Qt is great for building applications, but once or twice I have wanted to show a nice Qt dialog from an existing application that is written in pure C.
I want to describe how to create a Qt-based shared or static library that is usable by a Qt-unaware program. To narrow down the problem, let's assume that we call a function that shows some dialogs or widgets, and does not return until all dialogs have been closed. Otherwise it can't be done without some 'cooperation' from the hosting process - it has to at least run the message pump for Qt.
In this example I will use the Windows program rundll32.exe to load and execute a function that shows the Qt about dialog.
Let's say our DLL is called MyQtBasedDll.dll, and it has a single exported function - void MyEntryPoint().
We will run it using the command line rundll32.exe MyQtBasedDll.dll,MyEntryPoint. Note you need to give full paths for the exe and dll.
In order for rundll32.exe to load and use the program, MyEntryPoint() should have a C-calling convention. Additionally, we need a QApplication instance to do any GUI stuff, and rundll32.exe will obviously not create it for us.
To create the Qt library with Qt Creator, we can choose File->New File or Project->Libraries->C++ Library, and the project name should be MyQtBasedDll.
The code of the MyEntryPoint() function is pretty simple:
extern "C"
{
__declspec(dllexport) void MyEntryPoint()
{
if (!QApplication::instance()) {
QApplication a(__argc, __argv);
QMessageBox::aboutQt(0);
} else {
QMessageBox::aboutQt(0);
}
}
}
This function can be called multiple times by the same exe, and each time a new QApplication object is constructed, only for the duration of the function. Note that we check for an existing instance so our DLL will work even when called by a Qt application.
Now all you need to do is substitute QMessageBox::aboutQt(0); with your function that does useful stuff!
Collected from the Internet
Please contact [email protected] to delete if infringement.
Comments