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 :-)