Including isolated Qt objects in .cpp file - c++

Question

I want to use a library that utilizes the QT-class QVector2D to calaculate B-splines (library: https://github.com/ejmahler/SplineLibrary). But I can't figure out how to get the includes / linking right using a simple .cpp file and compiling with g++.
This is my attempt
// test.cpp
// My attempt at including the QVector2D class
#include </home/user/Qt/5.8/gcc_64/include/QtCore/qconfig.h>
#include </home/user/Qt/5.8/gcc_64/include/QtCore/qglobal.h>
#include </home/user/Qt/5.8/gcc_64/include/QtGui/qtguiglobal.h>
#include </home/user/Qt/5.8/gcc_64/include/QtGui/qvector2d.h>
// For spline library
#include <SplineLibrary/spline_library/spline.h>
#include <SplineLibrary/spline_library/vector.h>
#include <SplineLibrary/spline_library/splines/uniform_cr_spline.h>
int main()
{
std::vector<QVector2D> splinePoints{
QVector2D( 0, 1),
...
};
UniformCRSpline<QVector2D> mySpline(splinePoints);
QVector2D interpolatedPosition = mySpline.getPosition(0.5f);
}
Compiling with g++ 5
$ g++ -std=c++17 test.cpp -o test
gives
/home/user/Qt/5.8/gcc_64/include/QtCore/qglobal.h:63:28: fatal error:
QtCore/qconfig.h: No such file or directory compilation terminated.
I have verified that the file qconfig.h exists and that it is in the path as included.
I'm guessing my error is either in how I try to include the QT-headers, that I need to link it somehow.
How do I properly include a single Qt component (in this case QVector2D) and compile the project using g++?

Answer 1

First of all, you should never specify absolute paths in you includes. If someone else wants to use your code, they would need to modify all the files to point to the correct headers on their system. Apart from that you're using internal headers that you're not supposed to include yourself. At the very top of the Qt documentation for QVector2D you can find what header you're supposed to include to use the QVector2D class.
So test.cpp should look like this:
#include <QVector2D>
// For spline library
#include <SplineLibrary/spline_library/spline.h>
#include <SplineLibrary/spline_library/vector.h>
#include <SplineLibrary/spline_library/splines/uniform_cr_spline.h>
int main()
{
std::vector<QVector2D> splinePoints{
QVector2D( 0, 1),
...
};
UniformCRSpline<QVector2D> mySpline(splinePoints);
QVector2D interpolatedPosition = mySpline.getPosition(0.5f);
}
Compiler flags
Now, since you have a custom installation path for Qt5.8, the compiler can't find the header in the default search path, so when compiling this file you need to tell the compiler where to search for the Qt headers. As indicated by the line QT += Gui in the documentation I linked above, QVector2D is part of the QtGui sublibrary, so you need to include the headers in the directory /home/user/Qt/5.8/gcc_64/include/QtGui/. The compiler flag to do so is -isystem /home/user/Qt/5.8/gcc_64/include/QtGui.
However, the QtGui headers themselves depend on QtCore headers, but their path is given relative to the /home/user/Qt5.8/gcc_64/include directory, so in order for the compiler to find the header files the QtGui headers depend on you also need to add the compiler flag -isystem /home/user/Qt/5.8/gcc_64/include. Btw this is also the reason why you got the error QtCore/qconfig.h: No such file or directory compilation terminated., there is no such file QtCore/qconfig.h in the directory where test.cpp is located, nor is there one in the system include path, and you didn't give the compiler the additional path where to look for it.
So the compilation should be successful when executing g++ -std=c++14 -isystem /home/user/Qt/5.8/gcc_64/include -isystem /home/user/Qt/5.8/gcc_64/include/QtGui -fPIC -c test.cpp (assuming that the compiler previously didn't have any problem to find SplineLibrarys includes).
Linker flags
The previous command just compiles test.cpp (note the -c flag), but doesn't produce a binary that you can run. The linker has the same problem as the compiler - it doesn't know where to find the precompiled Qt libraries to link against unless you tell it where to look for them. The flag to do so is -L/home/user/Qt/5.8/gcc_64/lib.
In addition to telling the linker where to look for the precompiled libraries, you also need to tell it which ones you're using. As I've mentioned before, QVector2D is part of the QtGui sublibrary, so you need to add -lQt5Gui. Since QtGui depends on QtCore, -lQt5Core also needs to be added.
Assuming you used the default installer, your installation of Qt only includes dynamic libraries, so you also need to tell the linker to store the information where to find the library files in the created binary. The flag you need to add for this is -Wl,-rpath=/home/user/Qt/5.8/gcc_64/lib.
So putting everything together, the final command you need to run is g++ -std=c++14 -fPIC -isystem /home/user/Qt/5.8/gcc_64/include -isystem /home/user/Qt/5.8/gcc_64/include/QtGui -L /home/user/Qt/5.8/gcc_64/lib -Wl,-rpath=/home/user/Qt/5.8/gcc_64/lib test.cpp -lQt5Core -lQt5Gui.
If you use clang instead of gcc, the same command runs just fine if you replace g++ by clang++.