undefined reference to own library function - c++

Question

I'm using QT(5)-Creator with 2 projects, that compile both with MinGW my C++ programs under Windows 7. One project generates a static library, and the other is just there to verify that the library is alright.
The error:
When building the library no errors are generated and it creates successfully the static lib MyClassName.a.
In turn, when trying to import the library, I get error messages durring building/compilation:
undefined reference to 'MyClassName::MyClassName()'
undefined reference to 'MyClassName::MyClassFunction()'
The files:
here's the main.cpp of my importer project
#include <QCoreApplication>
#include "MyClassName.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
MyClassName *mainThread = new MyClassName();
mainThread->MyClassFunction();
return a.exec();
}
and it's .pro file
QT += core
QT += widgets
QT -= gui
TARGET = TESTerase
CONFIG += console
CONFIG -= app_bundle
CONFIG +=extserialport
CONFIG +=staticlib
TEMPLATE = app
HEADERS += \
MyClassName.h
SOURCES += main.cpp
unix:!macx: LIBS += -L$$PWD/../MyClassName-Release/ -lMyClassName
INCLUDEPATH += $$PWD
INCLUDEPATH += $$PWD/../MyClassName-Release/release
DEPENDPATH += $$PWD/../MyClassName-Release/release
unix:!macx: PRE_TARGETDEPS += $$PWD/../MyClassName-Release/release/MyClassName.a
btw I've used QT-Creator's include external library function
My guesses:
Since I'm able to [Strg+Click] the functions in the main, I assume the error is thrown by the Linker.
It may also be a manner of importing/creating/using libraries too, since I just started with that topic.
Anyways searching the Internet about it usually just results in the hint, to use the same compiler for both projects - which shouldn't matter in my case (since I'm building both projects with the same Creator and the same settings).
Has anybody an idea?
Thanks for your support
Kind Regards
[Update for Bogdans request]
the new .pro file
#-------------------------------------------------
#
# Project created by QtCreator 2015-04-29T19:46:22
#
#-------------------------------------------------
QT += core
QT += widgets
QT -= gui
TARGET = TESTerase
TEMPLATE = app
CONFIG += console
CONFIG -= app_bundle
CONFIG +=extserialport
HEADERS += \
MyClassName.h
SOURCES += main.cpp
unix:!macx: LIBS += -L$$PWD/../MyClassName-Release/ -lMyClassName
win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../MyClassName-Release/release/ -lMyClassName
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../MyClassName-Release/debug/ -lMyClassName
else:unix:!macx: LIBS += -L$$PWD/../MyClassName-Release/ -lMyClassName
INCLUDEPATH += $$PWD/../MyClassName-Release/release
DEPENDPATH += $$PWD/../MyClassName-Release/release
win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../MyClassName-Release/release/MyClassName.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../MyClassName-Release/debug/MyClassName.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../MyClassName-Release/release/MyClassName.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../MyClassName-Release/debug/MyClassName.lib
else:unix:!macx: PRE_TARGETDEPS += $$PWD/../MyClassName-Release/MyClassName.a

Answer 1

Your .pro file defines library dependencies only for unix platform unix:!macx:. I suppose there should be also win32 for Windows.

Answer 2

Had the same problem just now. Build was working fine on macOS/OSX but not on Windows with the same codebase.
It was caused by the fact I was not exporting my symbols, which is necessary on Windows.
Use Q_DECL_EXPORT on functions when building the lib.
And use Q_DECL_IMPORT when using the functions from the lib.
Example how to do this here: https://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application#Creating_a_shared_library
Solved the problem in my case, hope it helps!