编写字符串运算符“+=”的重载函数，使运算符“+=”用于字符串的拼接运算，实现字符串的S1+=S2操作运算。分别用友元函数和成员函数实现。

友元函数实现：

#include <iostream>
#include <cstring>

using namespace std;

class String {
private:
    char *str;
public:
    String(const char *s = NULL) {
        if (s == NULL) {
            str = new char[1];
            str[0] = '\0';
        } else {
            str = new char[strlen(s) + 1];
            strcpy(str, s);
        }
    }

    ~String() {
        delete[] str;
    }

    void display() const {
        cout << str << endl;
    }

    friend String operator+=(String &s1, const String &s2);  // 友元函数
};

String operator+=(String &s1, const String &s2) {
    char *temp = new char[strlen(s1.str) + strlen(s2.str) + 1];
    strcpy(temp, s1.str);
    strcat(temp, s2.str);
    delete[] s1.str;
    s1.str = temp;

    return s1;
}

int main() {
    String s1("Hello, ");
    String s2("world!");
    s1 += s2;
    s1.display();

    return 0;
}

成员函数实现：

#include <iostream>
#include <cstring>

using namespace std;

class String {
private:
    char *str;
public:
    String(const char *s = NULL) {
        if (s == NULL) {
            str = new char[1];
            str[0] = '\0';
        } else {
            str = new char[strlen(s) + 1];
            strcpy(str, s);
        }
    }

    ~String() {
        delete[] str;
    }

    void display() const {
        cout << str << endl;
    }

    String &operator+=(const String &s) {  // 成员函数
        char *temp = new char[strlen(str) + strlen(s.str) + 1];
        strcpy(temp, str);
        strcat(temp, s.str);
        delete[] str;
        str = temp;

        return *this;
    }
};

int main() {
    String s1("Hello, ");
    String s2("world!");
    s1 += s2;
    s1.display();

    return 0;
}