Re: How do I create a class from a string at runtime ?

"Daniel T." <>
Mon, 08 Oct 2007 23:30:09 -0400
TazaTek <> wrote:

Base* newObject( const string& s )
   Base* result = 0;
   if ( s == "classA" )
      result = new ClassA;
   else if ( s == "classB" )
      result = new ClassB;
   return result;


That makes sense.

Is there a way to implement this dynamically?

I think the answer to that depends on the development environment rather
than the language.

for example, if I later determined that I could have 100 classes
(instead of the 5 I mention previously) that could be used (not likely
as I'd re-design) and a case statement would become quite burdensome
to maintain.

There are ways of avoiding the case statement. For example creating a
map where the key is the string and the value is an object of the
appropriate type (or a builder of an object of the appropriate type.)
Then you could do something like this:

class Builder {
   virtual Base* create() = 0;

class Factory {
   map<string, Builder*> builders;
   void addBuilder( string s, Builder* b ) {
      builders[s] = b;
   Base* build( string s ) {
      base* result = 0;
      map<string, Builder*>::iterator it = builders.find( s );
      if ( it != builders.end() )
         result = it->second->create();
      return result;

With just a little bit of creativity, each of your 100 classes can have
a global object associated with it that automatically inserts itself
into the Factory as a builder of that class. In this way classes can be
added and removed from the system at link time without having to change
the Factory code. This doesn't really count as dynamic because it can
only be done during program construction though...

Frankly though, I don't see the point. The code that knows what string
to pass to the Factory could just as easily new the object it needs
directly since it obviously knows what the type of the object is.
(Unless of course, the string was input from outside the system.)

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