Important: Please read the Qt Code of Conduct -

Include header issue

  • Hi! I have two windows, main window and settings window (second).

    For example:

    On main window I have a button that will open settings window. On settings window I get some data from registry and need to set this data to main window. The problem is that I can't use connect do to this because I get error: C2143: syntax error: missing ';' before '*'.

    This error tells that I use circle includes.

    In main window header:
    #include "appsettings.h"

    In Settings window header:
    #include "mainwindow.h

    Without includes I can't create the objects from the classes. How to fix this issue? Thanks.

  • Hi @Cobra91151

    Can you share your code?

  • Most probably a forward declaration can be used in (at least) one of your header file. Then in the implementation file you can have your headers.

    AFAIK, C2143: syntax error: missing ';' before '*' doesn't tell you anything about circular dependencies.

  • @JohanSolo

    This error only appears when include headers, so I think the problem is with circle includes. What do you mean by forward declaration? Can you show an example?

  • This is basic C / C++ usage, see for instance forward declaration on wiklipedia or on stackoverflow.

  • @mostefa

    I can't share the code but I can create an example and upload it here.

  • Test project is available here - Mega. Thanks.

  • @JohanSolo

    I know about forward declaration but it's not working in my case.

  • Hi Cobra91151,

    if you forward declarate TestWindow in appsettings.h like @JohanSolo suggested

    #ifndef APPSETTINGS_H
    #define APPSETTINGS_H
    #include <QWidget>
    class TestWindow;
    namespace Ui {
    class AppSettings;
    class AppSettings : public QWidget
        explicit AppSettings(QWidget *parent = 0);
        Ui::AppSettings *ui;
        TestWindow *mainWindow;
    #endif // APPSETTINGS_H

    and include testwindow.h in apsettings.cpp your project can be compiled

    #include "appsettings.h"
    #include "ui_appsettings.h"
    #include "testwindow.h"
    AppSettings::AppSettings(QWidget *parent) :
        ui(new Ui::AppSettings)
        this->setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowMaximizeButtonHint | Qt::WindowCloseButtonHint);
        mainWindow = new TestWindow();
        delete ui;

    But a short look at your code showed, that your are creating a AppSettings object in TestWindow constructor.
    In AppSettings construtor you create a TestWindowobject, where you create a AppSettingsobject, where you
    create a TestWindow object, where you create a AppSettingsobject, ...

    I doubt this was your intention.

  • @sneubert

    Yes, it compiles but not working. I just want for example, to set the main window title from settings window (or other communications between two windows). So how to do it?

  • Hi @Cobra91151

    In your case take one more class(mainClass) that contains both headerfile and all connection between these two window should happens throgh this class only.

    whenever window1 sends any signal catch it in your mainClass and connect that to window2 and vice versa. for this, your both window objects shoul create in mainClass.
    this will give solution for your question

  • @Cobra91151

    If it solved then make it solved.

  • @Venkatesh-V

    I have added main class but the issue is still present.

    Test project is available here - Mega.

  • @Cobra91151

    still you made mistake,

    you have created communicationWorker object in AppSetting and in communicationWorker you created testWindow object and then in testwindow you created appsetting object, so it again became cycle.

    what i suggest you is,
    in communicationWorker create both appsetting and testwindow objects,

    in communicationWorker made connections between appsetting and testwindow

    whenever any signal emited in any of these two class catch those on communicationworker class and pass to required class.

    what you made in above example that is wrong.

  • @Venkatesh-V

    OK. Then in what file I should include communicationWorker.h? Thanks.

  • I don't think you have clear the difference between a class and an instance of the class.

    Personally I think that guerilla programming is evil so:

  • @VRonin

    class AppSettings is a class
    AppSettings *testSettings = new AppSettings() - is the instance of the class (heap);
    AppSettings testSettings; - is the instance of the class (stack).

    But to get the instance of a class you should include the appropriate header file.
    So I'm not right?

  • @Venkatesh-V

    I have changed code to your suggestion but it's not emitting a signal.

    Test project - Mega.

    Can you show an example? Thanks.

  • @Cobra91151
    you need to include the header file only when you instantiate an object.

    in your header file, you only declare pointers, so forward declaration is enough. you only need to include header file in cpp file

    and in your code, your main function just have a TestWindow, there is neither CommunicationWorker nor AppSettings, so the connection between TestWindow and AppSettings do not exist.

    I think @Venkatesh-V's suggestion is creating a CommunicationWorker as the top object, so you only need to create CommunicationWorker in the main function.

    but you can just declare AppSettings as TestWindow's member, and connect them in the constructor of TestWindow

  • @Flotisable

    I changed code to yours suggestions but windows are not communicate.

    My codes and test project are available - Mega.

    Can someone post an example or fix my test project? Thanks in advance.

  • I have fixed it by adding a button as a trigger, so now everything works.

Log in to reply