Question

I have an ABC with several derived classes. To create these derived classes I use the factory pattern:

.h file:

class derivedFactory { public:     base* createInstance(); }; 

.cpp file:

base* derivedFactory::createInstance() {     return new derived(); } 

Is there any advantage to this over just having a free function:

.h file:

base* derivedFactoryFunction(); 

.cpp file:

base* derivedFactoryFunction() {     return new derived(); } 

Also: I use the abstract factory pattern for dependency injection. I might use an inheritance hierarchy based on the ABC:

class objectCreator { public:     base* create() = 0; }; 

Is there any advantage to using this over a function pointer:

boost::function<base* ()> factory_ptr; 

Using boost::bind/lambda this seems to make my code more composable, and if I wish I can wrap a real factory object in it. I can see that there may be a slight performance decrease but this is much to worry about as it is only called during startup.

Answer 1

It depends on how flexible your factory needs to be. If the factory needs external information (like from a configuration file, program options, etc) to determine how to construct objects, than an object makes sense. If all you will ever need is in the arguments to factory, than a function is probably fine.

The only advantage I can see to having a pointer is for testing, where you can use a different factory function.