From: use_my_alias_here@hotmail.com   
      
   >> Sometimes you need to inform that the value is missing. Some use   
   >> pointers to objects where NULL represents the missing value. Another   
   >> solution is to use boost::optional.   
   >   
   > The only other variant I could imagine is an empty container, which is   
   > rather similar to boost::optional.   
   >   
   >   
   >> However, sometimes you need to provide more second level information   
   >> than just missing value. How do you solve that?   
   >>   
   >> I came up with an idea below. Is that a good solution or are there   
   >> better ways to solve it? Does any library have anything similar? What   
   >> can be improved? Any feedback is welcome.   
   > [...]   
   >> enum Signal   
   >> {   
   >>     NOT_AVAILABLE,   
   >>     AVAILABLE,   
   >>     WEAK,   
   >>     DISTORTED   
   >> };   
   >>   
   >> typedef Multivalence<   
   >>     double,   
   >>     Signal,   
   >>     NOT_AVAILABLE,   
   >>     AVAILABLE> SignalDouble;   
   >>   
   >> SignalDouble getSample()   
   >> {   
   >>     //return SignalDouble();            // Missing value.   
   >>     return SignalDouble( 8.75 );        // Good value.   
   >>     //return SignalDouble( DISTORTED ); // Distorted value.   
   >> }   
   >>   
   >> int main( int argc, char* argv[] )   
   >> {   
   >>     SignalDouble sample = getSample();   
   >>   
   >>     switch( sample )   
   >>     {   
   >>        case NOT_AVAILABLE:   
   >>           std::cout << "No sample available" << std::endl;   
   >>           break;   
   >>   
   >>        case AVAILABLE:   
   >>           std::cout << "Sample: " << *sample << std::endl;   
   >>           break;   
   >>   
   >>        case WEAK:   
   >>           std::cout << "Signal too weak" << std::endl;   
   >>           break;   
   >>   
   >>        case DISTORTED:   
   >>           std::cout << "Signal distorted" << std::endl;   
   >>           break;   
   >>     }   
   >>   
   >>     return 0;   
   >> }   
   >   
   > I would certainly suggest this code as replacement here:   
   >   
   > try   
   > {   
   >   double d = getSample();   
   > }   
   > catch(std::exception const& e)   
   > {   
   >   std::cout << "failed to read sample: " << e.what() << std::endl;   
   > }   
   >   
      
   I agree that exceptions is the right way to deal with problem that arises,   
   maybe my example wasn't the best. But consider you would like to have an enum   
   like this.   
      
   enum SciVal   
   {   
     NOT_AVAILABLE,   
     AVAILABLE,   
     NEG_INF,   
     POS_INF   
   }   
      
   What I mean is that there might be values not representable with ordinary   
   types.   
      
   > Assuming that the example usage is just not representative for what you   
   > are actually doing, there is another version of writing what you do there:   
   >   
   > typedef boost::variant SignalDouble;   
   >   
   > The point is that reading a sample either returns a value, or it returns   
   > an errorcode explaining why there was no value. It does so using a   
   > well-known mechanism (are variants in C++11, btw?), which makes code   
   > clearer and more maintainable.   
   >   
      
   In a way, but it doesn't keep good track of missing values.   
      
   I think the C++11 union is unrestricted, making boost::variant obsolete.   
   http://en.wikipedia.org/wiki/C++11#Unrestricted_unions   
      
   Regards,   
   Daniel   
      
      
      
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