On 26 September 2011 09:32, Jon Grant wrote: > Hello > > Jonathan Wakely wrote, On 26/09/11 08:10: >> >> On 26 September 2011 05:29, Ian Lance Taylor wrote: >>> >>> Jon Grant<j...@jguk.org> writes: >>> >>>> Currently gcc, and g++ don't give a warning when a pointer was >>>> converted to a bool, in the same way it is for other types. >> >> At least in C++, it's not really true to say "in the same way it is >> for other types" because you cannot convert from a pointer to any >> integer type except bool. Your test uses NULL for the other integer >> types, which is an integral constant expression with value zero, so >> it's ok to convert that to an integer type. That's not true for >> general pointer values: if your test used m_int(g_glob) then it >> wouldn't compile. > > Good point. My test should have used g_glob due to NULL being a macro of 0 > in C++.
That wouldn't compile. A pointer to boolean conversion is safe and well-defined. >>> There is a lot of code which uses >>> if (p) >>> where p is a pointer to test whether p is not NULL. I don't think we >>> could reasonably have gcc warn about such a case. >>> >>> We might be able to separate out conversion to bool on assignment from >>> conversion to bool in a test, though. >> >> That would still break this: >> >> Base* p = getObj(); >> bool is_derived = dynamic_cast<Derived*>(p); >> >> What problem is the warning supposed to solve? > > A programmer assigning a bool with a pointer, there's an implicit evaluation > there isn't there? rather than: > > bool invalid = (NULL == p); Why is that preferable? > I expect this depends on what the standard allows then. 4.12 Boolean conversions [conv.bool] 1 A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.
