Library project and template class gives unresolved external

[nefarious]

I have a library project im building in windows that exports a number of classes which all work. I just added a template class and when I attempt to instantiate it I get the following message:

unresolved external symbol "__declspec(dllimport) public: __cdecl Filter<class FooBlock>::Filter<class FooBlock>(void)"

The class header file is:

template <class Block>
class CCMSHARED_EXPORT Filter
{
public:
    Filter();
    void add(Frame* frame);
    Block* get();
    virtual void process();
private:
    Block* mBlock;
};

the implementation class is

template <class Block>
Filter<Block>::Filter()
{
}

template <class Block>
void Filter<Block>::add(Frame* frame)
{
    mBlock = new Block(
                frame->getData(),
                frame->getRate(),
                0xFFFFFFFF,
                frame->getSize(),
                frame->getDomain()
                );
}

template <class Block>
Block* Filter<Block>::get()
{
    return mBlock;
}

template <class Block>
void Filter<Block>::process()
{}

and Foo block is a class that is derived from Block as

class FooBlock : public Block
{
public:
    FooBlock(int* ipData, DWORD dwRate, DWORD dwIndex, DWORD dwSize, Domain domain);
    void process();
};

[SGaist]

Hi,

You also need to export Block and FooBlock.

[nefarious]

I do export Block. FooBlock is in the application project that uses the library. I have never used templates before and I think this is where the problem is. I built a version where everything is in the application project and get the same message. Maybe because I explicitly named the template class the compiler can't use a derived class. I even made a function pure virtual and the message is the same.

[nefarious]

I figured out the issue with the unresolved external symbol. The template needs to be contained all in a single header file, there can be no .cpp files for the implementation. I of course have to understand this better.

I now get the error:

CcmFilter.h:26: error: C2491: 'CcmFilter<CcmBlock>::CcmFilter' : definition of dllimport function not allowed
CcmFilter.h:40: error: C2491: 'CcmFilter<CcmBlock>::add' : definition of dllimport function not allowed

for the header file ccm_global.h, CCM_LIBRARY is defined in the .pro

#if defined(CCM_LIBRARY)
#  define CCMSHARED_EXPORT Q_DECL_EXPORT
#else
#  define CCMSHARED_EXPORT Q_DECL_IMPORT
#endif

the header file for the template

template <class CcmBlock> class CCM SHARED_EXPORT CcmFilter
{
public:
    CcmFilter();

    void add(CcmFrame* frame);
    CcmBlock* get();

private:
    CcmBlock* mBlock;
};

template <class CcmBlock> CcmFilter<CcmBlock>::CcmFilter()
{ }

template <class CcmBlock> void CcmFilter<CcmBlock>::add(CcmFrame* frame)
{
    mBlock = new CcmBlock(
            frame->getFrameData(),
            frame->getSampleRate(),
            0xFFFFFFFF,
            frame->getBlockSize(),
            frame->getDomain()
            );
}

template <class CcmBlock> CcmBlock* CcmFilter<CcmBlock>::get()
{
    return mBlock;
}

It seems that the CCM_LIBRARY is not being used for the definitions of the member functions?

[SGaist]

Didn't thought about that one but indeed, template method implementation should be at the same place the template is declared.

For more about templates see here.

[jsulm]

@nefarious Remove SHARED_EXPORT as it is not needed.

[SGaist]

Just a precision, @jsulm means that for the template class.

[jsulm]

@SGaist Yes :-)