為什麼要過載賦值運算子?

在使用者定義類後, 編譯器會預設提供無參構造(空實現), 析構(空實現), 拷貝構造(值拷貝), 過載賦值運算子(值賦值)

但是若類成員屬性為指標變數時, 編譯器提供的過載賦值運算子只會簡單的把指標變數的值賦值, 而不會申請一份新的記憶體空間, 將指標指向的變數賦值給這片記憶體空間,

當我們在析構中釋放記憶體時, 就會出現一段的記憶體被重複釋放的情況, 從而導致程式執行出錯. 故需要自己給出過載賦值運算子函式

示例程式碼:

#include <iostream>
#include <string>


using namespace std;
 
/**
 * recipe n.祕訣;處方;食譜;
 * 為什麼要過載賦值運算子?
 *在使用者定義類後, 編譯器會預設提供無參構造(空實現), 析構(空實現), 拷貝構造(值拷貝),  過載賦值運算子(值賦值)
 *但是若類成員屬性為指標變數時, 編譯器提供的過載賦值運算子只會簡單的把指標變數的值賦值, 而不會申請一份新的記憶體空間, 將指標指向的變數賦值給這片記憶體空間,
 *當我們在析構中釋放記憶體時, 就會出現一段的記憶體被重複釋放的情況, 從而導致程式執行出錯. 故需要自己給出過載賦值運算子函式
 * 語法:
 * 類名 &operator=(const 類名 &變數名)
 * {
 *  函式體
 * }
 * 注意事項:
 * 1, 對於這種含有指標型別的成員變數的類, 其建構函式中應該都有申請記憶體空間的語法
 * 因此不需要再過載賦值運算子函式中再申請記憶體
 * 2, 函式返回值型別應該引用型別, 返回值為呼叫物件的的引用(使用*this指標返回)
 
 
*/
const int MAXSIZE = 100;

double abs(double a) {
    if(a < 0) return -a;
    return a;
}

class Item {
    string name;
    double price;
    int quantity;
public:
    Item() {

    }

    Item(const string &name, double price, int quantity) : name(name), price(price), quantity(quantity) {}
};

 
struct Goods {
    string name;
    double price;

    string getInfo() {
        return "name: " + name + ", price: " + to_string(price);
    }
};

ostream &operator<<(ostream &cout, const Goods &goods);

class Store {
    friend ostream &operator<<(ostream &cout, const Store &store);

private:
    double balance;
    Item *itemList;
    Goods *menu;
    int goodsQuantity;
public:
    const string name = "MINI STORE";

    Store() : balance(0),goodsQuantity(0) {
        this->itemList = new Item[MAXSIZE];
        this->menu = new Goods[MAXSIZE];
    }

    Store(double balance, Item *itemList, Goods *menu, int goodsQuantity) : balance(balance),
                                                                            goodsQuantity(goodsQuantity) {
        this->itemList = new Item[MAXSIZE];
        this->menu = new Goods[MAXSIZE];
        for (int i = 0; i < goodsQuantity; ++i) {
            this->itemList[i] = itemList[i];
            this->menu[i] = menu[i];
        }
    }

    ~Store() {
        cout << "Close the Store" << endl;
    }
    Store &operator=(const Store &store) {
        balance = store.balance;
        goodsQuantity = store.goodsQuantity;
        for (int i = 0; i < goodsQuantity; ++i) {
            this->itemList[i] = store.itemList[i];
            this->menu[i] = (store.menu)[i];
        }

        return *this;
    }

};


ostream &operator<<(ostream &cout, const Goods &goods) {
    cout << "name:" << goods.name << ", price:" << goods.price;
    return cout;
}

ostream &operator<<(ostream &cout, const Store &store) {
    cout << "name:" << store.name << endl;
    cout << "MENU:" << endl;
    for (int i = 0; i < store.goodsQuantity; ++i) {
        if(abs(store.menu[i].price-0) > 1e-6)
        cout << store.menu[i] << endl;
    }
    cout << "balance:" << store.balance << endl;
    return cout;
}

void test1() {
    Item itemList[] = {Item("apple", 2.5, 10), Item("watermelon", 1.2, 100), Item("grape", 10, 15)};
    Goods menu[] = {Goods{"apple", 2.5}, Goods{"watermelon", 1.2}, Goods{"grape", 10}};
    Store store = {100, itemList, menu, 3};
    Store s2;
    cout << "Before assign, the object s2 is :" << endl;
    cout << s2;
    s2 = store;
    cout << "After assign, the object s2 is :" << endl;
    cout << s2;

}

int main() {
    test1();
    system("pause");

    return 0;
}