Re: const pointer to class problem

On Thu, 17 Apr 2008 19:33:58 -0600, t.wanat 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;};
       //           ^                  ^
       // drop the "void"          extraneous ";"

       int Version() const {return m_ver;}
       //              ^
       //         add this

>     FOO(){m_ver = 1;};
       //               ^
       //         extraneous ";"
> 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().

This is because "const FOO * fp" declares a pointer to a const FOO
object; so *fp cannot be modified in the function PrintVer(). But the
function FOO::Version() does not guarantee that the FOO object it is
invoked on will not be modified, so the compiler complains. You can
guarantee that FOO::Version() will not modify its invoking object by
declaring it "const" (see above).

Note that declaring the parameter to PrintVer() as "FOO* const fp" would
also solve your problem; it says that the *pointer* fp is const (rather
than the object it points to). It depends what you want, really... if the
function FOO::Version() really does not modify its invoking object it
should be declared const; and if PrintVer() does not modify the pointer
fp *or* the object it points to (as I suspect is your case), then the
parameter should probably be declared as "const FOO* const fp".

> 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.

Hope this helps,

-- 
Lionel B

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