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

James Kanze <>
Fri, 4 Jan 2008 01:11:55 -0800 (PST)
On Jan 4, 12:49 am, 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(...).

I'm not sure I understand what you're asking for. You have
different derived classes of Foo, each with its own function
create. You want the compiler to somehow call the correct
instance of create, according to the dynamic type. But the
dynamic type of what? How do you determine which Foo??::create
should be called?

How should I design this properly?


class Foo {
  // virtual static std::string type() const = 0; // not
  // virtual static Foo* create(const std::string& dataLocation) =
0; // not allowed
  // other functions that constiute the Foo

class Foo1 : public Foo {
  static std::string type() {
    static const std::string t = "Foo1";
    return t;
  static Foo* create(const std::string& dataLocation) {
    return new Foo1(/* parameters according to data read from

class Foo2 : public Foo1 {
  static std::string type() {
    static const std::string t = "Foo2";
    return t;
  // function "create" is derived from

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);
} // end namespace

Wouldn't some sort of table be more appropriate? Say an
std::map< std::string, Foo* (*)( std::string ) >. With:

    typedef std::map< std::string, Foo* (*)( std::string ) >
                        FooFactoryMap ;
    FooFactoryMap ourMap ;

        std::string const& dataLocation )
                = ourMap.find( getType( dataLocation ) ) ;
        return entry == ourMap.end()
            ? NULL
            : (*entry->second)() ;

You'll have to arrange to initialize the map somehow, but that
shouldn't be any more of a bother than maintaining all of your

James Kanze (GABI Software)
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