Re: CComPtr subtlety

"Igor Tandetnik" <>
Mon, 29 Jan 2007 14:10:44 -0500
Jason S <> wrote:

There are two constructors and two assignment operators that sort of
do the same thing:

[in the CComPtr class in atlbase.h, this is the msvc 6 version]
CComPtr(T* lp)
if ((p = lp) != NULL)
CComPtr(const CComPtr<T>& lp)
if ((p = lp.p) != NULL)

why the 2nd one in each pair?

They take different arguments. One supports

IInterface* p;
CComPtr<IInterface> q = p;

the other supports

CComPtr<IInterface> p;
CComPtr<IInterface> q = p;

I tried creating a similar class without
the 2nd pair and it seems to not work (it compiles but the reference
count does not change for some reason)

The second constructor is a copy constructor. If you don't provide one,
compiler automatically generates a copy constructor that simply performs
member-wise copy. Similarly for copy assignment operator. If member-wise
copy is the wrong thing to do (as it is in this case), you have to
explicitly write copy constructor and copy assignment operator.

C++ Rule of Three: if your class requires a non-trivial destructor, it
also requires a non-trivial copy constructor and copy assignment
With best wishes,
    Igor Tandetnik

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