c++ - Factory methods in derived classes -

i have 2 kinds of messages: messagea , messageb, both derived abstract class imessage containing pure virtual method std::string tostring(). can convert each message string representation pointer base class. that's fine. however, need somehow construct message (concrete type of message) string, example: messagea* msg = [something].fromstring( str ). null if given string isn't suitable construction of messagea. can see 2 approaches task:

a) messagefactory dynamic_cast

    class messagefactory     {         imessage* fromstring( const std::string& str );     };      ...     messagefactory mf;     messagea* msg = dynamic_cast< messagea* >( mf.fromstring( str ) );     if ( msg ) { ... }  

however, uses dynamic_cast avoid.

b) factory method in each derived class

static messagea* fromstring( const std::string& str ) {     return stringisok( str ) ? new messagea() : null; } 

is there better solution? should change in general design? thank you.

update

sometime know kind of message should string, i.e.

void sendrequest() {  ...  std::string response;  messagea* msg = fromstring( response );  // here should check if string valid messagea } 

but sometime don't know come me:

void processmessage( const std::string& str ) {      imessage* msg = fromstring( str );      if ( msg )      {          messagea* msga = dynamic_cast< messagea* >( msg );          if ( msga )          ...      }  } 

you make classes derived imessage register (or rather specific factory) messagefactory class statically @ program start. can achieved using static instance. imessage interface needs have pure virtual method canconstructfrom(string&) such when string comes in, pass messagefactory , factory find out of derived classes can constructed string , instantiate correct one.