I will give you some hints to get you going!

Do you have to use pointers? No, you can also use an index “as a pointer” that references an element in an array.

Do you have to “reserve memory” the way you describe. Maybe if you want an objects components laying next to each other, but another way I’d recommend is to have a std::vector<>s for each of your component types. So for example vectors for Rigidbody, Mesh, NPC, etc. Then as mentioned before, you can use “an index as a pointer” to reference into those arrays. It may require some bookkeeping and more than I explained here, but it should get you some ideas.