Is there a #define for checking if the compiler is Qt?

JonB

@hskoglund
Where does QT_VERSION get defined, please? Isn't it via #include <QtGlobal>, so it tells you whether or not you have already included that file? How do you decide whether to include that or not? That's the bit I don't get....

@kitfox
OK, to be fair/clear, surrounding some Qt-only code in #if defined(QT_VERSION) looks like a good model for most of your code. It's a clean symbol. So that's good.

You still have to "get going" somewhere, to decide whether to define that or include QtGlobal.h or not in the first place.

kitfox

@JonB Yeah, I realized that after I made the last post. For now I'll add an extra define to my build file.

JonB

@kitfox
:) :) Which is what I've been trying to get at all along :)

J.Hilk

@JonB said in Is there a #define for checking if the compiler is Qt?:

Where does QT_VERSION get defined, please? Isn't it via #include <QtGlobal>, so it tells you whether or not you have already included that file? How do you decide whether to include that or not? That's the bit I don't get....

nope, its part of qmake, and the define is available inside the *.pro file therefore influencing the generated make file.

Its generally used to check if some features are available in the Qt version you're using or not, to therefore include specific files(or not)

JonB

@J-Hilk
Ah, now when I Googled yesterday I found a post (stackoverflow??) which said that QT_VERSION was defined in QtGlobal. Admittedly I think the post was from years ago, but I often find Qt is behaving the same as at the turn of the millennium :) Ah, yes, see https://stackoverflow.com/a/24903583/489865. And that was as recent as 2015. If you think that is inaccurate, you might wish to post there :)

What you say is actually a bit strange: the header files etc. belong to a particular Qt version, hence it would make sense if they define the Qt version for which they work. Defining your own value for QT_VERSION isn't going to help much when they were written for a different version....

jsulm

@JonB qmake you're using defines which Qt version is used :-)
So, it is perfectly valid if qmake defines QT_VERSION.

JonB

@jsulm
Since you posted, I have edited mine to link to the reference I used for this statement. Are the answers there wrong? Do you wish to correct them? :)

Also, I have just looked at https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html. There, bold as brass, unconditional (e.g. it is not inside #ifndef QT_VERSION):

#define QT_VERSION      QT_VERSION_CHECK(QT_VERSION_MAJOR, QT_VERSION_MINOR, QT_VERSION_PATCH)

Does that not define the QT_VERSION, yes or no?

J.Hilk

@JonB you're also correct,

the first thing one does, when building the Qt libs, is building qmake, and IIRC that uses the QtGlobal of the source tree it belongs to...

kshegunov

@JonB said in Is there a #define for checking if the compiler is Qt?:

Does that not define the QT_VERSION, yes or no?

Correct. The qmake bootstrap has little to do with your code. I can't say for sure that qmake doesn't define the macro for your project, but it shouldn't (provided you've used CONFIG -= qt).

VRonin

@kitfox said in Is there a #define for checking if the compiler is Qt?:

That's right. I'm having trouble thinking of a way to rewrite ffmpeg-cpp_global.h so that it would work either inside of outside of a Qt environment. Currently FFMPEGCPP_LIBRARY is defined in the .pro file and acts as a switch - if it's included, then FFMPEGCPP_EXPORT transforms into instructions to export the definition to a dll. However, if it's not included (which it presumably would not be to anyone who wanted to use the library) it turns into a declaration to import the definition - instructions which depend on the Qt API to work.


#ifdef IM_USING_QT

#include <QtCore/qglobal.h>

#if defined(FFMPEGCPP_LIBRARY)
#  define FFMPEGCPP_EXPORT Q_DECL_EXPORT
#else
#  define FFMPEGCPP_EXPORT Q_DECL_IMPORT
#endif

#else 

#define FFMPEGCPP_EXPORT

#endif

Sorry to take a step back here but I think we got sidetracked.

In the code posted the only "Qt" code is Q_DECL_EXPORT and Q_DECL_IMPORT. They can easily be replaced with native code

#  if (defined(WIN64) || defined(_WIN64) || defined(__WIN64__) || defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__))
#    define NQ_DECL_EXPORT     __declspec(dllexport)
#    define NQ_DECL_IMPORT     __declspec(dllimport)
#  else
#    define NQ_DECL_EXPORT     __attribute__((visibility("default")))
#    define NQ_DECL_IMPORT     __attribute__((visibility("default")))
#  endif

#if defined(FFMPEGCPP_LIBRARY)
#  define FFMPEGCPP_EXPORT NQ_DECL_EXPORT
#else
#  define FFMPEGCPP_EXPORT NQ_DECL_IMPORT
#endif

Now it should build and link with any build system. If you are building the library itself the build system (qmake, qbs, CMake, Visual Studio, etc.) will need to add a compiler definition for FFMPEGCPP_LIBRARY while user of the library will not. This is 100% standard in C/C++ libraries

SimonSchroeder

@VRonin is correct that other ways are preferrable to define DLL import/export.

I also assumed that QT_VERSION had to be defined by qmake. However, I had a quick look at the generated Makefile. And what is defined are things like QT_CORE_LIB, QT_WIDGETS_LIB, etc. My guess is that QT_WIDGETS_LIB, e.g., will appear if you put QT += widgets into your .pro file. And I would expect that QT_CORE_LIB is always defined when you compile with Qt.