Three Files: An Aggregation That Shows Later Changes

1// ===== Part.h =====
2#ifndef PART_H
3#define PART_H
4#include <iostream>
5 
6class Part {
7    int m_id;
8    int m_qty;
9public:
10    Part(int id, int qty) : m_id(id), m_qty(qty) {}
11    void setQty(int qty) { m_qty = qty; }
12    friend std::ostream& operator<<(std::ostream& os, const Part& p) {
13        return os << "#" << p.m_id << " x" << p.m_qty;
14    }
15};
16 
17#endif
18 
19// ===== Kit.h =====
20#ifndef KIT_H
21#define KIT_H
22#include <iostream>
23#include "Part.h"
24 
25class Kit {
26    const Part* m_parts[4];
27    int m_count = 0;
28public:
29    Kit& operator+=(const Part& part) {
30        for (int i = 0; i < m_count; i++) {
31            if (m_parts[i] == &part) {
32                return *this;
33            }
34        }
35        m_parts[m_count] = &part;
36        m_count++;
37        return *this;
38    }
39    friend std::ostream& operator<<(std::ostream& os, const Kit& kit) {
40        static int printed = 0;
41        printed++;
42        os << "Kit " << printed << " (" << kit.m_count << ")" << std::endl;
43        for (int i = 0; i < kit.m_count; i++) {
44            os << "  " << *kit.m_parts[i] << std::endl;
45        }
46        return os;
47    }
48};
49 
50#endif
51 
52// ===== main.cpp =====
53#include <iostream>
54#include "Kit.h"
55using namespace std;
56 
57int main() {
58    Part bolt(1, 10), nut(2, 20);
59    Part bolt2(1, 10);
60    Kit kit;
61    kit += bolt;
62    kit += nut;
63    kit += bolt;
64    kit += bolt2;
65    cout << kit;
66    bolt.setQty(99);
67    cout << kit;
68    return 0;
69}