C++ linker problem, inline template method

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Hello Everyone!

I have a problem with an old project, that they want me to compile in Visual Studio. This application was working in QtCreator with gcc compiler, but while in VS10 with Qt implementation it has some compile errors. The linker doesn't find the methods in the dll. This is a sample from the problematic classes:

C++

template <class SW> //SettingsWidget
class RCC_EXPORT_IMPORT SettingsWidgetList : public ObjectList<SW>
{
    public:
        inline RCC::Result apply()
        {
            bool isOk=true;
            for(int i=0, l=ObjectList<SW>::count(); i < l; ++i)
            {
                if(!rcc_resultToBool((*this)[i]->apply()))
                    isOk=false;
            }
            return rcc_boolToResult(isOk);
        }
};

This is from the dll's public header.
...and the linker tells this:

Error   794 error LNK2019: unresolved external symbol "__declspec(dllimport) public: enum ReworkClient::RCC::Result __thiscall ReworkClient::SettingsWidgetList<class ReworkClient::ProfileSettingsWidget>::apply(void)" (__imp_?apply@?$SettingsWidgetList@VProfileSettingsWidget@ReworkClient@@@ReworkClient@@QAE?AW4Result@RCC@2@XZ) referenced in function "private: void __thiscall ProfileDialog::on_OkButton_clicked(void)" (?on_OkButton_clicked@ProfileDialog@@AAEXXZ) C:\!projects\c++\ReworkClient\2.0.x\ReworkClient\rcprofile.obj  ReworkClient

It seems the problem with the apply() method, but I don't understand what can be the difference between the VS and gcc compilers working. Anyone has any idea what can be wrong with the apply method's declaration?
Any idea are welcomed (even critiques)! Im out of clues...

Thank you for reading!
Regards
Adam

Posted 7-Feb-14 23:15pm

CzimerA

Updated 7-Feb-14 23:53pm

Richard MacCutchan

v2

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2 solutions

Solution 1

I suspect it may be that the DLL's exports are standard C style, but the code you are generating is C++. You need to use the following construct around the QT headers.

C++

extern "C"{
// QT includes go here
}

See http://msdn.microsoft.com/en-us/library/0603949d.aspx[^] for a full description.

Posted 7-Feb-14 23:57pm

Richard MacCutchan

Comments

CzimerA 8-Feb-14 6:35am

Unfortunately it does not seem to be working. I got much more errors than before. Anyway I dont think templates are working in C style.

CzimerA 8-Feb-14 6:35am

...but good point anyway! Thank thee :)

Richard MacCutchan 8-Feb-14 6:43am

Did you rebuild the DLL with the Microsoft compiler and linker?

CzimerA 8-Feb-14 7:09am

Yes but it still not working, I got some compile errors from QT includes.

Richard MacCutchan 8-Feb-14 7:12am

Well I'm sorry, but no one here can guess what those errors could be.

CzimerA 8-Feb-14 7:49am

I keep trying! Thanks the help anyway!

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nv3 · Accepted Answer · 2014-02-08T00:28:00

Solution 2

Your SettingsWidgetList class is a template class and hence the compiler generates code for it at the moment you instanciate an object of it with a concrete template parameter. When you compiled your DLL, there probably was no need to instantiate any such object in your library code. And hence the compiler did not produce the member functions in your DLL.

In your client code, however, you instantiate an object of type SettingsWidgetList<profilesettingswidget></profilesettingswidget>, something the compiler has not seen, when you created the DLL. Hence it cannot find the apply method in the DLL.

If ProfileSettingsWidget is the only template parameter that you intend to use for the SettingsWidgetList class, you can force the compiler to generate the required code by
instantiating an object of type SettingsWidgetList<profilesettingswidget></profilesettingswidget> in your DLL code.

In general, however, the concepts of DLLs and template classes are somewhat excluding each other. A template class is usually distributed just by the header file that defines it and not via precompiled code of a DLL.

Posted 8-Feb-14 0:28am

nv3

Comments

CzimerA 8-Feb-14 6:47am

Yes normally it seems the header file should be enough, because of the inline declaration, I wonder why the client side trying to pull out from the dll. Unfortunately ProfileSettingsWidget is not the only class that uses this class, but I will try the instantiation. Thanks!

nv3 8-Feb-14 7:52am

It's probably the RCC_EXPORT_IMPORT declaration of your class that tells the compiler to look for the apply function in the DLL. If you ramove that everything should also work fine.

CzimerA 8-Feb-14 8:01am

I tried that before, somehow the compiler starts to argue against the "apply" of the iterator. It cannot find it! Is it possible that, the compiler ignores the 'apply' method of the SettingsWidgetList, if the class is exported?

nv3 8-Feb-14 8:35am

I wouldn't think so. Which error message do you get when you remove the RCC_EXPORT_IMPORT from the class declaration?

CzimerA 8-Feb-14 8:50am

"left of '->apply' must point to class/struct/union/generic type" this belongs to the iterator,
but the most weird is this:
"use of undefined type 'ReworkClient::BarcodeValidator'" this error may belongs to SettingsWidgetForObject class, that is used by the MOC from UI file.

nv3 8-Feb-14 9:18am

Looks like the left of ->apply is either not of type SettingsWidgetList<..>* or you used the -> operator where the "." operator should have been used.

CzimerA 8-Feb-14 9:46am

All classes that are templates of the SettingsWidgetList, are inherited from SettingsWidget. This class has an apply() method. Anyway this was a working code under gcc, Im not sure it has syntax errors, but I have never seen it working so anything can happen :)