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 // Single instance
 int main(int argc, char **argv)
 {
     QtSingleApplication app(argc, argv);

     if (app.isRunning())
         return 0;

     MyMainWidget mmw;

     app.setActivationWindow(&mmw);

     mmw.show();
     return app.exec();
 }

 // Single instance
 int main(int argc, char **argv)
 {
     QtSingleApplication app(argc, argv);
     QMainWindow mw;

     if (app.isRunning()) 
     {
         mw.setDestPath(argv[1]); // обновили путь
         return 0; // вышли
     }

     app.setActivationWindow(&mw);

     mw.show();
     return app.exec();
 }

qtsingleapplication.h:47: ошибка: QtGui/QApplication: No such file or directory
qtlocalpeer.h:48: ошибка: QtNetwork/QLocalServer: No such file or directory
qtlockedfile.h:50: ошибка: QtCore/QFile: No such file or directory

#-------------------------------------------------
#
# Project created by QtCreator 2012-11-26T19:54:12
#
#-------------------------------------------------

QT       += core gui network

TARGET = untitled
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    qtsingleapplication/qtsinglecoreapplication.cpp \
    qtsingleapplication/qtsingleapplication.cpp \
    qtsingleapplication/qtlockedfile_win.cpp \
    qtsingleapplication/qtlockedfile_unix.cpp \
    qtsingleapplication/qtlockedfile.cpp \
    qtsingleapplication/qtlocalpeer.cpp

HEADERS  += mainwindow.h \
    qtsingleapplication/qtsinglecoreapplication.h \
    qtsingleapplication/qtsingleapplication.h \
    qtsingleapplication/QtSingleApplication \
    qtsingleapplication/qtlockedfile.h \
    qtsingleapplication/QtLockedFile \
    qtsingleapplication/qtlocalpeer.h

FORMS    += mainwindow.ui

qtsingleapplication.h:82: ошибка: 'QString' does not name a type
qtsingleapplication.h:84: ошибка: 'QWidget' has not been declared
qtsingleapplication.h:85: ошибка: expected ';' before '*' token
qtsingleapplication.h:89: ошибка: 'Q_UNUSED' was not declared in this scope

qtlockedfile.h:77: ошибка: expected ',' or '...' before '&' token
qtlocalpeer.h:58: ошибка: ISO C++ forbids declaration of 'Q_OBJECT' with no type

qtsingleapplication.cpp:145: ошибка: 'appId' was not declared in this scope
qtsingleapplication.cpp:173: ошибка: ISO C++ forbids declaration of 'QString' with no type