Daniel Frey wrote:
On Sun, 2008-08-10 at 17:36 +0200, Mathias Gaunard wrote:
Isaac Dupree wrote:
I'm looking at all these "operator unspecified_bool_type"s in Boost. They vary:
It's hard to get all of these right, and various Boost libraries have made changes in the past that got it wrong: type-safety compiler workarounds zero runtime overhead (a.k.a. more compiler workarounds) At some time, there was a safe-bool thingy in Boost.Operators, but it was removed. I honestly do not know why.
The code that landed in CVS for a time is not based on the safe-bool idiom, but on an alternative approach: It simply adds a private declaration of an "operator signed char() const". When the used implements "operator bool() const", conversion to bool works while conversion to other types are ambiguous (e.g. int) or private (e.g. signed char).
The problem was that it broke is_convertible: is_convertible<A,int>::value resulted in a compile-time error instead of returning false - which is the reason why the code was removed from CVS before it was ever released.
argh! And there was a good reason for that private operator signed char. Which is equivalent to providing several operator some-int()s for every builtin integral type. If we did that, at least it would be just an access error rather than an ambiguity error*. But I'm not sure if that helps us. Conceivably, safe_bool could friend is_convertible and is_convertible could special-case for it. But how could it detect when a derived class actually does implement an operator? Can is_convertible<T,int> where T is directly or indirectly derived from safe_bool<>, take the address &T::operator int, and then see whether the class type of the resulting value is of type safe_bool<> (in which case it's not convertible), or else it'll be of some derived type, in which case it is convertible. I'm not sure if anything like that is possible in the standard, let alone the implementations we support. But maybe it is? (for a large penalty in hack-ness.) * (Which might make the error messages better? or worse?) -Isaac