> One is conversion to a pointer type: in this case you can have
> a source expression of any integer type, but it must be constant
> and have the value zero. Example:
>
> int * p = '\0';
>
> enum { e = 5 };
> int (*pf)() = 4 % ( e - '\001');
>
> If you use the indeterminate article "a"/"an" in the wording above
> ("an" rvalue of type From) does it mean: "all" expressions of type
> From or "at least one" expression of type From? Of course, to
> exclude int->pointer-to-object convertibility one would say "all"
> expressions, but I'm not sure whether this is ok for UDTs.
>
>
> The other case is the deprecated conversion const char [N] -> char*
> which is valid only if the source expression is a string literal.
OK, both of those are special conversions of literals - I think we can
dismiss these by taking John Spicer's suggestion, and saying that the source
type is an lvalue of type From. (in other words is_convertible does not
detect those conversions).
>
> - Another point that I would like to understand is "where"
> convertibility is checked. Currently convertibility is checked in the
> scope of is_convertible<> but is this the intent? This makes a
> difference when you have e.g. a private conversion operator:
>
>
> class X {
> operator int(); // private
> friend class Y;
> };
>
> class Y {
> void f() {
> is_convertible<X, int>::value; // true or false??
> // or error?
> }
> };
Now that one is just plain nasty :-( It's a similar problem to what happens
if the To type has a private constructor taking the From type, my gut
feeling is that both these cases have to be listed as "undefined behaviour".
Clearly is_convertible<T,U>::value must have a single value under the one
definition rule, so there can be no ambiguous context-sensitive answers.
Thanks for bringing these up, I'll try and produce some better wording over
the weekend, and then I'll see if you guys can shoot it down again :-)
John Maddock
http://ourworld.compuserve.com/homepages/john_maddock/index.htm
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