类、封装与访问控制

1#include <iostream>
2#include <string>
3
4struct BankAccount {
5    std::string owner;
6    double balance;
7}; // 分号是必需的！
8
9int main() {
10    // 1. 创建结构体的实例（对象）
11    BankAccount account;
12
13    // 2. 我们可以直接访问成员，因为它们默认是公共的
14    account.owner = "Alice";
15    account.balance = 100.0;
16
17    std::cout << account.owner << " 拥有 $" << account.balance << "\n";
18
19    // 问题：缺乏安全性。我们很容易破坏逻辑
20    account.balance = -5000.0; // 无效状态！
21    
22    std::cout << "损坏的余额: " << account.balance << "\n";
23
24    return 0;
25}

1#include <iostream>
2#include <string>
3
4class BankAccount {
5    std::string owner; // 默认私有
6    double balance;    // 默认私有
7};
8
9int main() {
10    BankAccount account;
11
12    account.balance = 100.0; // 编译错误：'balance' 是私有的
13    return 0;
14}

1#include <iostream>
2#include <string>
3
4class BankAccount {
5private: 
6    // 数据被隐藏。只有这个类可以访问它
7    std::string owner;
8    double balance = 0.0; // 类内初始化（良好实践）
9
10public:
11    // 初始化函数（我们将在下一课用构造函数替换它）
12    void init(std::string name, double startAmount) {
13        owner = name;
14        if (startAmount > 0) balance = startAmount;
15    }
16
17    // 公共接口：受控访问
18    void deposit(double amount) {
19        if (amount > 0) {
20            balance += amount;
21            std::cout << "存入 $" << amount << "\n";
22        }
23    }
24
25    // 获取器：只读访问余额
26    double getBalance() {
27        return balance;
28    }
29};
30
31int main() {
32    BankAccount account;
33    account.init("Alice", 100.0); // 使用公共函数进行初始化
34
35    account.deposit(50.0);        // 使用公共函数修改
36    // account.balance = -500;    // 错误：仍然为私有！
37
38    std::cout << "当前余额: $" << account.getBalance() << "\n";
39
40    return 0;
41}

1#include <iostream>
2#include <string>
3
4class BankAccount {
5private: 
6    std::string owner;
7    double balance = 0.0; 
8
9public:
10    void init(std::string name, double startAmount) {
11        owner = name;
12        if (startAmount > 0) balance = startAmount;
13    }
14
15    void deposit(double amount) {
16        if (amount > 0) {
17            balance += amount;
18        }
19    }
20
21    // 逻辑：强制执行“余额不能为负”的规则
22    void withdraw(double amount) {
23        if (amount > balance) {
24            std::cout << "错误：取款 $" << amount << " 时资金不足\n";
25        } else if (amount <= 0) {
26             std::cout << "错误：无效的取款金额。\n";
27        } else {
28            balance -= amount;
29            std::cout << "成功：取款 $" << amount << "\n";
30        }
31    }
32
33    double getBalance() { return balance; }
34};
35
36int main() {
37    BankAccount account;
38    account.init("Alice", 100.0);
39
40    account.withdraw(40.0);  // 正常：余额变为 60.0
41    account.withdraw(500.0); // 被阻止：类自身保护自己
42
43    std::cout << "最终余额: $" << account.getBalance() << "\n";
44    return 0;
45}

1#include <iostream>
2#include <string>
3#include <vector>
4
5class BankAccount {
6private:
7    std::string owner;
8    double balance = 0.0;
9    std::vector<std::string> history; // 我们也隐藏了历史记录日志
10
11    // 内部辅助函数：用户不需要直接调用
12    void logTransaction(std::string msg) {
13        history.push_back(msg);
14        std::cout << "[LOG]: " << msg << "\n";
15    }
16
17public:
18    void init(std::string name, double startAmount) {
19        owner = name;
20        balance = startAmount;
21        logTransaction("为 " + name + " 创建账户");
22    }
23
24    void deposit(double amount) {
25        if (amount > 0) {
26            balance += amount;
27            // 在这里使用私有辅助函数
28            logTransaction("存入 " + std::to_string(amount));
29        }
30    }
31
32    void withdraw(double amount) {
33        if (amount <= balance && amount > 0) {
34            balance -= amount;
35            // 在这里使用私有辅助函数
36            logTransaction("取款 " + std::to_string(amount));
37        }
38    }
39    
40    double getBalance() { return balance; }
41};
42
43int main() {
44    BankAccount account;
45    account.init("Alice", 100.0); // 自动记录创建日志
46    account.deposit(50.0);        // 自动记录存款日志
47    
48    // account.logTransaction("伪造日志"); // 错误：私有函数！
49    
50    return 0;
51}