float as a key in std::map

[S__B]

Both have their advantages and disadvantages.

The advantage of using coordinates is that it results in a unique ordering. The disadvantage to it is that the order of your nodes can change drastically if one of them is moved even just a little bit.

The advantage of using vector length is that it uses an actual mathematical/physical value, which is the Euclidean Norm (although almost any 3D norm will do). An advantage to doing this is that for starters, it makes sense by looking at a graph which nodes are order before what other nodes, and the ordering is more robust to tiny changes in point positions.

I don't think this would work, as infinitely many points on "the sphere" will have the same key. So in the end the "find" will be indeterminate.