Re: const pointer to class problem

On Apr 17, 8:33 pm, t.wa...@[EMAIL PROTECTED]
 wrote:
> I'm trying to slowly convert some C code to C++ and have encounter
> what's probably a trivial problem.
>
> The easiest way to describe the problem is with an example:
>
> class FOO
>   {
> public:
>     int Version(void){return m_ver;};
>     FOO(){m_ver = 1;};
> private:
>     int m_ver;
>   };
>
> void PrintVer(const FOO * fp)
>   {
>   int v = fp->Version();  /* error here */
>
>   cout << "The version is " << v << endl;
>   }
>
> When I try to compile, I get the error
> 'FOO::Version' cannot convert 'this' pointer from 'const FOO' to 'FOO
> &'
>
> on the line containing fp->Version().
>
> If I remove the const from the routine declaration, it works fine, but
> I'd rather not do that.   I guess I don't understand what the compiler
> is trying to tell me, and have no idea of how to work around this
> without removing the const.

{ edits: quoted banner (see the end of this article) removed, please don't
quote 
extraneous material  --  even if the clc++m banner is a very fine one.
-mod }

You have a constant pointer, but your Version method does not promise
the compiler that it will not alter any member data. Any call made
with a constant pointer must promise not to alter member data. To make
such a promise change to:

class Foo
{
public:
    const int Version() const  // That const after the param list makes
the promise
    {
       // Note you are returning by value
       // making a copy of the member data, adhering to your promise
       return m_ver;
    }

    Foo()
    :
    // Initializer list is better form
    m_ver(1)
    {
    }

private:
    int m_ver;
};

-- 
      [ See http://www.gotw.ca/resources/clcm.htm
for info about ]
      [ comp.lang.c++.moderated.    First time posters: Do this! ]