Re: C++ 101 dumb question

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>
Thu, 21 Jun 2007 12:18:46 -0500
"Anthony Jones" <> wrote in message

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam> wrote in message

"Anthony Jones" <> wrote in message

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam> wrote in message

Define operator= to close the handle previously held in the object,


DuplicateHandle to get a copy of the right-hand-side's handle. But be
careful about x = x, when (this == &rhs) you should just do nothing.

Thanks Ben. Now that I'm beginning to under the copy constructor and
assigment operator a little better. The question of dealing with


which specifically are handles did cross my mind.

In a copy constructor I merely have to duplicate the handle in the src.

In an assignment I need to release any existing handle and duplicate
from the source. A special case arises where src and this are the same
object. In this case do nothing otherwise I'll be attempting to


handle I've just released.

The specific case for Cryptographic contexts duplicate handle doesn't
CryptContextAddRef is used instead to increase the number of
CryptoReleaseContexts needed to actually release the context.

Crypto& Crypto::operator = (const Crypto& src)
   if (this != src)
       if (m_hProv) CryptReleaseContext(m_hProv, 0);

   m_hProv = src.m_hProv;

   CryptContextAddRef(m_hProv, 0, 0);

Getting close.

Crypto& Crypto::operator = (const Crypto& src)
   if (this != &src)
       if (m_hProv) CryptReleaseContext(m_hProv, 0);

       m_hProv = src.m_hProv;

       if (m_hProv) CryptContextAddRef(m_hProv, 0, 0);

Thanks dumb logic mistake and & on the src

A couple of issues I'm not sure of:-

Do I need to implement a != operator?

Does comparing Crypto objects to each other make sense? Remember != is


opposite of ==, not of =.

I guess I asked that since I wasn't sure how this != src (or this != &src)
was working.
This would be testing the identity of the object as in its address in

That's correct.

"this" has type Crypto*
"src" has type const Crypto&, so that "&src" has type const Crypto*
The != operator is already defined for pointers, so you don't have to (and
can't) define it yourself.

Ultimately I don't know what const Crypto& src is actually doing. I've
& to make calls to something that uses * but I'm not familiar with & in
function signature. What's that called?

(C++ is frustrating in that in order to understand such a thing you need
be able to look it up in the documentation and in order to do that you
to know what text to look for.)

In Visual C++ help, if you type "&" into the index, it will take you to "&
operator". You'll find two flavors, "& (Address-of operator)" and "&
(Reference operator)" which link to each other. This usage is the reference

A reference is just like a pointer, except that taking the address when
making the reference, and dereferencing when using the reference, are
automatic and silent. For this reason using non-const references as
function parameters is discouraged, because there is no indication that the
parameter is passed by reference and changed inside the function. const
references preserve the caller's copy like pass-by-value, but avoid the
extra copy, by allowing the function to read (but not overwrite) the
caller's copy directly. And by extra copy, I mean a call to the copy
constructor, which could be quite expensive.

Also, because a reference always names the object pointed to, you cannot
rebind a reference to point to a different object.

Can I access the m_hProv private member of src with src.m_hProv?

Yes, any member of Crypto can access private members of any Crypto
not just "this".

Wah hey that was a good guess then ;)

Thanks for your patience.

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