C++函数返回局部变量/拷贝构造/移动构造/拷贝赋值/移动赋值*

#include <iostream>

using namespace std;

int g_base_num = 1;

class Teacher {

   public:

    int id;

    int* studentIds{nullptr};

    int count;

   public:

    // 构造函数

    Teacher(int id, int count) {

        this->id = id;

        this->count = count;

        this->studentIds = new int[count];

        for (int i = 0; i < count; i++) {

            this->studentIds[i] = (i + 1) * g_base_num;

        }

        g_base_num *= 10;

        cout << "contrustor" << endl;

    }

    // 拷贝构造函数：尽量使用深拷贝

    Teacher(const Teacher& other) {

        this->id = id;

        this->count = other.count;

        this->studentIds = new int[count];

        for (int i = 0; i < count; i++) {

            /* code */

            this->studentIds[i] = other.studentIds[i];

        }

        cout << "copy constructor" << endl;

    }

    //移动构造函数

    Teacher(Teacher&& other) {

        this->id = other.id;

        this->count = other.count;

        this->studentIds = other.studentIds;

        other.studentIds = nullptr;

        cout << "move constructor" << endl;

    }

    // 移动赋值

    Teacher& operator=(Teacher&& ch) {

        cout << "yi dong fu zhi yun suan fu" << endl;

        // 释放this中动态分配的内存

        delete this->studentIds;

        this->id = ch.id;

        this->count = ch.count;

        this->studentIds = ch.studentIds;  // 转移动态内存到this

        ch.studentIds = nullptr;

    }

    // 拷贝赋值

    Teacher& operator=(Teacher& ch) {

        cout << "copy fu zhi yun suan fu" << endl;

        // 释放this中动态分配的内存

        delete this->studentIds;

        // 深拷贝

        this->studentIds = new int[ch.count];

        for (int i = 0; i < ch.count; i++) {

            this->studentIds[i] = ch.studentIds[i];

        }

        this->id = ch.id;

        this->count = ch.count;

    }

    ~Teacher() {

        if (this->studentIds) {

            delete this->studentIds;

            this->studentIds = nullptr;

        }

    }

};

Teacher GetTeacher() {  // 借助了移动copy构造函数的能力，相当于返回了局部变量

    Teacher t(1, 4);

    for (int i = 0; i < t.count; i++) {

        t.studentIds[i] = i;

    }

    cout << "begin to return" << endl;

    return t;

}

void PrintTeacher(Teacher&& ch) { cout << endl; }

int main(int argc, char const* argv[]) {

    // 这是一个临时变量(右值)构造左值的情况，所以这里的赋值操作会调用移动够早函数

    cout << "=============begin to get one obj=============" << endl;

    // 在C++中返回对象的时候，对象需要经过2次拷贝构造函数的调用才能够完成返回对象的传递。一次拷贝到栈上的临时对象里，另一次把临时

    // 对象拷贝到存储返回值的对象里

    Teacher p = GetTeacher();

    for (int i = 0; i < p.count; i++) {

        cout << p.studentIds[i] << " ";

    }

    cout << endl;

    cout << "=============end get one obj=============" << endl;

    cout << endl;

    cout << "=============begin to copy constructor=============" << endl;

    // 拷贝构造函数要使用深拷贝

    Teacher tt(p);

    cout << "=============end copy constructor=============" << endl;

    for (int i = 0; i < tt.count; i++) {

        cout << tt.studentIds[i] << " ";

    }

    cout << endl;

    cout << "=============begin to move= function=============" << endl;

    // 使用移动构造函数，经过移动后，p本身的地址并没有发生变化

    p = Teacher(1, 2);

    for (int i = 0; i < p.count; i++) {

        cout << p.studentIds[i] << " ";

    }

    cout << endl;

    cout << "=============end move= function=============" << endl;

    cout << endl;

    cout << "=============begin to copy= function=============" << endl;

    p = tt;

    for (int i = 0; i < p.count; i++) {

        cout << p.studentIds[i] << " ";

    }

    cout << endl;

    cout << "=============end copy= function=============" << endl;

    cout << endl;

    cout << "end main" << endl;

    return 0;

}