使用C程序中的Qt库

Question

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.

Answer 1

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!