Question

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Why should the implementation and the declaration of a template class be in the same header file?

My header file has

template <typename T>
class AA : public BB<T>
{
public:
    AA()
    { ... }

this is working fine. But I need to separate the constructor implementation from header file.

So in cpp, I have

template <typename T>
AA<T>::AA()
{ ... }

When I compile this, it compiled but I get unresolved external symbol error. What am I missing here?

Answer 1

You can explicitly instantiate templates in your implementation file using :

template class AA<int>;

this will generate a definition from a template, but it is of use only if you know what types your class clients will use