RE: Inherited member( void )const - const to be or not to be

[lrtaylor@micron.com]

Basically, you can't have a const function returning a non-const pointer
or reference to that object, because that would then allow you to call a
non-const function on the object, violating the fact that you declared
the first function as const.  If you want to get a non-const pointer to
the object, then you can't declare your getProvider function as const,
because it really isn't, even though it is not changing any members of
the class.

Thanks,
Lyle


-----Original Message-----
From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
Behalf Of SVisor
Sent: Tuesday, September 07, 2004 10:35 AM
To: gcc-help@gcc.gnu.org
Subject: Re: Inherited member( void )const - const to be or not to be


Hi and thanks for the reply, but I do not agree...

...
> Don't use _CONST ... that's a reserved symbol.

Well that was news. Anyway it was just "dummy" code.

...
> All symbols starting with underscore followed by a capital letter are 
> reserved.  All symbols with two underscores in a row anywhere are
reserved.

name__more is not reserved. __name is reserved. Usage of _Name is not 
recomended, but what I know not reserved. At least thats what I have 
been taught. No one has _ever_ shown proof of anything else (heck no one

has shown me proof of what Ive learned, I just have accepted it ;-). Do 
you have any link to a list of which combinations are reserved and which

are not?

> I've fixed your code, works now just fine.  Compiled either way.

NO! You _broke_ it! Now you return a pointer to constant, and thats 
_NOT_ what I wanted. I wanted to hint the compiler that a function does 
not change members, but I can still change members (and call non 
constant member-functions) using the returned pointer (and its legal 
C++). The question was more about cast from "this", like this:

virtual CProvider* getProvider( void )const{ return (CProvider*)this; }

It will compile and provide sane working code, but its C (not C++) 
styled. And the cast is not _safe_: The code would compile even if the 
class would not inherit from CProvider.

...
> BTW:  using the -DCONST=const is preferred.  So much so, that you
should 

I do not agree as thats not portable across all compilers. While my 
construct is (well 99.9% of them at least). The code was just dummy code

to show a point, and make it easy to test. You usually know if something

is const or not :-).

// Jarmo

-- 

> just have const in the code.  Const correctness should be worked in
from 
> the beginning.
> 
> HTH,
> --Eljay
> 
> - - - - - - - - -
> // Compile #1:  g++ -DCONST=const foo.cpp
> // Compile #2:  g++ -DCONST= foo.cpp
> #include <cstdio>
> 
> class CProvider
> {
> protected:
>     const char* str;
> public:
>     CProvider(const char* s)
>     : str(s)
>     {
>         printf("CProvider(%s)\n",str);
>     }
> 
>     void echo() CONST
>     {
>         printf("CProvider(%s)::echo()\n", str);
>     }
> };
> 
> 
> class CAbstract
> {
> public:
>     CAbstract()
>     {
>         printf("CAbstract\n");
>     }
> 
>     virtual ~CAbstract()
>     { }
> 
>     virtual CProvider CONST* getProvider() CONST = 0;
> };
> 
> 
> class CAllocated : public CAbstract
> {
> protected:
>     CProvider* ptr;
> 
> public:
>     CAllocated()
>     {
>         printf("CAllocated\n");
>         ptr = new CProvider("Allocated");
>     }
> 
>     virtual CProvider CONST* getProvider() CONST
>     {
>         return ptr;
>     }
> };
> 
> 
> class CInherited : public CAbstract, public CProvider
> {
> public:
>     CInherited()
>     : CProvider("Inherited")
>     {
>         printf("CInherited\n");
>     }
> 
>     virtual CProvider CONST* getProvider() CONST
>     {
>         return this;
>     }
> };
> 
> 
> int main()
> {
>     CAllocated tmp1;
>     CInherited tmp2;
> 
>     tmp1.getProvider()->echo();
>     tmp2.getProvider()->echo();
> 
>     CAbstract* pTmp1 = &tmp1;
>     CAbstract* pTmp2 = &tmp2;
>     pTmp1->getProvider()->echo();
>     pTmp2->getProvider()->echo();
> }
> 
>