CODEWITHSUNDEEP
c++factory-pattern
user2946316
user2946316

Reputation:

Factory class driven by a map for determining the object type

I'm having a brain freeze and can't figure out how to best solve this issue. I'm creating objects from my factory class by calling 

CreateEnvironment<T>(ARGS);

Now lets say that i want to save alot of class-types into a map and iterate through the map and call the method at runtime like this:

ITERATION: CrateEnvironment<(*it)>(world);

(*it) should be the class type, which could be FOO or BAR for example. How do i achieve this instead of having alot of if statements?

Best regards

Upvotes: 0

Views: 221

Answers (1)

Christophe
Christophe

Reputation: 73406

For each class you could have a function that would serve as generator and create a new object and return a pointer to it (or better, a shared_ptr).

In your container you could then store a the generator function pointers.

Step by step explanations:

Suppose you have these classes to populate your world:

struct GO { virtual void say_hello()=0; };  // Game Object
struct A:GO { void say_hello() { cout<<"I'm a werewolf\n";} };
struct B:GO { void say_hello() { cout<<"I'm a soldier\n";}};

You can then define a generic GO generator:

template <class T> 
shared_ptr<GO> generator() {
    return make_shared<T>(); 
};

This would serve as subsititue for your "type" container (for the simplicity of the example I've used a vector, but you could easily opt for a map):

typedef shared_ptr<GO> (*gen_fn)();
vector <gen_fn> generators{generator<A>, generator<B>};

You could then populate your universe like this, without any if:

vector<shared_ptr<GO>> universe; 

default_random_engine generator;
uniform_int_distribution<int> distribution(0,generators.size()-1);

for (int i=0; i<10; i++) {   
    int mytype = distribution(generator); 
    universe.push_back(generators[mytype]()); 
}

for (auto x: universe) 
    x->say_hello();

And here an online demo.

Statistical remark: As the distribution is uniform, you will have a high probability of having the roughly the same proportion of each type of object. If you'd like to have different distribution, you could add several times generators of the same type. For example, with generators{generator<A>, generator<B>, generator<B>}; you'd have around 66% of soldiers and 33% of werewolves.

Upvotes: 1

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