From:	Jack Klein <jackklein AT spamcop DOT net>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: FINALY: delete/delete[] with build-in-types/user-classes
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References:	<8q1tn2$iv6$1 AT info DOT cyf-kr DOT edu DOT pl>
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Date:	Sun, 17 Sep 2000 08:16:04 GMT
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On Sun, 17 Sep 2000 10:00:28 +0200, "Rafał Maj" <r_maj AT poczta DOT onet DOT pl>
wrote in comp.os.msdos.djgpp:

> Hi,
> because my last question about delete / delete[] caused big discusion with
> meany diffrent opinions, I wan't to check, if finaly I understand it...
> (probably not ;)
> 1) "s" is a pointer to array, maked like:  myclass* s = new myclass[100];
>    a)  delete []s    will not only free memory, but alsow call destructor
> for each of 100 objects "myclass"

Yes, that is correct.

>    b) using delete s;  is probably wrong, because none destructor will be
> called. But all memory 100*sizeof(myclass) will be freed ?

Maybe, maybe not.  The C++ language standard states that items created
with new[] be destroyed by delete[], and those with new by delete.
Combining new[] with delete or new with delete[] is undefined
behavior, which means anything can happen.  Even if this happens to
work with one particular, it might not with the next one.

> 2) with   char *s = new char[100]  I can use :
>   a) delete []s;  but it isn't necessarly, when char doesn't have any
> destructor
>   b) delete s;  is good

No, delete after new[] is not good.  It is illegal code, it is a bug,
it is not valid C++.  Even if it seems to work.

> 3) and what with   char* s = malloc(sizeof(char)*100); ?

Actually you can't even compile this as C++, although you can as C.
In C++ you can't assign a void * to another type of pointer without a
cast.  And you never, never need to multiply anything by sizeof(char),
since sizeof(char) is 1 by definition, and always will be.  The safest
way to use malloc() is:

char *s = (char *)malloc(100 * sizeof *s);

In C, don't use the (char *) cast.

>    a) free - typical

Yes, or realloc() with a size of 0.

>    b) can I use delete ?

No.

>    c) can I use delete [] ?

No.

> 4) because strdup() uses malloc,   char *s=strdup(S); should be "deleted"
> same way as in question 3
> 
> Many Thanks,
> Rafal

There are three pairs of memory management functions, and you should
never, NEVER, mix them:

new & delete
new[] & delete[]
malloc() or calloc() & free()

You can use all of these different methods in the same program for
different allocations, but for any given memory block you must free it
with the corresponding type you used to allocate it.

Jack Klein
-- 
Home: http://jackklein.home.att.net

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