Re: A little confused with this (const ptrs)

Rolf Magnus <>
Mon, 15 May 2006 18:37:11 +0200
Bit byte wrote:

const Bar* temp = NULL ;
//retrieve and use
temp = Foo::getBar();
delete temp ; // <- compiler did not complain (what's happend ? - has
Foo::Bar really been deleted?)

Yes, it has. Do you have any reason to believe it hasn't?

The line 'const Bar* temp = NULL ;' declares a variable temp which is
allocated on the stack. I later on ASSIGN (not allocate) the address of
a constant ptr to temp (or atleast thats what I thought I was doing) -
and then it seems I can just go ahead and destroy the const object
pointed to by ptr. So what's the bloody use of const? It it does not
protect others from modifying your 'const' object.

It does. However, note that destroying an object doesn't count as modifying

Consider this:

#include <iostream>

class X
    X(int i) : i_(i) {}
    int i_;

void f()
    const X* x = new X(3);

    std::cout << x->i_ << '\n';

    delete x;

Wouldn't you say that the object pointed to by x is never modified in f()?
If, not, let's go one step further:

void f()
    const X x = 3;
    std::cout << x.i_ << '\n';

Has the object been modified in f() now? It was created at the beginning of
the function and destroyed at the end, just like in the previous example.
If destruction counted as modification, you could basically not define any
constants at all, because they couldn't been destroyed when they go out of

Basicaly, this is what I want to do:

1). store a copy of Bar 'safely' in Foo
2). Be able to request a ptr to Bar and use it without being able to
modify it
3). I want Foo::setBar() to be able to replace (i.e. destroy first, then
replace) the original Foo::Bar* variable


1). How do I do that?

Well, you already are doing that. I don't see your problem.

My problem is that I want to be able to hand a objet which I can create
and modify at will within the (container class), but when handing the
object to another class etc, I want to explicitly mark the object as
'handle with care' - do not modify or else ...
It would seem that I was labouring under a misaprehension about 'const'
- since the object I had decorated as a const was summarily executed as
soon as I let it out of the confines of my class - and you do not seem
suprised, and do not see what my problem is. That is what my problem is.

I could have looked more carefully. Then I would probably have seen that you
thought destruction counted as modification.

I want to have a class that contains an object (actually a structure) -
lets call it data type A. I want the container class to have a method
which allows it to accept a new variable of data type A, and then
replaces its previous variable of data type A with the newly received one.

This same class, will receive requests by other classes, who want to use
the object. However, I do not want any class receiving this
class/structure, to be able to modify (and certainly not delete) the
received object.

Well, my question would be, why anyone would want to delete an object he
hasn't created himself and wasn't told to delete? Seriously, when I get a
pointer from some API function, I would never get the idea to delete it
unless the function's documentation explicitly tells me to do that.
Anything else is plain stupid.

This is the problem I'm trying to solve. Any pointers (no pun intended)
wll be much appreciated.

One thing you can do is return a reference to the object. However, the user
of your class can still take its address and delete the object. But with
the reference, the user might not have the idea to delete the object just
for fun.
C++ only offers features to protect against accidental errors, not about
deliberate sabotage.

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