用c++实现：设计一个字符串类重载“+=”运算符,使其实现两个字符串的连接并赋值。试分别用成员函数和友元函数实现。

成员函数实现：

#include <iostream>
#include <cstring>

using namespace std;

class String {
public:
    String(const char* str = "") {
        len = strlen(str);
        data = new char[len + 1];
        strcpy(data, str);
    }

    String(const String& other) {
        len = other.len;
        data = new char[len + 1];
        strcpy(data, other.data);
    }

    ~String() {
        delete[] data;
    }

    String& operator+=(const String& other) {
        char* temp = new char[len + other.len + 1];
        strcpy(temp, data);
        strcat(temp, other.data);
        delete[] data;
        data = temp;
        len += other.len;
        return *this;
    }

    void print() {
        cout << data << endl;
    }

private:
    char* data;
    int len;
};

int main() {
    String s1("hello");
    String s2("world");
    s1 += s2;
    s1.print();
    return 0;
}

友元函数实现：

#include <iostream>
#include <cstring>

using namespace std;

class String {
public:
    String(const char* str = "") {
        len = strlen(str);
        data = new char[len + 1];
        strcpy(data, str);
    }

    String(const String& other) {
        len = other.len;
        data = new char[len + 1];
        strcpy(data, other.data);
    }

    ~String() {
        delete[] data;
    }

    friend String operator+(const String& str1, const String& str2) {
        String result;
        result.len = str1.len + str2.len;
        result.data = new char[result.len + 1];
        strcpy(result.data, str1.data);
        strcat(result.data, str2.data);
        return result;
    }

    void print() {
        cout << data << endl;
    }

private:
    char* data;
    int len;
};

int main() {
    String s1("hello");
    String s2("world");
    String s3 = s1 + s2;
    s3.print();
    return 0;
}