Enums and Templates

I am wondering if I can somehow return enum values from a method, using templates or otherwise. Here is an outline of what I want to do. The specific enums are declared outside the scope of the State class.

class State {
map<string, int> values;
//...

template <enum E>
E getEnumValue(const string& key) const;
}

I have never written any code using either templates or C++ enums before, so I don't really know where to start.

Thanks.

Why not test this idea with your compiler?

Code:

enum numbers { zero, one, two, three };

template<typename T> T foo(T t) { return t; }

int main()
{
	std::cout << foo<numbers>(zero);
	std::cout << foo<numbers>(one);
	std::cout << foo<numbers>(two);
	std::cout << foo<numbers>(three);
	return 0;
}

Yah.

Why do you need a template? Is it because you want your class to work with different enum types? In that case I would expect the class would need to be templated and the values map would have to be a map<string, E>. You would still use typename or class inside the template declaration instead of enum.

Tonto: Yes, I know that enum values are integers.

Daved: I want instances of the State class to be able to return enum values instead of the integers it stores.

This is what I want to do (but in C++). And I know it uses reflection and an unchecked cast, but all I want to know is if this is possible in C++ at all.

Code:

import java.util.Map;
import java.util.HashMap;

public class State {

	public Map<String, Integer> values = new HashMap<String, Integer>();

	public enum MyEnum {MY_CONSTANT};

	public <E extends Enum<E>> E getEnumValue(String key, Class<E> enumType) {
		try {
			E[] enumValues = (E[])enumType.getMethod("values").invoke(null);
			return enumValues[values.get(key)];
		} catch (Exception e) {
			return null;
		}
	}

	public static void main(String[] args) {
		State s = new State();
		s.values.put("!", State.MyEnum.MY_CONSTANT.ordinal());
		System.out.println(s.getEnumValue("!", State.MyEnum.class));
		System.out.println(s.getEnumValue("!", State.MyEnum.class).getClass().getName());
	}
}

Not without manually writing some metadata - the compiler won't store the names of the enum values, just their numeric values.

You could try the experimental Boost.Enum, which uses macros to automatically generate the metadata.