How to randomly select a class to instantiate without using switch?

Question

I\'m refactoring a single 3000+-line class with a tangled web of conditionals and switches into a set of worker classes. Previously part of the constructor would select whic

Answer 1

I would suggest creating a common base class (if you've not already got one) and then using a factory class to encapsulate the creation process. The factory would just return a pointer to your base class which has the prototype run method.

Something along these lines:

class Type
{
    virtual void Run() = 0;
};

class TypeFoo : public Type
{
public:
    TypeFoo() {};
    virtual void Run() {};
    static Type* Create() { return new TypeFoo(); };
};

class TypeBar : public Type
{
public:
    TypeBar() {};
    virtual void Run() {};
    static Type* Create() { return new TypeBar(); };
};

class TypeBaz : public Type
{
public:
    TypeBaz() {};
    virtual void Run() {};
    static Type* Create() { return new TypeBaz(); };
};

class TypeFactory
{
    typedef Type* (*CreateFn)();

public:
    static Type* RandomTypeFooWeighted()
    {
        CreateFn create[] = 
        {
            TypeFoo::Create, 
            TypeFoo::Create,   // weighted towards FOO
            TypeBar::Create, 
            TypeBaz::Create
        };   
        const int fncount = sizeof(create)/sizeof(*create);
        return create[ rand()%fncount ]();
    }
};

So to use it you can just call:

Type *t = TypeFactory::RandomTypeFooWeighted();

Credit to Nawaz for the function pointer bits and bobs.