Member Function Templates

The fun of templates does not stop at global functions. Oh, no. You can create member function templates as well. As of Visual C++ 6.0, the definition of IUnknown has been augmented with a member function template for QueryInterface:

struct IUnknown {
...
  template <class Q>
  HRESULT STDMETHODCALLTYPE QueryInterface(Q** pp)
  { return QueryInterface(__uuidof(Q), (void**)pp); }
}

Before the member function template, all calls to QueryInterface had to be sure to match up the interface type and the interface identifier:

void Fly(IUnknown* punk) {
  IBird* pbird = 0;
  punk->QueryInterface(IID_ICat, (void**)&pbird); // Oops!
  punk->QueryInterface(IID_IBird, (void**)pbird); // Double oops!
  punk->QueryInterface(IID_IBird, (void**)&pbird); // OK
  pbird->Fly();
}

On the other hand, with the QueryInterface member function template, the type of the interface suffices:

void Fly(IUnknown* punk) {
  IBird* pbird = 0;
  punk->QueryInterface(&pbird); // __uuidof uses to determine IID
  pbird->Fly();
}

In effect, member function templates add new member functions of a class, just as function templates add new global functions based on use.