I agree that single particles are unusual. However, the overhead entailed by the current model is pretty small so it is not clear to mean having vectors would be noticeably faster. To access a Particle attribute the internal pseudo-code looks something likeHowever, I suggest rather than a single pointer attribute we use a vector<Particle pointer> attribute. This would certainly make things like hierarchies much easier to implement (plus, much faster, since you could just cache a 'child'ParticleVectorKey rather than having to do a lot of lookups for child1,child2, child3 etc.) Pointers to single particles are probably sufficiently unusual that the overhead of a single-element list is negligible.
check that the vector of child keys is long enough get the key from the vector get the particle If we have a vector it looks like get the key for the children get the vector of children get the particleso you just save one check which generally succeeds. As a result I would vote for not complicating the interface by having the particle attributes look different from the others. Mostly an aesthetic preference, so not very strong.