Automatic static invocation of derived types

Question

Does anyone know of a way to make derived classes automatically instantiate a static variable with a template type (this either has to require nothing from the writer of the der

Answer 1

I'd recommend a CTRP-backed approach:

// Entity.h
class EntityBase
{ // abstract
};

template
class Entity
  : public EntityBase
{ // also abstract thanks to the base
  static char _enforce_registration; // will be instantiated upon program start
};

// your actual types in other headers
class EntityType1
  : public Entity
{ // automatic registration thanks to the _enforce_registration of the base
  // ...
};

// Entity.cpp
#include "Entity.h"

template
char RegisterType(){
  GetGlobalFactory().registerEntityType();
  return 0; // doesn't matter, never used.
}

template
char Entity::_enforce_registration = RegisterType();

Though, as seen, you now need to get your factory through a GetGlobalFactory function, which lazy initializes the factory to ensure that it has been initialized before the enforced registration happens:

Factory& GetGlobalFactory(){
  static Factory _factory;
  return _factory;
}