Re: Template Base Class with Factories

Vidar Hasfjord <>
Wed, 28 Jan 2009 21:51:11 -0800 (PST)
On Jan 28, 5:18 pm, wrote:

Hi, I'm going to use a simplified example to ask my question.

Suppose I have a base class:

template <class T> class BaseClass {


with a couple of classes which inherit it

class Example1 : public BaseClass<int> {


class Example2 : public BaseClass<float> {


Question: How do I write a Factory class to return a reference to
BaseClass? Such as:

class FactoryClass {

          template <class T> BaseClass<T> *Create(int option) {

 (option) {


 case 0 : return(new Example1);


 case 1: return(new Example2);



Where in my code I would call it as such.....

  FactoryClass thisFactory;
  BaseClass<???> *thisBaseClass1 = thisFactory.Create(0);
  BaseClass<???> *thisBaseClass2 = thisFactory.Create(1);

Now, I know that BaseClass is defined with a template parameter, but
in this case I do not know what that parameter is until the factory
creates it. Is there a way to define the local reference to BaseClass
in the code such that "I don't know what the type is and I don't care
at this point"? Go easy on me, I'm relatively new to templates.

You lack a common base class. Despite its name BaseClass is a template
that generates unrelated base classes. One possible solution is to
derive the generated base classes from a common ancestor:

  class GrandBase {/*...*/};

  template <class T>
  class BaseClass : public GrandBase {/*...*/};


  FactoryClass f;
  GrandBase* a = f.Create (0);
  GrandBase* b = f.Create (1);

Vidar Hasfjord

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