Factory design

[Carlos Santander]

John C escribió:
> So I've got an abstract class with a factory method. Users call the 
> factory method to instantiate an instance. It calls a function which 
> modules that contain the contrete class must implement.
> 
> But I'm getting linking errors. OptLink thinks 'createXmlReaderImpl' is 
> undefined, presumably because it's expecting the body to be contained in 
> the same module.
> 
> Note that if 'createXmlReaderImpl' is declared with 'extern(C)', then it 
> links without errors.
> 
> I've seen a similar technique used before. The main entry point in 
> dmain.d in phobos, for example. Ideas?
> 
> module model;
> 
> // Modules containing the concrete class implement this.
> XmlReader createXmlReaderImpl(Stream input);
> 

How about putting it in an abstract class, say a singleton?

> abstract class XmlReader {
> 
>   static XmlReader create(Stream input) {
>     return createXmlReaderImpl(input);
>   }
> 
>   // Abstract methods...
> 
> }
> 
> // impl1 implementation wraps some C library.
> module impl1;
> import model;
> 
> XmlReader createXmlReaderImpl(Stream input) {
>   return new XmlReaderImpl(input);
> }
> 
> class XmlReaderImpl : XmlReader {
> 
>   // Implements XmlReader's abstract methods
> 
>   private this(Stream input) ...
> 
> }
> 
> // impl2 implementation wraps some other C library.
> module impl2;
> import model;
> 
> XmlReader createXmlReaderImpl(Stream input) {
>   return new XmlReaderImpl(input);
> }
> 
> class XmlReaderImpl : XmlReader {
> 
>   // Implements XmlReader's abstract methods
> 
>   private this(Stream input) ...
> 
> }
> 
> // User code that chooses which implementation to use.
> module program;
> import model, impl1; // Choosing impl1
> 
> void main() {
>   Stream s = ...;
>   XmlReader reader = XmlReader.create(s);
> }
> 
> Thanks,
> John.


-- 
Carlos Santander Bernal