builder 패턴?과 관련된 질문드립니다.(C++)

//foo.h
 
class Bar; // declared other file.
 
class Foo{
public:
    const std::vector<int> GetNums(){
        return nums;
    }
 
private:
    friend class Bar;
    std::vector<int> nums;
};

//bar.h
#include "foo.h"
 
class Bar{
public:
    Bar(Foo *foo);
 
    void AddNums(int num);
 
private:
    const Foo *foo;
};

//bar.cc
#include "bar.h"
 
void Bar::Bar(Foo *foo) : foo(foo){}
 
void Bar::AddNums(int num){
    foo->nums.push_back(num);
}