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Posted 12 Jan 2010

Reusable safe_bool Implementation

, 12 Jan 2010
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Reusable safe_bool implementation
safe_bool - what & why[^]?

In my understanding their implementation is broken (if fixable), but in the course I came up with my own that appears slightly more elegant to me:

template <typename T>
class safe_bool 
   typedef void (safe_bool::*bool_type)() const;
   bool_type to_bool_type(bool b) const 
    { return b ? &safe_bool<T>::safe_bool_true : 0; }
   void safe_bool_true() const {}
   bool operator ==(safe_bool<T> const & rhs); 
   bool operator !=(safe_bool<T> const & rhs); 

Can be used like this:

struct A : public safe_bool<A>
   operator bool_type() const { return to_bool_type(true); }
   // replaces:
   // operator bool() const { return true; }

From all samples I found that's the cleanest to write and read, while only the two symbols bool_type and to_bool_type "escape" to derived classes.

I am putting this up for review, since I'm clearly at the limits of my understanding of the standard. However, the following test cases pass on VC 2008:

struct A : public safe_bool<A>
   operator bool_type() const { return to_bool_type(true); }
struct B : protected safe_bool<B> // protected is ok, too
   operator bool_type() const { return to_bool_type(false); }
struct C : public B
   operator bool_type() const { return to_bool_type(rand() < RAND_MAX / 2); }

void SafeBootCompileFails()
   A a, a2;
   B b;
   C c;
   if (a) {}      // Yes!
   if (!a) {}     // Yes!

   a=a;           // Yes! (default assignment op should be allowed)
   a=a2;          // Yes! (default assignment op should be allowed)

   long l = a;    // No!

   if (a==a2) {}  // No!
   if (a!=a2) {}  // No!
   if (a<a2)  {}  // No!

   if (a==b) {}   // No!
   if (a!=b) {}   // No!
   if (a<b)  {}   // No!
   if (b==c) {}   // No!
   if (b!=c) {}   // No!
   if (b<c)  {}   // No!

   a=b;           // No!

   b = c;         // Yes, slicing. Not my problem today.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Germany Germany
Peter is tired of being called "Mr. Chen", even so certain individuals insist on it. No, he's not chinese.

Peter has seen lots of boxes you youngsters wouldn't even accept as calculators. He is proud of having visited the insides of a 16 Bit Machine.

In his spare time he ponders new ways of turning groceries into biohazards, or tries to coax South American officials to add some stamps to his passport.

Beyond these trivialities Peter works for Klippel[^], a small german company that wants to make mankind happier by selling them novel loudspeaker measurement equipment.

Where are you from?[^]

Please, if you are using one of my articles for anything, just leave me a comment. Seeing that this stuff is actually useful to someone is what keeps me posting and updating them.
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Comments and Discussions

GeneralKind of short on the why Pin
Tim Craig12-Jan-10 9:30
memberTim Craig12-Jan-10 9:30 
GeneralRe: Kind of short on the why Pin
peterchen12-Jan-10 10:49
memberpeterchen12-Jan-10 10:49 
JokeRe: Kind of short on the why Pin
Jörgen Sigvardsson13-Jan-10 12:40
memberJörgen Sigvardsson13-Jan-10 12:40 
GeneralRe: Kind of short on the why Pin
peterchen13-Jan-10 13:24
memberpeterchen13-Jan-10 13:24 
Laugh | :laugh:

I got to keep that in mind for next time!

Personally, I love the idea that Raymond spends his nights posting bad regexs to mailing lists under the pseudonym of Jane Smith. He'd be like a super hero, only more nerdy and less useful. [Trevel]
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