On Fri, 07 Mar 2003 11:46:24 +0100, Sam Partington wrote:
> Hate to sound pushy, but I've no answer on this, were the patches ok?
>> >> Daniel Frey wrote:
>> > No problem. IIRC it was Peter Dimov who came up with the safe-bool
>> > idiom first. At least I saw it first from him. Another way which
>> > works but results in worse error messages is this:
>> >
>> > template <class T, class B = ::boost::detail::empty_base> struct
>> > bool_testable : B
>> > {
>> > private:
>> > operator int() const;
>> >
>> > public:
>> > operator bool() const
>> > {
>> > return !!static_cast< T& >( *this );
>> > }
>> > };
One problem I just found with your implementation (and thus with Peter's
idiom in general) is, that is doesn't provide an operator bool(). This
may sound weird, an example hopefully clears things up:
#include <iostream>
using namespace std;
#include <boost/operators.hpp>
struct A : boost::bool_testable< A >
{
bool operator!() const { cout << "operator!" << endl; return false; }
operator int() const { cout << "operator int()" << endl; return 42; }
};
int main()
{
A a;
if( a )
cout << "Hello, world!" << endl;
// int i = a;
// long l = a;
}
The code above happily compiles without warnings but it calls operator
int(). Several options come to mind:
- Document it that the class T of bool_testable< T > shall not have any
other operators that might conflict.
- Use the above alternative implementation I showed. It is not as nice as
yours/Peter's wrt error-messages, but it works as expected.
- Convince the standard committee that "explicit operator bool" is needed
:) See:
http://groups.google.de/groups?hl=de&lr=&ie=UTF-8&frame=right&th=d0969f0fa2460dd4&seekm=3A2D10EE.35544D0A%40aixigo.de#link1
Thoughts?
Regards, Daniel
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