Re: const pointer to class problem

> 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.

It *is* a bad error message, but it's trying
to tell you that you're not respecting the
promise you made with by qualifying the pointer
with "const" in the parameter list: not to
change the object either by direct access to
its data or by calling a method on it that hasn't
made that promise, such as Version().

The way to tell the compiler that Version() promises
not to change the object it's called on is
with this unfortunate syntax:

  int Version(void) const
  {return m_ver;}

That "const" after the parameters means that
the implicit "this" pointer will be treated as
if declared "const FOO*" rather than just "FOO*",
which is the mismatch that the compiler is ineptly
trying to tell you about.  It just throws in
the ampersand to throw you off.

Sean

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