Is this program legal?

   #include <algorithm>

   struct Useless {};

   struct Evil {
      operator bool() const { return true; }
      Useless operator!() const { return Useless(); }
   };

   struct Comp {
      Evil operator()( int, int ) { return Evil(); }
   };

   int main() {
      int a[10];
      std::binary_search( a, a+10, 5, Comp() );
   }

I think it is; I think std::binary_search only requires that its
comparator argument have a result type that is "convertible to bool" and
thus "Evil" counts.

Nevertheless, I have seen implementations that work along the lines of

   template <class FI, class T, class Comp>
   bool binary_search(FI first, FI last, const T& val, Comp comp) {
     FI i = lower_bound(first, last, val, comp);
     return i != last && !comp(val, *i);
   }

and such a library implementation makes my program fail to compile.  My
impression is that the standard would require a conforming
implementation to do something like

   template <class FI, class T, class Comp>
   bool my_binary_search(FI first, FI last, const T& val, Comp comp) {
     FI i = lower_bound(first, last, val, comp);
     bool b = comp(val, *i);
     return i != last && !b;
   }

This makes me glad I am not a library implementer.

Comments?

--
Brian McNamara

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"Brian McNamara!" wrote:
>
> Is this program legal?
>
>    #include <algorithm>
>
>    struct Useless {};
>
>    struct Evil {
>       operator bool() const { return true; }
>       Useless operator!() const { return Useless(); }
>    };
>
>    struct Comp {
>       Evil operator()( int, int ) { return Evil(); }
>    };
>
>    int main() {
>       int a[10];
>       std::binary_search( a, a+10, 5, Comp() );
>    }
>
> I think it is; I think std::binary_search only requires that its
> comparator argument have a result type that is "convertible to bool" and
> thus "Evil" counts.

The standard specifies in 25.3p2 that the function object must return
either true or false. Not a value of a type convertible to those values;
it must actually return one of those values.

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