【问题描述】不允许修改主函数，请将以下代码补充完整。 class String { char *ptr; public: String(char *s) { ptr=new char[strlen(s)+1]; strcpy(ptr, s); } ~String() { delete []ptr; } void print() { cout<<ptr<<endl; } }; int main( ) { String p1("book"), p2("pen"),p3("good"),p4; p4 = p4 = p1 ; p3 = p1 = p2; cout<<"p2:"; p2.print(); cout<<"p1:"<<p1; cout<<"p3:"<<p3; p4+=p3; cout<<"p4:"<<p4; return 0; } 【样例输入】 无 【样例输出】 p2:pen p1:pen p3:pen p4:bookpen

【参考代码】

#include <iostream>
#include <cstring>
using namespace std;

class String {
    char *ptr;
public:
    String(char *s) {
        ptr = new char[strlen(s) + 1];
        strcpy(ptr, s);
    }
    ~String() {
        delete []ptr;
    }
    void print() {
        cout << ptr << endl;
    }
    String& operator=(const String &s) {
        if (this == &s) return *this;
        delete []ptr;
        ptr = new char[strlen(s.ptr) + 1];
        strcpy(ptr, s.ptr);
        return *this;
    }
    String& operator+=(const String &s) {
        char *temp = ptr;
        ptr = new char[strlen(ptr) + strlen(s.ptr) + 1];
        strcpy(ptr, temp);
        strcat(ptr, s.ptr);
        delete []temp;
        return *this;
    }
    friend ostream& operator<<(ostream &os, const String &s) {
        os << s.ptr;
        return os;
    }
};

int main() {
    String p1("book"), p2("pen"), p3("good"), p4;
    p4 = p4 = p1;
    p3 = p1 = p2;
    cout << "p2:";
    p2.print();
    cout << "p1:" << p1 << endl;
    cout << "p3:" << p3 << endl;
    p4 += p3;
    cout << "p4:" << p4 << endl;
    return 0;
}