Re: can a const pointer be deleted?

James Kanze <>
Wed, 3 Dec 2008 01:29:44 -0800 (PST)
On Dec 2, 4:05 pm, Maxim Yegorushkin <>

On Dec 2, 1:55 pm, abir <> wrote:

  In the std containers erase takes a const_iterator.

This is not so.

In std containers erase member function is a non-const member
function, because it changes the container contents. It would
not be logical for that function to accept const_iterator.

Why not? It don't modify anything through the iterator; the
iterator is only used for determining the position.

The original standard required iterator; this was recognized as
an error, and has been corrected; insert and erase now take
const_iterator for the position argument.

I have my own container and there const_iterator is a const
pointer to the item. I want to free the memory for the item
on erase (i.e both destroy & deallocate). However
deallocate function for allocator (and so delete, free etc)
takes a pointer rather than a const pointer. Is that mean i
have to cast it back to a pointer?

Make your_container::erase() accept non-const iterators.

In this case, I'd use const_cast, if nothing else was readily
available. You're in a non-const function of the container, so
you have a right to modify it, so once you've asserted that the
iterator does point into this container (and not some other
container), I seen no harm in de-constifying it. (A comment
would be in order, however.)

I was looking at std::list. There even a const_iterator
returns a non const node pointer (not very sure though).
Also how this works ?

The const-ness that the compiler understands is very
superficial. The nodes aren't part of the object, as far as the
compiler is concerned, so even a const member function (e.g. the
one which returned the const_iterator) can obtain non-const
pointers to the node. After that, it's the responsibility of
the classes involved (the container, iterators, nodes, etc.) to
ensure that this lack of const doesn't "leak" out at the
interface somewhere.

does the operator delete automatically cast it to a non
const type ?
const testobj* to = new testobj(1,1);
delete to;

There are two different things in C++ named delete: a) delete-
expression; b) operator delete. Here you are using
delete-expression and it ignores const-ness and volatile-ness.
What delete-expression a) does is it calls the destructor of
the object and then invokes operator delete b).

In C, while this works
char* ch = (char*)std::malloc(5);
for a const pointer i explicitly need to cast it back like,
const char* ch = (const char*)std::malloc(5);
 std::free((char*)ch);//or std::free(void*)ch);

In C++ the equivalents of malloc/free are operator new/delete,
not new/ delete-expression:

    #include <new>
    int main()
        char const* c = (char const*)operator new(5);
        operator delete((void*)c); // need a cast here

The main argument here is that since const-ness doesn't prevent
calling the destructor in something like:

        MyClass const object ;
        // ...
        // object.~MyClass() called here.

it shouldn't be an issue in:

        MyClass const* pObject = new MyClass ;
        // ...
        delete pObject ;

(That's the argument given. Up to you to decide whether it is
convincing or not.)

James Kanze (GABI Software)
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