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



  • So there's no defines automatically set when using Qt to indicate that? I have to put a define in my .pro file?



  • @kitfox
    Sorry, I still don't think you're getting it. By the time you have a .pro file to put anything in, you're already compiling for Qt, aren't you?



  • I'm trying to format the code so that it can build either by clicking build within Qt Creator or by whatever compiler the original author was using.


  • Lifetime Qt Champion

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

    @kitfox
    Sorry, I still don't think you're getting it. By the time you have a .pro file to put anything in, you're already compiling for Qt, aren't you?

    No, you can build a Qt-less project.

    @kitfox yes that's a define you have to enable yourself like shown in the Shared Library chapter of Qt's documentation.



  • @SGaist
    Why would you start with a .pro file if you don't have Qt anywhere? What are you going to process it with?


  • Lifetime Qt Champion

    @JonB do you mean if you don't use Qt in your project ? Maybe because you are used to use qmake to manage your projects and are satisfied with it.



  • @SGaist
    I envisage the OP is asking: he gives all his code plus Makefiles to someone else, who may not possess Qt at all, no qmake etc. And they can compile the code some other way with nothing Qt anywhere.

    Then at one point he said

    either by clicking build within Qt Creator or by whatever compiler the original author was using.

    So maybe he'd like to know if he's compiling from Qt Creator or not.

    The point I'm trying to get across is he will need to define some symbol somewhere if he wants to compile for/with Qt or some symbol somewhere if he wants to compile otherwise. QT_IS_AVAILABLE has got to be defined somewhere in those cases where it's wanted, else it'll never any effect. Different makefiles, an environment variable, something so the Qt or non-Qt compilation can be performed.



  • @kitfox What kind of modifications are you doing?



  • 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. So what am I supposed to use if someone it trying to compile this with another compiler? I'd like a way to essentially nullify everything in this header file if I can tell that the person trying to compile it is not using Qt.

    As far as I can tell, I'm going to have to have everyone using Qt who wants to use this library add some define in their .pro file so that the imports are generated. Ie, something like:

    
    #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
    


  • Just trying to get the library to build under Qt. Then would like to write a few simple apps to test it to make sure it's working.



  • @kitfox It would be nice if there were some equivalent of IM_USING_QT automatically defined so that I would not have to have all QT users explicitly specify it.


  • Moderators

    If you're using a decent compiler you can use __has_include to check if Qt headers are available and define some macro constant if they are.
    That doesn't solve any linker and toolchain (moc etc.) issues, but that's how you can detect if Qt source is available in your project.



  • @kitfox
    If your definition of "I'm using Qt" is you have the Qt include files on your box, @Chris-Kawa's suggestion looks great. Is that what you wanted? Or did you say you wanted to know whether the user is sitting inside Qt Creator? Or does the user choose whether he wants to compile for Qt or not (e.g. like you will want to while testing each route of this code)? What exactly is the deciding factor?



  • @kitfox why not use CMake so that you can specifically define whether or not to use Qt, and then decide which implementation source file to add to compilation?



  • If the user wants to compile with the Qt API using my modifications, I want the DLL import/export tags to resolve to the definitions defined in QtCore/qglobal.h. If they wish to use the original environment they were developed in, or some other non Qt environment, I would like the FFMPEGCPP_EXPORT tag to resolve to the empty string and the #include <QtCore/qglobal.h> left out so that they can compile with their libraries. Basically as this is an open source library, I'm trying to make minimal changes and leave it in a state such that it can still compile in its original environment, while also making it capable of compiling using the Qt environment.



  • I'm not familiar with cmake. I suppose I should learn it, but I haven't had the time.



  • Try

    #if defined(QT_VERSION)
    


  • This post is deleted!


  • @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.



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



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


  • Moderators

    @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)



  • @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....


  • Lifetime Qt Champion

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



  • @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?


  • Moderators

    @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...


  • Moderators

    @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).



  • @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



  • @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.


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