Qt project files: win32 or win 64 - windows

Question

I am confused about this:
My Qt creator is 32bit while my windows is 64 bit. In .pro file, should I use win 32{...} or win 64 {...}? If I want to link OpenCV libraries, in which one should I use, x64 or x86?
It would be better if you could give me a little explanation about this.
Thanks!

Answer 1

Qmake has some platform and compiler variables like: win32, unix, win32-msvc, ... which are available on mkspecs directory in the installed Qt directory. You can have conditional .pro file based on platform and compiler. For example you can have project which is able to compile both on Windows and Linux. You can linked to an external library conditionally :
win32: LIBS += -L$$PWD/Windows/Path/To/Library/ -lTheLibrary
else:unix: LIBS += -L$$PWD/Linux/Path/To/Library/ -lTheLibrary
If you want to detect whether you are compiling for 32-bit or 64-bit compilers on Windows and linked to relevant libraries accordingly you can use :
win32:contains(QMAKE_HOST.arch, x86_64) {
LIBS += -L$$PWD/Path/To/Library_64Bit/ -lTheLibrary
} else {
LIBS += -L$$PWD/Path/To/Library_32Bit/ -lTheLibrary
}

Answer 2

There are lot of posts that covered these differences, but the important part to remember is that you have to match the architecture between the compiler and the 3rd party libraries that you want to use.
For instance, I have a computer with Windows 64-bit where I use Qt Creator 3.2.1 (opensource) based on Qt 5.3.1 (MSVC 2010, 32 bit). Since my compiler is 32 bit, it's important that I use the 32 bit version of the OpenCV libraries.
So in the .pro file I'll use:
LIBS += -L"C:\\opencv\\build\\x86\\vc12\\lib" \
-lopencv_world300d