The future of STL and the ranges library

This section deals with a library that is proposed for the upcoming C++ standard, C++20. As of now, it is available at https://github.com/ericniebler/range-v3. Compiling these examples will require an updated compiler.

Limitations of the iterators in STL

Although the iterator and algorithm concepts in STL have quite a few good properties, they lack composability.

Let's say, we have some sort of a Warrior class with an ability and a level of ability, as implemented below:

enum class EAbility { Fencing, Archery };
class Warrior {
public:
  EAbility ability_{};
  int level_{};
  std::string name_{};
};

Now, let's say we want to find the Warrior with the highest level of Archery in the list of warriors.

If we were to use STL, the algorithm we'd use is std::max_element(), operating on level_, but as we only want to take the warriors with the ability of Archery into account, it gets tricky. Essentially, we want to compose a new algorithm out of a combination...