Re: How to cast from const to non-const
From: Liviu Andron (landron_at_bitdefender.com)
Date: 11/23/04
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Date: Tue, 23 Nov 2004 19:51:47 +0200
"Liviu Andron" <landron@bitdefender.com> wrote in message
news:#Ial$PX0EHA.3596@TK2MSFTNGP12.phx.gbl...
> "Simon Trew" <noneofyour@business.guv> wrote in message
> news:#Tmts2I0EHA.1524@TK2MSFTNGP09.phx.gbl...
> > "Mike Gleason Jr Couturier" <mikejrMAPSON@videotron.ca> wrote in message
> > news:%2310O4nnzEHA.2012@TK2MSFTNGP15.phx.gbl...
> > > First I totally agree that one should never have
> > > to use const_cast.
> > >
> > > But doesn't const checkings are only at compile time ?
> > > Why would that produce undefined behavior then ? Why
> > > would const_cast exists ?
> > >
> >
> > const qualification affects the mangled (decorated) signature of the
> > function in the linker, so it's not quite purely a compile-time thing.
But
> > yes, the constness rules are in general enforced by the compiler.
> >
> > The problem is that if you want your code to be rigorously const
correct,
> > every API that it uses must also be const correct. If you're dealing
with
> > 3rd party APIs and technologies this may not be the case. (For example,
> COM
> > doesn't have the notion of const.) const_cast was introduced explicitly
> to
> > enable constness (and only constness) to be cast away when an API wants
a
> > non-const pointer/reference but your own code has a const one. However,
as
> a
> > programmer you have to be sure that the API really doesn't modify the
> data,
> > for this to be safe.
> >
> > It is also fairly common to use a data structure to pass data between an
> app
> > and both "getter" and "setter" functions. For example, ListView_SetItem
> and
> > ListView_GetItem both take a LVITEM structure. In this structure, there
is
> a
> > pointer to the string of text, LPTSTR pszText. The getter method will
fill
> > this in, but the setter item will use whatever you supply. You'd like to
> be
> > able to supply a const version of LPTSTR, i.e. LPCTSTR, but you can't of
> > course. So const_cast comes in quite handy in these cases, again, when
you
> > know the API doesn't actually modify the data.
> >
> > Another use is to get a non-const version of the "this" pointer in a
const
> > member function, for example, where a member function that is logically
> > const actually mutates the object's member data, e.g. by cacheing
> > information. It can often be undesirable to remove the constness of the
> > member function. However, in this case it's usually better to use the
> > mutable keyword on the member data declaration.
> >
> >
>
> Another use (when you don't control received parameters) ?
>
> struct MyStruct{
> ...
> TCHAR path[MAX_PATH];
> ....
> };
>
> bool func( const MyStruct &st)
> {
> ...
> (const_cast<MyStruct&>st).path[MAX_PATH - 1] = _T('\0');
> ...
> }
>
>
and another ( it's actually a 'hack', but...): a string fragment (char* +
size) when calling a function that receives a null-terminated string ( save
the last character, force a const_cast, use the function, restore the last
character)
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