why i am getting this error: use of undeclared identifier 'MainWindow' ?

[Qt embedded developer]

@JonB i have not manually created any file. this is by default file added by qt

for your reference my mainwindow.h file contain this class defination

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

[JonB]

@Qt-embedded-developer
That looks as I would expect. So far I cannot see why you would get the error you reported. Assume there were no other errors reported.

What about the generated ui_mainwindow.h file from the .ui file, which is in the compilation output (usually Build-...) directory? Force a clean build; you could just delete all the files in that output (not your sources!) directory. Though that should not be the issue....

[Christian Ehrlicher]

There is another mainwindow.h somewhere around which the compiler is using - seeing the other posts about the include path mess this is at least a very good option here...

[JonB]

@Christian-Ehrlicher Ooohhhh! :)

[Qt embedded developer]

@JonB i have deleted the all the files from output directory.

my compile output is:

10:55:52: Running steps for project part1...
10:55:52: Starting: "/usr/bin/x86_64-linux-gnu-qmake" /home/mangal/JAYMOGAL/MYFIRST/part1/part1.pro -spec linux-g++-64 CONFIG+=debug CONFIG+=qml_debug
10:55:52: The process "/usr/bin/x86_64-linux-gnu-qmake" exited normally.
10:55:52: Starting: "/usr/bin/make" -f /home/mangal/JAYMOGAL/MYFIRST/build-part1-Qt_5_12_8_System-Debug/Makefile qmake_all
make: Nothing to be done for 'qmake_all'.
10:55:52: The process "/usr/bin/make" exited normally.
10:55:52: Starting: "/usr/bin/make" -j2
/usr/lib/qt5/bin/uic ../part1/mainwindow.ui -o ui_mainwindow.h
x86_64-linux-gnu-g++ -c -m64 -pipe -g -std=gnu++11 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../part1 -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtWidgets -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 -o main.o ../part1/main.cpp
x86_64-linux-gnu-g++ -m64 -pipe -g -std=gnu++11 -Wall -W -dM -E -o moc_predefs.h /usr/lib/x86_64-linux-gnu/qt5/mkspecs/features/data/dummy.cpp
x86_64-linux-gnu-g++ -c -m64 -pipe -g -std=gnu++11 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../part1 -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtWidgets -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 -o mainwindow.o ../part1/mainwindow.cpp
/usr/lib/qt5/bin/moc -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB --include /home/mangal/JAYMOGAL/MYFIRST/build-part1-Qt_5_12_8_System-Debug/moc_predefs.h -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 -I/home/mangal/JAYMOGAL/MYFIRST/part1 -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/include/c++/9 -I/usr/include/x86_64-linux-gnu/c++/9 -I/usr/include/c++/9/backward -I/usr/lib/gcc/x86_64-linux-gnu/9/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include ../part1/mainwindow.h -o moc_mainwindow.cpp
x86_64-linux-gnu-g++ -c -m64 -pipe -g -std=gnu++11 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../part1 -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtWidgets -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 -o moc_mainwindow.o moc_mainwindow.cpp
x86_64-linux-gnu-g++ -m64 -o part1 main.o mainwindow.o moc_mainwindow.o   -L/usr/X11R6/lib64 /usr/lib/x86_64-linux-gnu/libQt5Widgets.so /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread   
10:55:59: The process "/usr/bin/make" exited normally.
10:55:59: Elapsed time: 00:07.

but still i get this warning in issue window:
mainwindow.h:10: error: expected class name

[JonB]

@Qt-embedded-developer said in why i am getting this error: use of undeclared identifier 'MainWindow' ?:

but still i get this warning in issue window:
mainwindow.h:10: error: expected class name

I don't know what the "issue" window is, maybe this is code model warning not from compiler?

[Qt embedded developer]

@JonB what to do to stop code model warning in qt creator ?

[jsulm]

@Qt-embedded-developer said in why i am getting this error: use of undeclared identifier 'MainWindow' ?:

what to do to stop code model warning in qt creator ?

Disable the Clang plug-in (in Help/About Plugins...)

[Qt embedded developer]

@jsulm when i am disable it my building of project process not get stop. it continuously running.

[jsulm]

@Qt-embedded-developer Clang plug-in has actually nothing to do with the build.
Try complete rebuild: delete build folder, run qmake and then build.