Safe Boolean Idiom cannot compile with Visual C++ 10(2010)

[CBasicNet]

Hey guys, I have derived my class from the C++ safe bool idiom class from this page : The Safe Bool Idiom by Bjorn Karlsson

class Element : public safe_bool<>
{
public:
	bool Exists() const;
	bool boolean_test() const { return Exists(); }; // boolean_test() is a safe_bool method
}

When I tried to use it in the if expression like below

Element ele;
...
if(ele)

I got an error like error C2451: conditional expression of type 'Element' is illegal. If I try to cast it to bool, I got this error,

Element ele;
...
if((bool)ele)

error C2440: 'type cast' : cannot convert from 'Element' to 'bool'

This is the 1st time I am using safe bool idiom, I am not sure if this is not allowed or a bug in Visual C++ 10. Any comments?

[Amleto]

need to see your safe_bool<> code

also, you have semicolon bool boolean_test() const { return Exists(); }; <-- here. Why?

and you missed semicolon from the end of your class dec.

Your code examples wont compile due to syntax error. suggest you copy your code here, not re-writre it.

[itsmeandnobodyelse]

The sample code at end of the article compiled well in VS2008 after I made the member function this_type_does_not_support_comparisons a public function.

Did you also copy the template functions?

[Amleto]

The sample code at end of the article compiled well in VS2008 after I made the member function this_type_does_not_support_comparisons a public function.

Did you also copy the template functions?

the article says the whole point of that function is that it isnt public.

[monarch_dodra]

If you've been following the evolution of C++, then you should now that safe bool is almost obsolete:

C++0x provides explicit operators. This means you can cast your object to a bool, but ONLY if you explicitly ask for it.

This is combined by the fact that in C++0x, expressions that expect a boolean are allowed to make implicit calls to the explicit operator bool.

The end result, is that your object will never be implicitly cast to bool, but writing "if(my_object)" will still be valid.

...

In my professional experience, I would advise against using a cast to bool though, unless your object models a pointer concept. Providing a member is_valid() is just as easy, and more explicit/clear to users who are not used to using your class.

As mentioned by the article, the notion of "cast to bool" can be ambiguous, and may be dangerous.

Its a case by case issue of course, so you'll have to be the judge.

[itsmeandnobodyelse]

the article says the whole point of that function is that it isnt public.

VS2008 won't return a function pointer of a protected member function.

  void f()
  {
     int i = 0;
     Testable_with_virtual twf;
     if (twf)   // compile error
         i++;
     Testable_without_virtual twof;
     if (twof)  // compile error 
         i--;

  }

error C2248: 'safe_bool_base::this_type_does_not_support_comparisons' : cannot access protected member declared in class 'safe_bool_base'

[CBasicNet]

In my professional experience, I would advise against using a cast to bool though, unless your object models a pointer concept. Providing a member is_valid() is just as easy, and more explicit/clear to users who are not used to using your class.

As mentioned by the article, the notion of "cast to bool" can be ambiguous, and may be dangerous.

Its a case by case issue of course, so you'll have to be the judge.

I guess you are right! I have dropped the idea of using the safe bool idiom. My library users can call Exists() method.