B and bar. But each of those could have declare split from define if you choose.

You cannot avoid declaring A::foo() within defining A, and you cannot avoid declaring B::bar() within defining B.

The way the code looks, it is very clear:
1) Declaring B must precede declaring foo.
2) Declaring A must precede declaring bar.
It seems pretty clear (despite paulsm4's example):
3) Defining B must precede defining foo.
4) Defining A must precede defining bar.

My example sorts all that out by the following sequence:
Declaring B
Defining A including declaring foo
Defining B including declaring and defining bar
Defining foo.

It may be better style (though not needed in this example) to also split defining bar from declaring bar.

paulsm4 · 05-28-2011, 10:40 PM

Hi, Mtk358 -

Glad you're squared away. Let me elaborate a bit on what johnsfine was trying to say:

1. You should always strive to separate your interface from your implementation (one of the few things I dislike about Java and C# is they tend to make it harder to do this).

2. If you did this, your interface would go in a .h header, your implementation in one or more .cpp modules ... and the syntax I gave you should work perfectly.

3. Otherwise, you might have problems trying to access any members of the class you forward referenced.

Here's an alternate example:

Code:

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H

class B;

class A
{
public:
  A ();
  void someMethod ();
  void someOtherMethod ();
...
  B foo;
};

class B
{
public:
  B ();
  ...
  A bar;
};
#endif
/* MYHEADER_H */

Code:

// mymodule.cpp
#include "myheader.h"

A::A ()
  : foo()
{
  ...
    B foo() {}
};

B::B ()
  : bar()
{
public:
    A bar() {}
};

int main()
{
  ...
}

MTK358 · 05-29-2011, 08:29 AM

I do separate the interface and implementation. I didn't in the example becasue it's simpler and still causes the same error.

Sergei Steshenko · 05-29-2011, 01:06 PM

http://stackoverflow.com/questions/3...ursive-classes - pay attention to pointers to classes.

paulsm4 · 05-29-2011, 01:28 PM

MTK358 -

Quote:

I do separate the interface and implementation.

Sorry - I didn't mean to imply that you wouldn't ordinarily do that. I just wanted to make sure you were crystal-clear with all the responses to your original question (including mine).

Again - glad you're squared away

Sincerely .. PSM