That's great ta:

Firstly I was just trying to encapsulate the code as it seemed the natural progression if I were creating objects in objects. Is it wrong to try this kind of nesting? Would I be better off just creating a separate class?

I tend to use nested classes in cases where the classes never need to be accessed publicly (i.e. they are only used in private areas of a class). It's just a stylistic thing really, and there's no real problem doing either.

Secondly I was trying to avoid vectors as I thought that you couldn't save them in .bin files. only vars and arrays! Is that a falsehood?

#include <cstdio>
 
void save(std::FILE* fp, const std::vector<cTroupObj>& objs)
{
 uint32_t num = objs.size();
 std::fwrite(&num, 1, sizeoff(uint32_t), fp);
 if(objs.size()) //< important! (otherwise you'll hit NULL if empty)
 {
    std::fwrite(&objs[0], objs.size(), sizeof(cTroupObj), fp);
 }
}
 
void load(std::FILE* fp, std::vector<cTroupObj>& objs)
{
 uint32_t num = objs.size();
 std::fread(&num, 1, sizeoff(uint32_t), fp);
 objs.resize(num);
 if(objs.size()) //< important! (otherwise you'll hit NULL if empty)
 {
    std::fread(&objs[0], objs.size(), sizeof(cTroupObj), fp);
 }
}

There is but one problem with it. When reading in the data, the constructors will be called when you call resize, and then the values will be written over with your file data. If you had 6 million troups, this may cause a (very small) slow down due to memory bandwith / CPU cache issues. If however it's just for a few troups (i.e. <10000) you'll be fine. Ultimately, you've got to look at the simplicity of using a std::vector here to add and remove troups, vs the faffery of managing that yourself. It works, It's simple, and you can spend your time worrying about the more important aspects.

If the binary data you're reading in is generally going to be constant (for example all of the mesh & collision data in your level), then that could be an area where you might want to consider something else (if, and only if, you're writing a game for xbox/ps4 and expect to be loading that data off DVD). For data being loaded off hard drive, that effort isn't really going to be worth it. For data such as a save game, it definitely isn't worth it.