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. Regards, Daniel