C++ class传值和传引用的详细介绍

#include <iostream>
#include <string>
#include <vector>
class Student {
public:
    std::string name;
    int age;
    Student(std::string n, int a) : name(n), age(a) {}
    // 拷贝构造函数（用于演示传值时的拷贝行为）
    Student(const Student& other) : name(other.name), age(other.age) {
        std::cout << "📋 [拷贝构造函数被调用]" << std::endl;
    }
};
// --- 1. 传值 ---
// 这里的 s 是 originalStudent 的副本
void modifyByValue(Student s) {
    s.name = "Modified_Value";
    s.age = 99;
    std::cout << "函数内(值): " << s.name << ", " << s.age << std::endl;
}
// --- 2. 传引用 (非常用) ---
// 这里的 s 是 originalStudent 的别名
void modifyByReference(Student& s) {
    s.name = "Modified_Reference";
    s.age = 88;
    std::cout << "函数内(引用): " << s.name << ", " << s.age << std::endl;
}
// --- 3. 常量引用 (最佳实践) ---
// 既避免了拷贝，又防止了修改
void printStudent(const Student& s) {
    // s.name = "Error"; // 编译错误：不能修改 const 对象
    std::cout << "函数内(只读): " << s.name << ", " << s.age << std::endl;
}
int main() {
    Student originalStudent("Alice", 20);
    std::cout << "原始数据: " << originalStudent.name << ", " << originalStudent.age << std::endl;
    std::cout << "--- 调用传值函数 ---" << std::endl;
    modifyByValue(originalStudent); 
    // originalStudent 未被修改，但触发了拷贝构造
    std::cout << "调用后: " << originalStudent.name << ", " << originalStudent.age << std::endl << std::endl;
    std::cout << "--- 调用传引用函数 ---" << std::endl;
    modifyByReference(originalStudent);
    // originalStudent 被修改了
    std::cout << "调用后: " << originalStudent.name << ", " << originalStudent.age << std::endl;
    return 0;
}

class Animal { public: virtual void speak() { cout << "..." << endl; } };
class Dog : public Animal { public: void speak() { cout << "Woof!" << endl; } };
void makeSound(Animal a) { a.speak(); } // 错误：传值会导致切片，Dog 变成 Animal
void makeSound(Animal& a) { a.speak(); } // 正确：传引用保留多态性