Re: C++ 101 dumb question

"Anthony Jones" <>
Thu, 21 Jun 2007 12:25:44 +0100
"MrAsm" <> wrote in message

On Thu, 21 Jun 2007 10:29:38 +0100, "Anthony Jones"
<> wrote:

Class CMyClass
   char * m_str;
   CMyClass(char * str)
       m_str = str;
       // Do I need to anything with m_str

In addition to what SvenC wrote, I'd like to add that, if you use raw
pointers (like tha 'char *'), you also need to implement a copy
constructor and operator= in your class, e.g.:

  // Copy ctor
  CMyClass( const CMyClass & );

  // operator=
  CMyClass & operator=( const CMyClass & );

and maybe also define your destructor to be 'virtual', to be safe e.g.
with inheritance:

  virtual ~CMyClass();

C++ has several subtle things, that you don't find in C# or
Javascript... You'd better taking a good C++ book and study it and
*write* lots of code (and debug it, to learn a lot also from your

I wish you a great experience learning C++!


Thanks guys for your input. I had my suspicion that the answer is
ultimately 'go read a book' however that (along with using the Web) is what
I am doing. As you've guessed I spend most of my time writing Javascript
and I will continue to do so. However I'm also required to maintain and
developer an ISAPI filter which up to now has be fairly basic but is getting
more complex.

I dumbed down the example in a (failed) attempt to focus on a particular
area which so far in my studies seems somewhat ambigous. I'm hoping that
some one can say something in such that the penny will drop. Let me bring
up the detail a little (and ditch the distracting string stuff in real code
I'm using CString).

My main focus is the behaviour of the return of a function. Nothing I've
read so far explains step by step what actually happens when returning a
object. Here is better example:-

class Crypto
 //Public methods here

#include "StdAfx.h"
#include "Crypto.h"

 m_hProv = 0;
 CryptAcquireContext(&m_hProv, 0, 0, PROV_RSA_FULL,
                      CRYPT_MACHINE_KEYSET | CRYPT_SILENT |
 if (m_hProv != 0) CryptReleaseContext(m_hProv, 0);
// Other method implementations here

Now I use this class like this:-

void a()
    Crypto oCryptoInA;
    oCryptoInA = b();

Crypto b()
    Crypto oCryptoInB;
    return oCryptoInB;

Now at guess the sequence of events on return in b is:-

Destructor of Crypto is called where this is oCryptoInA.

Data members at oCryptoInB are copied in to oCryptoInA (this is a simple mem
copy since no operator= is defined).

Destructor of Crypto is called where this is oCryptoInB.

This leaves m_hProv in oCryptoInA holding a handle that has now been

Have I got the sequence right? What represents a good solution?

If I were to substitute the simple handle of m_hCrypto with a COM Smart
pointer would I be the same sort of bother. That is without an
implementation of operator= the smart point will not track the correct
number of references and destory the object prematurely.

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