Re: C++ 101 dumb question

From:

David Wilkinson <no-reply@effisols.com>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 21 Jun 2007 09:06:58 -0500

Message-ID:

<eXno#TAtHHA.4612@TK2MSFTNGP04.phx.gbl>

Anthony Jones wrote:

What does the signature of a copy constructor look like?

CMyClass(CMyClass src);

OR

is it:-

CMyClass& operator=(CMyClass& src);

What if the member was an instance of class that does
have it's own copy constructor would the default copy constructor call that
to copy the member or would it blindly copy the members of that object as
well? That to me is the critical question.

Surely that could be optimised away. The existing object is already on the
stack. Are you saying a second copy gets added to the stack, then the
original has it's destructor called, then after returning execution to the
caller another copy is performed, then presumably this temporary copy in the
stack has it's destructor called?

Perhaps I'm showing my ignorance again but since the callee knows its to
return an object it has on the stack it could defer its destruction to the
caller. The caller could perform the copy constructor once then destroy the
object from the callee function.

Copy constructor:

CMyClass(const CMyClass& src);

Assignment operator

CMyClass& operator = (const CMyClass& src);

When you copy or assign, the copy constructor and assignment operator
that you have defined are used. If you have not defined one, the default
one is used. The default one does member-wise copy or assignment. This
rule is applied recursively.

There is a thing called Return Value Optimization, which in some cases
can eliminate unnecessary copying. However, your classes must be
designed so that all the unnecessary copying would be done correctly if
it were done.

--
David Wilkinson
Visual C++ MVP