Re: Design Pattern for Dynamic Class Loading

From: (Pascal J. Bourguignon)
Tue, 29 Jul 2008 16:56:40 +0200
digz <> writes:


Apologies if this is the wrong group for this question. I want to
design an interface , where for a custom functionality , the client
writes a new class with the function implementation and then puts the
name of the Class as a configuration parameter or something and then
the main program loads the new class like a module and the client can
now write code using the new Functions. I suppose this problem has
been solved before , if so what is the best way to do this .


 Example :

I have already provided him with :
struct printInt {
void operator ()(int i)
{ Print(i); }

Now the user wants to write a class and start using the implementation
in the class which is
struct printDbl{
void operator ()(double i)
{ Print(i); }

int main()
Config* c = readConfig();
For name in c->classNames():
    Load( name ) ;

printDbl pd;

The easiest way to do that would be to use lisp. Really why are you
bashing your brains this way? Just use lisp, there's nothing
simplier to let the user add functions to a running lisp program.

Otherwise, it would be easier to do it with a more dynamic language,
like Objective-C. Apple's Xcode allows you to do that, when debugging
a program, you can correct a class, and reload it.
Of course, you can dynamically load classes or parts of classes in an
Objective-C program at will, and you can even send messages that were
unknown before hand from old code (using a -perform: method with a
computed selector).

In C++, that may be possible, but only thru virtual methods (or thru
normal C-like functions).

It won't help the "user" to provide him with printInt. You will have
to provide him with a superclass and a factory entry point:

class PrintStuff {
  virtual ~PrintStuff(){}

  virtual void setArgument(int anInt)=0;
  virtual void setArgument(double aDouble)=0;
  virtual void setArgument(std::string aString)=0;
  virtual doIt(void)=0;


typedef PrintStuff* (*FactoryFun)(void);
extern PrintStuff* factory(void);

Then he will implement a concrete subclass, and provide an implementation for the factory:

class PrintDouble:public PrintStuff {
  double value;
  virtual ~PrintDouble(){}

  virtual void setArgument(int anInt){value=anInt;}
  virtual void setArgument(double aDouble){value=aDouble;}
  virtual void setArgument(std::string aString){std::istringstream s(aString);s>>value;}
  virtual doIt(void){std::cout<<"My value is "<<value<<std::end;}


PrintStuff* factory(void){
  return(new PrintDouble());

Well compiled as a dynamically loadable library, this could could be
loaded into your running C++ program, that could then do:

int main(void){
   Config* c=new Config("myapp.conf");
   FactoryFun factory=loadAndFindSymbol(c->libraryName(),"factory");
   PrintStuff* obj=factory();

to implement loadAndFindSymbol, on unix systems have a look at dlopen,
dlsym, dlclose. There may be some difficulties with name mangling
too, perhaps it would be best to declare:
extern "C"{
   extern PrintStuff* factory(void);

__Pascal Bourguignon__

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