在C语言中，struct仅能打包数据：
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// C语言风格 struct Student { char name[20]; int age; float score; };C++让struct脱胎换骨，不仅能包含数据，还能拥有"行为"（方法）：
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struct Student { char name[20]; int age; // 方法！C语言无法做到 void introduce() { std::cout << "我是" << name << "，今年" << age << "岁\n"; } };更关键的是，C++引入了class关键字，它与struct几乎相同，唯一区别是默认访问权限：
class默认成员为private（私有）struct默认成员为public（公有）
踩坑经历：刚学C++时，我常常忘记在
class中添加public关键字，导致所有成员函数都无法在类外调用，编译器报错看得我一头雾水。后来才明白，C++的设计哲学是"默认保守"——除非明确声明，否则不对外开放。
封装：数据与行为的完美融合
封装是面向对象的基石。它像保险箱一样保护内部数据，只通过特定"钥匙孔"（接口）与外界交互。我们用public和private访问限定符实现这一思想：
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class BankAccount { private: double balance; // 私有成员，外部无法直接访问 public: // 公有接口，控制对私有数据的访问 void deposit(double amount) { if (amount > 0) balance += amount; } bool withdraw(double amount) { if (amount > 0 && balance >= amount) { balance -= amount; return true; } return false; } double getBalance() const { return balance; } };这种设计有两大优势：
- 数据安全：防止外部代码直接修改
balance，避免设置为负数等非法操作 - 行为封装：将相关操作（存款、取款）与数据绑定在一起，代码更清晰
代码演示：银行账户类，展示封装的力量
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#include <iostream> using namespace std; class BankAccount { private: string accountNumber; double balance; public: // 构造函数：初始化账户 BankAccount(string number, double initialBalance = 0.0) : accountNumber(number), balance(initialBalance) { cout << "账户 " << accountNumber << " 已创建，初始余额: " << balance << endl; } // 存款 void deposit(double amount) { if (amount <= 0) { cout << "错误：存款金额必须大于0" << endl; return; } balance += amount; cout << "存入 " << amount << "，新余额: " << balance << endl; } // 取款 bool withdraw(double amount) { if (amount <= 0) { cout << "错误：取款金额必须大于0" << endl; return false; } if (balance < amount) { cout << "错误：余额不足，当前余额: " << balance << endl; return false; } balance -= amount; cout << "取出 " << amount << "，新余额: " << balance << endl; return true; } // 获取余额 double getBalance() const { return balance; } // 显示账户信息 void display() const { cout << "账户: " << accountNumber << ", 余额: " << balance << endl; } }; int main() { // 创建账户 BankAccount account1("ACCT-001", 1000.0); account1.display(); // 存款 account1.deposit(500.0); // 取款 account1.withdraw(200.0); // 尝试非法操作（无法直接访问私有成员） // account1.balance = -10000; // 编译错误！无法访问私有成员 cout << "当前余额: " << account1.getBalance() << endl; return 0; }类域与成员函数
当你在类内定义函数时，编译器会为每个成员函数隐式添加一个this指针作为第一个参数。这个指针指向调用函数的对象本身，让我们能在函数内部访问对象的成员变量。
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class Rectangle { private: int width, height; public: void setSize(int width, int height) { // this->width 指成员变量，width指参数 this->width = width; this->height = height; } };this指针是C++实现面向对象的关键机制，它让我们可以在同一个类的多个对象之间区分各自的成员变量。当你调用obj.method()时，编译器将其转换为method(&obj)，隐式传递对象地址。
易错点警告：如果你尝试通过空指针调用成员函数，当函数内部使用了成员变量时会导致程序崩溃。但如果成员函数不访问任何成员变量，空指针调用可能"侥幸"成功，这是危险的未定义行为！
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