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  4. Using macros cannot resolve recursive references either.
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Using macros cannot resolve recursive references either.

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  • K 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?

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by
    #3

    @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!

    https://forum.qt.io/topic/113070/qt-code-of-conduct

    K 1 Reply Last reply
    3
    • K 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?

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by JonB
      #4

      @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... ?

      1 Reply Last reply
      1
      • Pl45m4P Pl45m4

        @kuqipair

        What macros exactly are you talking about?

        K Offline
        K Offline
        kuqipair
        wrote on last edited by
        #5

        @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.

        Pl45m4P 1 Reply Last reply
        0
        • K 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.

          Pl45m4P Online
          Pl45m4P Online
          Pl45m4
          wrote on last edited by
          #6

          @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!

          If debugging is the process of removing software bugs, then programming must be the process of putting them in.

          ~E. W. Dijkstra

          1 Reply Last reply
          0
          • jsulmJ 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!

            K Offline
            K Offline
            kuqipair
            wrote on last edited by
            #7

            @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.

            JonBJ 1 Reply Last reply
            0
            • K 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.

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on last edited by
              #8

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

              K 1 Reply Last reply
              0
              • JonBJ JonB

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

                K Offline
                K Offline
                kuqipair
                wrote on last edited by
                #9

                @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!

                JonBJ 1 Reply Last reply
                0
                • K 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!

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #10

                  @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.

                  1 Reply Last reply
                  0
                  • Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by
                    #11

                    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.

                    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                    Visit the Qt Academy at https://academy.qt.io/catalog

                    1 Reply Last reply
                    1
                    • S Offline
                      S Offline
                      SimonSchroeder
                      wrote on last edited by
                      #12

                      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.

                      1 Reply Last reply
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