Using macros cannot resolve recursive references either.

[kuqipair]

I have three files: classA.h, classB.h, and all_header.h. The details are as follows:

//classa.h
#ifndef CLASSA_H
#define CLASSA_H

#include"all_header.h"

class ClassA
{
public:
    ClassA();
    ClassB b1;
};
#endif 

//classb.h
#ifndef CLASSB_H
#define CLASSB_H

#include"all_header.h"

class ClassB
{
public:
    ClassB();
    ClassA* a1;
};
#endif // CLASSB_H

//all_header.h
#ifndef ALL_HEADER_H
#define ALL_HEADER_H

class ClassA;
class ClassB;

#include"classb.h"
#include"classa.h"

#endif // ALL_HEADER_H

During compilation, the following error occurs:
error: C2079: 'ClassA::b1' uses undefined class 'ClassB'

However, when I use the above structure in a standard C++ compiler (whether MSVC or MinGW), it works fine.
So, why does this happen?

[Pl45m4]

@kuqipair

What macros exactly are you talking about?

[jsulm]

@kuqipair said in Using macros cannot resolve recursive references either.:

error: C2079: 'ClassA::b1' uses undefined class 'ClassB'

ClassA::b1 returns ClassB, not ClassB* - this cannot work with forward declaration!

[JonB]

@kuqipair
In addition to @Pl45m4 [I posted this before @jsulm's response]

During compilation, the following error occurs:

However, when I use the above structure in a standard C++ compiler (whether MSVC or MinGW), it works fine.

What does the first line mean? You must have used a "standard C++ compiler" when you got the error, but then you say that in a "standard C++ compiler" it works fine... ?

[kuqipair]

@Pl45m4 In both classA. h and classB. h, directly referencing the all_ceader. h file achieves the effect of forward declaration. However, when I do this on QT, I encounter compilation errors.

[Pl45m4]

@kuqipair said in Using macros cannot resolve recursive references either.:

However, when I do this on QT, I encounter compilation errors.

There is no "Qt"... You use a "standard C++ compiler" when using Qt in your project...
Also check as @jsulm pointed out:

@jsulm said in Using macros cannot resolve recursive references either.:

lassA::b1 returns ClassB, not ClassB* - this cannot work with forward declaration!

[kuqipair]

@jsulm I implemented this in a QT project. I wanted classA.h and classB.h to directly include the all_header.h file to achieve the effect of forward declarations, but this caused compilation failures. However, when I used the same approach in a regular C++ project compiled with both MSVC and MinGW compilers, it worked successfully.

[JonB]

@kuqipair
Then start by comparing the exact compilation lines executed in the Qt and non-Qt case, to see where the difference is.

[kuqipair]

@JonB Ah-ha, I figured it out!
I consulted DeepSeek-R1, and it suggested explicitly defining the precompiled header in the .pro file like this:

PRECOMPILED_HEADER = all_header.h
CONFIG += precompile_header

Big thanks to the amazing open-source AI models from my great motherland (China) - they're truly phenomenal! Highly recommended!

[JonB]

@kuqipair
Had no idea you were using, or wanted to use, precompiled headers.
Still doesn't tell me why you would need to: either it's legal C++ or it isn't, don't know why making it precompiled would alter that, but never mind.

[Christian Ehrlicher]

No need for some obscure AI here - simply fix your project structure to avoid recursive includes as this is a big sign for a design flaw. Also use forwarding of classes instead direct includes to reduce compile times.

[SimonSchroeder]

Headers alone don't do anything. Suppose you have a classb.cpp that just includes classb.h. This header first declares the macro CLASSB_H. Then you go on to include all_header.h from classb.h. This first includes classb.h followed by classa.h. However, when including classb.h from all_header.h (included from classb.h) CLASSB_H is already defined and the inclusion returns nothing. So far, ClassB has not been defined. Then you go on including classa.h which expects a full definition of ClassB.

This problem only occurs when including classb.h (before any of the other two headers). It is not a problem when including classa.h or all_header.h (first). Using all_header.h as a pre-compiled header will include all_header.h before everything else in every translation unit. That's why this solution works. But IMHO this is not a proper solution because source code should always also compile without precompiled headers. Especially in this scenario a change in ClassA or ClassB will trigger a recompile of the precompiled header and thus a recompile of your whole project. This is not a tenable solution for large projects. Precompiled headers should be reserved for third-party libraries that (almost) never change. It is not meant for your own source code. (Precompiled headers are supposed to reduce compilation times and not increase them.)

I am not sure what a good solution to this problem is. Most likely it works by removing the include guards for all_header.h. I believe these are (in this specific context) not actually needed.