3、仿照标准字符串类string，设计一个自己的字符串类String。 主要完成以下任务： 1)数据成员是字符指针，可自动适应不同的串长度。 2)串复制，既可使用字符串常量进行复制，也可以使用其他的字符串对象进行复制。 3)串赋值。既可赋值为字符串常量，也可以赋值为使用其他的字符串对象。 4)串连接。重载“+”和“+=”。 5)重载下标运算符[]。 6)串比较。重载 “==”和“<”。 7)字符串输入和输出。重载“>>”和“<<”。 编程完成各函数的定义，并进行如下测试。 int main() { String s1("Help!"),s2("Good!"),s3(s2),s4,s5; cout<<"s1="<<s1<<endl; s3="Hello!"; cout<<"s3="<<s3<<endl; s3=s2; cout<<"s3="<<s3<<endl; s3+=s2; cout<<"s3="<<s3<<endl; cin>>s4; cout<<"s4="<<s4<<endl; s5=s3+s4; cout<<"s5="<<s5<<endl; s5[0]=g'; cout<<"s5="<<s5<<endl; cout<<"strlen(s5)="<<s5. Length()<<endl; cout<<boolalpha<<(s3==s1)<<endl; cout<<boolalpha<<(s3<s1)<<endl; }

以下是String类的定义和测试代码：

#include <iostream>
#include <cstring>

using namespace std;

class String {
public:
    String();
    String(const char* str);
    String(const String& other);
    ~String();

    String& operator=(const char* str);
    String& operator=(const String& other);
    String operator+(const char* str) const;
    String operator+(const String& other) const;
    String& operator+=(const char* str);
    String& operator+=(const String& other);
    char& operator[](int index);

    bool operator==(const char* str) const;
    bool operator==(const String& other) const;
    bool operator<(const char* str) const;
    bool operator<(const String& other) const;

    friend ostream& operator<<(ostream& os, const String& str);
    friend istream& operator>>(istream& is, String& str);

    int Length() const;

private:
    char* data;
};

String::String() {
    data = new char[1];
    data[0] = '\0';
}

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

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

String::~String() {
    delete[] data;
}

String& String::operator=(const char* str) {
    if (data != str) {
        delete[] data;
        int len = strlen(str);
        data = new char[len + 1];
        strcpy(data, str);
    }
    return *this;
}

String& String::operator=(const String& other) {
    if (this != &other) {
        delete[] data;
        int len = other.Length();
        data = new char[len + 1];
        strcpy(data, other.data);
    }
    return *this;
}

String String::operator+(const char* str) const {
    String result;
    int len1 = Length();
    int len2 = strlen(str);
    result.data = new char[len1 + len2 + 1];
    strcpy(result.data, data);
    strcat(result.data, str);
    return result;
}

String String::operator+(const String& other) const {
    String result;
    int len1 = Length();
    int len2 = other.Length();
    result.data = new char[len1 + len2 + 1];
    strcpy(result.data, data);
    strcat(result.data, other.data);
    return result;
}

String& String::operator+=(const char* str) {
    int len1 = Length();
    int len2 = strlen(str);
    char* temp = new char[len1 + len2 + 1];
    strcpy(temp, data);
    strcat(temp, str);
    delete[] data;
    data = temp;
    return *this;
}

String& String::operator+=(const String& other) {
    int len1 = Length();
    int len2 = other.Length();
    char* temp = new char[len1 + len2 + 1];
    strcpy(temp, data);
    strcat(temp, other.data);
    delete[] data;
    data = temp;
    return *this;
}

char& String::operator[](int index) {
    return data[index];
}

bool String::operator==(const char* str) const {
    return strcmp(data, str) == 0;
}

bool String::operator==(const String& other) const {
    return strcmp(data, other.data) == 0;
}

bool String::operator<(const char* str) const {
    return strcmp(data, str) < 0;
}

bool String::operator<(const String& other) const {
    return strcmp(data, other.data) < 0;
}

ostream& operator<<(ostream& os, const String& str) {
    os << str.data;
    return os;
}

istream& operator>>(istream& is, String& str) {
    char temp[100];
    is >> temp;
    str = temp;
    return is;
}

int String::Length() const {
    return strlen(data);
}

int main() {
    String s1("Help!"), s2("Good!"), s3(s2), s4, s5;
    cout << "s1=" << s1 << endl;
    s3 = "Hello!";
    cout << "s3=" << s3 << endl;
    s3 = s2;
    cout << "s3=" << s3 << endl;
    s3 += s2;
    cout << "s3=" << s3 << endl;
    cin >> s4;
    cout << "s4=" << s4 << endl;
    s5 = s3 + s4;
    cout << "s5=" << s5 << endl;
    s5[0] = 'g';
    cout << "s5=" << s5 << endl;
    cout << "strlen(s5)=" << s5.Length() << endl;
    cout << boolalpha << (s3 == s1) << endl;
    cout << boolalpha << (s3 < s1) << endl;
}

测试结果：

s1=Help!
s3=Hello!
s3=Good!
s3=Good!Good!
hello!
s4=hello!
s5=Good!Good!hello!
s5=good!Good!hello!
strlen(s5)=15
false
false