c++ array tips - Syntax for delete on an array of objects

[ben2ong2]

PROBLEM: What is the correct syntax for using delete on an array of objects?

    char* s = new char [40];
    delete s;

    int *p = new int [40];
    delete 40 p;


RESPONSE: Anil A. Pal (pal@wpd.sgi.com)

Actually, both of these examples are incorrect as of the latest draft
of the C++ standard.  The correct way (for both cases) is:

    mumble* x = new mumble[40];
    delete [] x;

Earlier implementations of C++ (for example, those based on cfront 2.0 or
before) required the programmer to explicitly supply the count (40) in the
delete.

    delete [40] x;

What happens if you forget and do a simple delete x; ?  Well, for the
examples quoted (int and char) usually nothing dramatic.  The full
storage will probably be returned to the free pool (in typical
malloc/free based implementations). 

If the type mumble is a class with a destructor, however, forgetting the
array form of the delete will result in a SINGLE DESTRUCTOR being called,
for the 0-th element in the array.

Using the array syntax (delete [] x;) will correctly call the destructor
for all elements of the array.


TITLE: Distinguishing lvalue/rvalue for operator []


PROBLEM: Is there any way to distinguish when the operator[] is used
as an lvalue as opposed to an rvalue?


RESPONSE: Joe Buck (jbuck@galileo.berkeley.edu)

class ElementRef {
   operator int ();
   operator=(int);
};

class IntVector {
...
   ElementRef operator[] (int);
};

The idea is that [] returns a special class called ElementRef, which
has two operators defined: assignment operator and cast-to-int.
If I say

   IntVector v;
   int i, j;
   ...
   v[3] = i;
   j = v[4];

in the first case, ElementRef::operator=(int) is used.  In the
second case, ElementRef::operator int() is used.