struct my_obj_t {
Object * sort;

size_t size;
};

bool my_obj_cmp(const my_obj_t &a, const my_obj_t &b) {
   if(a.sort == b.sort) {
       return a.size < b.size;
   } else {
       return a.sort < b.sort;
   }
}

...
std::vector<my_obj_t> arr;
...
std::sort(arr.begin(), arr.end(), my_obj_cmp);
...

bool Object_cmp(const Object &a, const Object &b) {
       return a < b;
}

...
std::sort(sort, sort + size, Object_cmp);
...