In the code box after the paragraph that begins with the sentence “Now let’s do the overloaded assignment operator.”, I think line 1 of the code should be:

01. // Assignment operator

Not:

01. // Copy constructor

Thanks.

[ Good eye. Fixed! Thanks. -Alex ]

I should be slightly more clear about my terminology — when I said “valid memory address” in the above comment, I really meant “allocated memory address”. Also, to be a bit more clear, accessing an unassigned bit of memory won’t necessarily cause a crash. It may or it may not, depending on where the piece of memory is and whether the operating system has protected that memory. Windows, for example, prevents programs from reading and writing to certain memory addresses in order to prevent malicious code from overwriting parts of the operating system. If you try to do that, you will get an access violation.

For example, consider the following program:

// Assign some memory to pnValue
int *pnValue = new int;
// Write a value into that memory.  This is fine
*pnValue = 6;
// Delete the memory
delete pnValue;

// The following is not a good idea, but it probably
// won't crash
cout < < *pnValue;

// Now, let's set the pointer to a protected address
pnValue = (int*)0xFFFFFFFF;

// This will definitely crash
cout << *pnValue;

When we delete a pointer using delete or delete[], it does not contain a valid memory address any longer. However, that does not necessarily mean the program will crash if you use the pointer for reading.

Consequently, in the last example, the if statement will succeed because cString.m_pchString is non-zero. It will then allocate new memory for m_pchString (and cSource.m_pchString). Then it copies cSource.m_pchString, which is that uninitialized new memory (aka. garbage) into m_pchString, which is itself.

The end result of the self-assignment without the self-assignment check is that you’ll get a new string of the correct length, but it will be filled with garbage.

if (cSource.m_pchString)

or

strncpy(m_pchString, cSource.m_pchString, m_nLength)?

When we delete a pointer using delete [], does it still contain a valid memory address?

[ Fixed! Thanks. -Alex ]

– serg.