编写类String的构造函数、析构函数和赋值函数

#include <iostream>
#include <string.h>

using namespace std;

class MyString
{
public:
    MyString(const char *str = NULL);
    MyString(const MyString& str);
    MyString& operator=(const MyString& str);

    MyString(MyString&& str);
    MyString& operator=(MyString&& str);

    ~MyString() {
        delete[] pData;
    }

    char* getStr() { return pData; }
private:
    char* pData;
};

//注意参数为常量字符串指针
//当创建一个字符串对象时，此时不是将pData直接指向外部字符串的地址，
//而是在MyString内部重新创建一个新的字符串，并将pData指向这个新的字符串的地址，
//所以MyString不会修改外部传入的字符串的内容，这点要注意
//例如调用MyString("abcdef"), 虽然已经为字符串“abcd”分配了内存，
//但是在MyString 内部会重新创建一个新的“abcd”
MyString::MyString(const char *str) {
    cout << "Constructor: MyString is created!"<< endl;
    if (str == NULL) {
        pData = new char[1];
        pData[0] = '\0';
    }
    else
    {
        int len = strlen(str) + 1;
        pData = new char[len];
        strcpy_s(pData, len, str);
    }
}

MyString::MyString(const MyString& str) {
    cout << "Copy Constructor: MyString is created!" << endl;
    int len = strlen(str.pData); //strlen 碰到'\0'结束，返回的长度长度不包含'\0'
    pData = new char[len + 1];
    strcpy_s(pData, len + 1, str.pData);
}
//赋值函数中，上来比较 this == &other 是很必要的，因为防止自复制，这是很危险的，
//因为下面有delete []m_data，如果提前把m_data给释放了，指针已成野指针，再赋值就错了。
//返回值必须为引用类型MyString&，否则会调用一次拷贝构造函数
MyString& MyString::operator=(const MyString& str) {
    cout << "operator= Constructor is Called!" << endl;
    if (this == &str) {
        return *this;
    }

    delete[] pData;

    int len = strlen(str.pData);
    pData = new char[len + 1];
    strcpy_s(pData, len + 1, str.pData);

    return *this;
}

MyString::MyString(MyString&& other) : pData(other.pData) {
    cout << "Move Copy Constructor: MyString is created!" << endl;
    other.pData = nullptr;
}

MyString& MyString::operator=(MyString&& other) {
    cout << "Move operator= Constructor is Called!" << endl;
    if (this == &other)
        return *this;
    delete[] pData;
    pData = other.pData;
    other.pData = nullptr;
}

ostream& operator<<(ostream &output, MyString str) {
    return cout << str.getStr();
}

MyString getStr() {
    MyString str("Hello");
    return str;
}

void main() {

    MyString str1("hello");             //调用普通构造函数
    MyString str2("world");             //调用普通构造函数
    
    MyString str3(str1);                //调用拷贝构造函数

    str3 = str2;                        //调用赋值函数

    MyString str4 = getStr();
    
    MyString str5;
    str5 = getStr();

    getchar();
}

运行结果如下：

Constructor: MyString is created!

Constructor: MyString is created!

Copy Constructor: MyString is created!

operator= Constructor is Called!

Constructor: MyString is created!
Move Copy Constructor: MyString is created!

Constructor: MyString is created!
Constructor: MyString is created!
Move Copy Constructor: MyString is created!
Move operator= Constructor is Called!

这里的细节可以参考一下陈硕的

C++面试中string类的一种正确简明的写法

https://www.cnblogs.com/Solstice/p/3362913.html
https://github.com/chenshuo/recipes/blob/master/string/StringTrivial.h