Building the particle system class

With all of the building blocks in place, it's time to actually build the particle system class. Let's start with some type definitions:

using Updaters = std::unordered_map<std::string, 
  std::unique_ptr<BaseUpdater>>; 
using EmitterList = std::vector<std::unique_ptr<Emitter>>; 
using Emitters = std::unordered_map<StateType, EmitterList>; 
using GeneratorList = std::vector<std::unique_ptr<BaseGenerator>>; 
using Generators = std::unordered_map<std::string,GeneratorList>; 
using RemovedEmitters = std::vector<Emitter*>; 
using Particles = std::unordered_map<StateType, 
  std::unique_ptr<ParticleContainer>>; 
using ForceApplicatorList = std::vector<ForceApplicator>; 
using ForceApplicators = std::unordered_map<StateType, 
  ForceApplicatorList>; 
 
using GeneratorFactory = std::unordered_map<std::string, 
  std::function<BaseGenerator*(void)>>; 

In order to...