Re: minor confusion

From: Robert W Hand (rwhand_at_NOSPAMoperamail.com)
Date: 11/01/03 

Date: Sat, 01 Nov 2003 05:03:30 -0500

On 31 Oct 2003 19:10:24 -0800, forums_mp@hotmail.com (forums_mp)
wrote:

>and again my confusion. "gh" is a 'copy' (member wise) of t. hence
>when gh goes out of scope why does "t" care?

The compiler generated copy constructor makes gh from t. The pointer
to int, data, will have the same value in both objects. That value
points to the anonymous array that was formed in the default
constructor when t was formed. So t and gh are not independent
objects. They share that array. When the destructor of gh is called,
that array is destroyed. It is again destroyed when t goes out of
existence. That is an example of undefined behavior.

Reread Mark's notes. He gave good advice about writing your own copy
constructor.

>// consider
>int idx = 5;
>int jdx = test (idx);
>int test (int a)
>{
> return a++;
>}
>the fact that a has block scope means nothing to idx. so now "a" got a
>copy of idx. "a" post incremented the copy of idx. "a" is no longer
>in existence, however idx is preserved. the difference here is the
>parameter is of type int but pass by value is pass by value, hence my
>confusion.

When the object passed by value is a pointer, doing something to the
objected pointed to, may change the apparent semantics. In your OP
example, the array, that data pointed to, was deleted twice.

Best wishes,

Bob

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