Re: operator overload question

From:
Ulrich Eckhardt <eckhardt@satorlaser.com>
Newsgroups:
microsoft.public.vc.language
Date:
Thu, 19 Oct 2006 12:58:59 +0200
Message-ID:
<kc2k04-vu5.ln1@satorlaser.homedns.org>
Olli wrote:

1>c:\programme\microsoft visual studio 8\vc\include\xutility(256) :
error C2666: 'eoScalarFitness<ScalarType,Compare>::operator <' : 2
overloads have similar conversions


In some code it finds a use of the less-than operator where it could use
both the builtin one for double values and the one for eoScalarFitness.
What I wonder is what code that is, as it might lie in the call and not the
code that actually causes the error.

template<class _Ty1, class _Ty2> inline
bool __CLRCALL_OR_CDECL _Debug_lt(
     const _Ty1& _Left, _Ty2& _Right,

                          ^^^^^^^^^^^^^

const wchar_t *_Where, unsigned int _Line)


The above marked part seems strange to me, I would have expected the call to
use a const reference instead. Not sure if that changes anything.

template <class ScalarType, class Compare >
class eoScalarFitness
{
     public :

     eoScalarFitness() : value() {}
     eoScalarFitness(const eoScalarFitness& other) : value(other.value) {}
     eoScalarFitness(const ScalarType& v) : value(v) {}


This ctor should be explicit, I think....

     eoScalarFitness& operator=(const eoScalarFitness& other)
     { value = other.value; return *this; }
     eoScalarFitness& operator=(const ScalarType& v)
     { value = v; return *this; }

     operator ScalarType(void) const { return value; }


.... and this conversion should be explicit, too. Unfortunately C++ doesn't
recognize (yet) the 'explicit' keyword in conversion operators, so you
should instead use a get() function or somesuch. I guess that that is also
the problem: if you try to compare this object with an object of the
ScalarType, it could either use the non-explicit ctor to create another
oeScalarFitness or use the conversion operator to get a ScalarType.

     /// Comparison, using less by default
bool operator<( const eoScalarFitness & other) const { return
Compare()(value, other.value); } // THIS IS THE FAULTY? OVERLOAD


The overload itself is not faulty, although I personally would have made it
a free function. That way you can also implement the other two (possibly
necessary) overloads:

  wrapper < wrapper // given here as member
  wrapper < scalar // missing
  scalar < wrapper // missing
  scalar < scalar // provided by scalar type

The same of course applies to the other comparison operators (which lack ==
and != for completeness, btw).

If these comments don't help, please provide a minimal(!) example, as is
customary on the Usenet.

Uli

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