Problem including a.h in b.h when b.h is included in a.h

[lilactiger89]

Hi there,

I'm trying to construct two classes that need each other to work but am getting errors when compiling. I need to include settings.h in blocks.h and also to include blocks.h in settings.h
If I don't include blocks.h in settings.h (and remove all objects of type Blocks that are needed) then it compiles perfectly. Same vice versa

I'm using header guards etc, I believe that the problem is because when compiling settings.h, blocks.h needs to be compiled and that needs settings.h to be compiled.

blocks.h:30: error: ‘Settings’ does not name a type

Please help! Is there another way to use a file without including it?

[Thenero]

Well you're right about why the problem occurs.

http://www.cplusplus.com/forum/articles/10627/
It a long read. But I found it very informative.

[lilactiger89]

Brilliant that was exactly what I was looking for!
Thanks :-)

[lilactiger89]

Alas my program is still failing to work but I now have new errors.
I'm forward declaring the classes in each other like so:
class Settings;
In another piece of code (main.cpp) I am including both settings.h and blocks.h as I create objects of these types in it.

My error is now:
settings.h:138: error: invalid use of incomplete type ‘struct Blocks’
settings.h:6: error: forward declaration of ‘struct Blocks’
This relates to some code in settings.h which attempts to call a function of Blocks

If I switch over the order in which I include the files in main.cpp then my error becomes:
blocks.h:221: error: invalid use of incomplete type ‘struct Settings’
blocks.h:6: error: forward declaration of ‘struct Settings’
This relates to code which calls a function of Settings

What am I doing wrong???

[Disch]

You're trying to inline functions which use interdependencies in the h file. For example you're doing this:

class Blocks;

class Settings
{
  void AFunc(Blocks b)
  {
    b.foo = 1;  // ERROR
    // you can't inline this function because Blocks was not fully defined.
  }
};

The solution is one of the following:

1) Don't inline these functions. Put their implementation in a cpp file instead.

or

2) Inline them after the class is fully defined. See section 7 of previously linked article:

http://www.cplusplus.com/forum/articles/10627/#msg49682

[lilactiger89]

Thanks, it's working now. I'd inlined them after the class was defined but not realised that I needed to include the header file after that too!

class Blocks;

class Settings
{
  void AFunc(Blocks b)
};

#include "blocks.h"

void Settings::AFunc(Blocks b)
{
  b.foo = 1;  
}

[Disch]

don't forget the inline keyword or else you'll get linker errors when including that in multiple cpp files.