Importing libraries and including them still gives undefined references

RekTekk249

I'm quite new to c++ and Qt in general, so I might have made some obvious mistake here, but I spent the last couple hours trying to figure that out and still no luck. He're the code in question, from the bit7z library:

try {
        bit7z::Bit7zLibrary lib( L"7za.dll" );
        bit7z::BitExtractor extractor( lib, bit7z::BitFormat::Zip );
        //Trimming extension
        QString subDirectory = "sub/" +
                QString::fromStdString(fileName
                           .toStdString().substr(0,
                                       fileName.toStdString().find_last_of(".")));

        CreateDirectoryA(subDirectory.toLocal8Bit().data(), nullptr);
        extractor.extract(L"sub/" + fileName.toStdWString(), subDirectory.toStdWString());

    }catch (const bit7z::BitException& ex ) {
        qDebug() << ex.what();
    }

Getting undefined references on every instance of bit7z... I know the string manipulation are probably not good, but I'm used to python where everything "" is a string, while here you have char* char[] std::string QString wstring wchar* and such... Every function takes a different type of string and none of theses are compatible...
The .pro file:

...
...
DISTFILES +=

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/bit7z/lib/ -lbit7z64
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/bit7z/lib/ -lbit7z64_d
else:unix: LIBS += -L$$PWD/bit7z/lib/ -lbit7z64

INCLUDEPATH += $$PWD/bit7z/include
DEPENDPATH += $$PWD/bit7z/include

What am I missing here? I don't see any error in the IDE until compilation. I've added the library with the "Add library" option when you right click and "external library".

Christian Ehrlicher

So what error do you get? Since it's a C++ library - is Qt and the library compiled with the same compiler? You can't mix msvc and mingw here.

aha_1980

Hi @RekTekk249,

Just one addition to @Christian-Ehrlicher: Your program needs the same archticture (bitness) as your library, please check that too.

Regarding the second point: indeed string handling is horrible in C and only partly better in C++; but with QString you have a mighty and locale aware toolbox.

   QString subDirectory = "sub/" +
            QString::fromStdString(fileName
                       .toStdString().substr(0,
                                   fileName.toStdString().find_last_of(".")));

So this can be done better (uncompiled + untested):

const QString subDirectory = "sub/" + fileName.mid(0, fileName.lastIndexOf('.'));

CreateDirectoryA(subDirectory.toLocal8Bit().data(), nullptr);

Note that is is platform specific (Windows) API and only works for ASCII filenames. A bit better would be:

CreateDirectoryW(reinterpret_cast<const wchar_t *>(subDirectory.utf16()), nullptr);

extractor.extract(L"sub/" + fileName.toStdWString(), subDirectory.toStdWString());

And here it depends on which parameters extract() expects.

Hope that helps.

Regards

SGaist

Hi,

To add to @aha_1980 additions, QDir has QDir::mkdir which is cross-platform so you don't have to play with native API for that kind of action.

RekTekk249

Yes, thanks for that, I wasn't aware that compilers weren't compatible... The library is open source, I'll just recompile it. Also, I heard of Qt::mkdir, but apparently it's doesn't have good performances, but I got this from a year old SO thread, it might have been better now.. I'll give it a try. I know CreateDirectoryW is a windows api function, but it was the simplest to use, and I had my fileName filtered with regex to be letters/numbers only for other reasons. For the strings, I was totally unaware of the QString.mid() function... I was looking on the docs for sub subs sub_str substring and stuff like that.. Thanks for that, I got it working now! That's the first time I encounter a language with multiple compilers like that... C and its variants seem to be the only one. Honestly though it feels like Qt is it's own language, more than a framework...

Christian Ehrlicher

@RekTekk249 said in Importing libraries and including them still gives undefined references:

Also, I heard of Qt::mkdir, but apparently it's doesn't have good performances

micro-optimization - you call it once... it's just not worth the trouble.