10-2 存取控制——public 與 private

一、class 裡的東西，不是誰都可以動的

先看一個銀行帳戶的例子。

#include <iostream>

using namespace std;

class BankAccount {
public:
    string owner;
    int balance;
};

int main() {
    BankAccount acc;
    acc.owner   = "小明";
    acc.balance = 1000;

    acc.balance = -99999;   // 沒有任何阻擋，帳戶餘額變成負的！

    cout << acc.balance << endl;
}

這是因為我們之前在 class 裡用了 public 這個存取修飾詞。 public 的意思就是「公開的」，誰想看、誰想改都可以。

用 private 將存取限制改為「私有的」看看差別。

#include <iostream>

using namespace std;

class BankAccount {
private:
    string owner;
    int balance;
};

int main() {
    BankAccount acc;
    acc.owner   = "小明";
    acc.balance = 1000;

    acc.balance = -99999;   // 沒有任何阻擋，帳戶餘額變成負的！

    cout << acc.balance << endl;
}

修改之後，連編譯都失敗了，出現了大量如下的錯誤訊息。

main.cpp:13:9: error: 'std::string BankAccount::owner' is private within this context
   13 |     acc.owner   = "小明";
      |         ^~~~~
main.cpp:7:12: note: declared private here
    7 |     string owner;
      |            ^~~~~
main.cpp:14:9: error: 'int BankAccount::balance' is private within this context
   14 |     acc.balance = 1000;
      |         ^~~~~~~
main.cpp:8:9: note: declared private here
    8 |     int balance;
      |         ^~~~~~~
main.cpp:16:9: error: 'int BankAccount::balance' is private within this context
   16 |     acc.balance = -99999;   // 沒有任何阻擋，帳戶餘額變成負的！
      |         ^~~~~~~
main.cpp:8:9: note: declared private here
    8 |     int balance;
      |         ^~~~~~~
main.cpp:18:17: error: 'int BankAccount::balance' is private within this context
   18 |     cout << acc.balance << endl;
      |                 ^~~~~~~
main.cpp:8:9: note: declared private here
    8 |     int balance;
      |         ^~~~~~~