Re: const pointer to class problem

On 18 abr, 03:33, 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.

Hello,

FOO::Version() is not a const member function, so it cannot be invoked
by a const FOO object, or via a pointer (or a reference) to a const
FOO object. It makes sense to mark FOO::Version() as a const member
function, because it does not mutate its object (it merely inspects
the m_ver member variable of its object):

#include <iostream>

class FOO
{
public:
    int Version() const { return m_ver; }
    FOO() : m_ver(1) {}

private:
    int m_ver;
};

void PrintVer(const FOO& fp)
{
    int v = fp.Version();

    std::cout << "The version is " << v << std::endl;
}

int main()
{
    FOO f;
    PrintVer(f);

    return 0;
}

Note the use of an initialization list in the default constructor of
FOO [1] and the change of the parameter type of PrintVer() to a
reference to a const FOO (it is generally preferable to pass an
argument via a reference instead of passing it via a pointer unless
the argument can be legitimately null).

[1] See: http://www.para****ft.com/c++-faq-lite/ctors.html#faq-10.6

Regards,
David

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