Re: Design problem: Factory pattern needs 'static virtual'?

kwikius <>
Thu, 3 Jan 2008 16:38:50 -0800 (PST)
On Jan 3, 11:49 pm, Markus Dehmann <> wrote:

I need a simple object serialization, where loading an object from
file looks like this:

  Foo* foo1 = FooFactory::create("./saved/foo1.a321f23d");
  Foo* foo2 = FooFactory::create("./saved/foo2.eb287ac8");

Now, Foo is an abstract base class, and FooFactory contains a static
function which again calls static create functions on Foo1 or Foo2
(see code for all classes below). The problem is that all these
functions are static, but I also want them to be part of the (virtual)
Foo interface. But virtual static functions are not allowed.

You might say that Foo1::create(...) should not be a member function,
but just a global function outside of Foo1. But I want classes derived
from Foo1 to be able to inherit or override create(...).

How should I design this properly?

namespace FooFactory {
  Foo* create(const std::string& dataLocation) {
    const std::string type =
    if(type == Foo1::type()){
      return Foo1::create(dataLocation);
    else if(type == Foo2::type()){
      return Foo2::create(dataLocation);

look up "static polymorphism" or named conformance to an interface

Simply put , use a static create function and if a derived wishes to
override it adds its own static create function, which hides that in
the base class otherwise the compiler will find the base class
version, In fact there is no need for derivation, as any class with a
static create function , or whose base has a static create function
can be used, assuming the return type is viable.

Andy Little

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