Undefined reference errors linking a dll in QtCreator - c++

Question

I have built a simple dll using g++ under mingw:
We have an include file:
#ifndef __TESTDLL_H
#define __TESTDLL_H
class sineCalculator
{
double n;
public:
sineCalculator();
sineCalculator(double);
double sine();
void setAngle(double);
};
#endif
Then an implementation cpp:
#include <testdll.h>
#include <math.h>
sineCalculator::sineCalculator()
{
n = 0;
}
sineCalculator::sineCalculator(double x)
{
n = x;
}
double sineCalculator::sine()
{
return sin(n);
}
void sineCalculator::setAngle(double x)
{
n = x;
}
This I have compiled as a dll.producing both a .dll file and .a import library with the names visaTest.dll and libvisaTest.a
I can write a small program and link to this dll successfully just using the command line - no Qt just g++. I can create objects and run the methods all fine.
However, I now want to add this library to a Qt application within QtCreator. I have followed the advice of several questions and added these lines to my .pro file:
INCLUDEPATH += C:/msys64/home/hoyla/libs/
INCLUDEPATH +=C:/msys64/home/hoyla/includes/
DEPENDPATH +=C:/msys64/home/hoyla/libs/
win32:CONFIG(release, debug|release): LIBS += -LC:/msys64/home/hoyla/libs/ -lvisaTest
else:win32:CONFIG(debug, debug|release): LIBS += -LC:/msys64/home/hoyla/libs/ -lvisaTest
else:unix: LIBS += -L$$PWD/../../../../libs/ -lvisaTest
Although I use some absolute paths here I have also tried with relative paths. However, I keep getting undefined reference errors to my dll functions when building the Qt file. I would point out that the paths point to the location of my .a import library, the .dll itself is within the system path. What am I doing wrong here?

Answer 1

Mea Culpa! It seems I had compiled my library under 64-bit MinGW but Qt Creator was defaulting to the 32-bit toolchain. One would have thought that the linker might have been able to determine that was the error and generate a more useful error message but there we go. Lesson learned - make sure you're using the tools you think you're using.