exporting Template class in dll unresolved external - c++

Question

I am trying to create a class library dll that exports a template class. I have a local version of the same class and it must be coincidental, but it works correctly. I know this doesn't really mean much, clearly something is wrong.
The exported template class in the library dll is:
template <class CcmBlock>
class CCMSHARED_EXPORT CcmFilter
{
public:
CcmFilter()
{
mBlock = nullptr;
////mBlockList = new std::list<CcmBlock*>();
}
void add(CcmFrame* frame)
{
if (frame == nullptr)
return;
mBlock = new CcmBlock(
frame->getFrameData(),
frame->getSampleRate(),
0xFFFFFFFF,
frame->getBlockSize(),
frame->getDomain()
);
mBlockList->push_back(aBlock);
}
CcmBlock* get()
{
return mBlock;
}
private:
CcmBlock* mBlock;
////std::list<CcmBlock*>* mBlockList;
};
On the application side:
CcmFilter<FooBlock>* oneFilter = new CcmFilter<FooBlock>();
//Filter<Block>* filter = new Filter<Block>();
CcmFrame* frame = new CcmFrame(0, 50000, 40, 1024, Domain::Time);
oneFilter->add(frame);
CcmBlock* block = oneFilter->get();
FooBlock* fooBlock = dynamic_cast<FooBlock*>(block);
if (fooBlock == nullptr)
{ //Report Error }
else
{ // Do the work}
and the FooBlock class is derived from CcmBlock as follows:
class FooBlock : public CcmBlock
{
public:
FooBlock(int* ipblock, DWORD dwSampleRate, DWORD dwMicrophoneIndex, DWORD dwBlockSize, Domain domain);
void process();
};
The library compiles and builds the dll. When I attempt to build the application I get messages:
mainwindow.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl CcmFilter::CcmFilter(void)" (__imp_??0?$CcmFilter#VFooBlock####QEAA#XZ) referenced in function "public: __cdecl MainWindow::MainWindow(class QWidget *)" (??0MainWindow##QEAA#PEAVQWidget###Z)
mainwindow.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: void __cdecl CcmFilter::add(class CcmFrame *)" (__imp_?add#?$CcmFilter#VFooBlock####QEAAXPEAVCcmFrame###Z) referenced in function "public: __cdecl MainWindow::MainWindow(class QWidget *)" (??0MainWindow##QEAA#PEAVQWidget###Z)

Answer 1

DLL-s are loaded at runtime, while templates are instantiated at compile time.
A work-around which is not ideal but might work for a very limited use case is to explicitly instantiate a template somewhere after their declaration. That is, add something like:
template class CcmFilter<FooBlock>;
For details, see https://msdn.microsoft.com/en-us/library/by56e477(VS.80).aspx

Answer 2

Just do not use declspec. But in your case template will be compiled each time when you include it in *.cpp file. It wont be exported.
In order to export some well-known implementations of your template you probably should use template explicit instantiation definitions in tandem with __declspec(export).