Extend ResourceCache for factory functions

[DanRStevens]

Currently ResourceCache can only be used with constructors of the cached type. We should find a way to extend this to allow factory functions to be used instead. Really we want some function that takes a given fixed set of parameters, and returns the desired type, which can be added to the cache, and looked up using a parameter list tuple.

[DanRStevens]

Possibly relevant info:
StackOverflow: How do I expand a tuple into variadic template function's arguments?

[DanRStevens]

I found I can update the syntax slightly to change how parameters are specified. It's not quite what I want, but interesting.

Changing to this allows function style syntax:

template <typename FactoryFunction>
class ResourceCache;

template <typename Resource, typename... Params>
class ResourceCache<Resource(Params...)>
// ...

Example usage:

using AnimationCache = ResourceCache<AnimationSet(std::string)>;

Example usage:

NAS2D::ResourceCache<MockResource(std::string, int)> cache;

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The current syntax requires all types to be listed serially:

using AnimationCache = ResourceCache<AnimationSet, std::string>;

NAS2D::ResourceCache<MockResource, std::string, int> cache;

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This updated syntax isn't quite what I want, since it's still just a type definition, and doesn't allow specifying a specific factory function that matches that type.

Actually, the limitation is on how objects are instantiated and stored in the std::map:

const auto pairIterBool = cache.try_emplace(key, params...);

Here the try_emplace method is implicitly calling the object's constructor with the given parameters. If we use a factory function to first construct and return an object, we may find the insert method may make more sense. That will have slightly lower efficiency. I don't know of any way to emplace into a std::map using a factory function.

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Incidental note: We should probably be using std::forward for those parameters.

It may also make sense to use std::move with the std::tuple used as the key, particularly for large keys, or keys containing std::string objects. The try_emplace has an rvalue overload for the key parameter.