Re: how to avoid instantiation of template operator?

"John Moeller" <>
Tue, 27 Mar 2007 01:23:01 CST
On Mar 26, 5:19 am, "Michael Kilburn" <> wrote:

On Mar 26, 6:38 am, "Daniel Kr?gler" <>

Michael Kilburn schrieb:

The code should solve your problem, I guess.

   template<class C> friend
   typename enable_if<IsEqualComparable<T, C>::value,
   operator==(Self l, C& r) {
     return l.m_ref == r;

Thanks! Exactly what I needed :-). There are couple of challenges
left, though:

I think that I'm beginning to understand what you're going for.

- I'll need Is***<> template for every operator possible
- All of them should exclude implicit conversion (they'll be done by
corresponding conversion operator declared in accessor class) -- i.e.
if B and C can be converted to int IsEqualComparable(B,C) should be
"false", even if B() == C() will compile. This looks pretty
straightforward, actually... -- I guess I'll need to update your t() &
u() to return smth that could be converted (with user-defined
operator) to T & U respectively.

Ah. I think what you want is the double-conversion trick. No more
than one implicit conversion can be used in any one conversion, so I
believe that what you want is to replace t() == u() with a function
that takes arguments that are implicitly convertible from T and U, and
has the equivalent of t() == u() in the body. Something like:

// This class has only one implicit conversion; that is, to a W &
template <class W> class ImplicitConvert {
   W & ref_;
   // Make an explicit constructor
   explicit ImplicitConvert(W & ref) : ref_(ref) {}
   // Define an implicit conversion to W &
   operator W & () { return ref_; }
   // Don't define an operator==()!

You would replace "t() == u()" with "ImplicitConvert<T>(t()) ==
ImplicitConvert<U>(u())". This would ensure that the implicit
conversion is done by a step that you control (ImplicitConvert<T>(t())
to T &, for example), instead of the match for the operator==().

- and the latest and most scary -- I'll need to find result type of
every operator. So, at the end every operator should look like:

You'll probably need to define this explicitly for many operators.
What if A() + B() results in an instance of C?

template<class U, class restriction = SFINAE<is_XXX_defined<T, U> > >
typeof(t XXX u) operator XXX(Self l, U& r) { return l.m_ref XXX r; }

(not mentioning const versions & version where both arguments have
type 'Self'). I guess this is possible only under GCC, since MSVC does
not have 'reliable' typeof -- I am not sure if existing hacks will
help me here.

Since you're going through the effort of finding the return types of
all these operators, you could just specify it, instead of relying on
typeof, or maybe define a traits class for IsEqualComparable, or some
such. Additionally, you can usually count on (for example) operator==
taking const reference arguments, which will limit the specific cases
that you have to cover. It doesn't make sense for operator== to take
non-const arguments. The same kind of reasoning can be applied to
most operators. The case of T and U being the same class shouldn't
offer too much difficulty, but you could always define a
specialization that handled it if you had to.

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