模拟实现string类，重载运算符“+”、“-”、“+=”、“==”、“!=”、“<”、“<=”、“>”、“>=”、“[]”、“>>”、“<<”，实现两个字符串的赋值、连接、相等的判断、不等的判断、大小比较、访问、输入和输出运算。

以下是一个简单的模拟实现string类的代码：

#include <iostream>
#include <cstring>

class String {
public:
    // 构造函数
    String() : m_data(nullptr), m_size(0) {}
    String(const char* str) : m_data(nullptr), m_size(0) {
        if (str) {
            m_size = std::strlen(str);
            m_data = new char[m_size + 1];
            std::strcpy(m_data, str);
        }
    }
    String(const String& other) : m_data(nullptr), m_size(other.m_size) {
        if (other.m_data) {
            m_data = new char[m_size + 1];
            std::strcpy(m_data, other.m_data);
        }
    }
    // 析构函数
    ~String() {
        if (m_data) {
            delete[] m_data;
            m_data = nullptr;
        }
    }
    // 重载运算符
    String operator+(const String& other) const {
        String newString;
        newString.m_size = m_size + other.m_size;
        newString.m_data = new char[newString.m_size + 1];
        std::strcpy(newString.m_data, m_data);
        std::strcat(newString.m_data, other.m_data);
        return newString;
    }
    String operator-(const String& other) const {
        String newString;
        newString.m_size = m_size - other.m_size;
        newString.m_data = new char[newString.m_size + 1];
        std::strcpy(newString.m_data, m_data);
        std::strtok(newString.m_data, other.m_data);
        return newString;
    }
    String& operator+=(const String& other) {
        m_size += other.m_size;
        char* newData = new char[m_size + 1];
        std::strcpy(newData, m_data);
        std::strcat(newData, other.m_data);
        delete[] m_data;
        m_data = newData;
        return *this;
    }
    bool operator==(const String& other) const {
        return std::strcmp(m_data, other.m_data) == 0;
    }
    bool operator!=(const String& other) const {
        return std::strcmp(m_data, other.m_data) != 0;
    }
    bool operator<(const String& other) const {
        return std::strcmp(m_data, other.m_data) < 0;
    }
    bool operator<=(const String& other) const {
        return std::strcmp(m_data, other.m_data) <= 0;
    }
    bool operator>(const String& other) const {
        return std::strcmp(m_data, other.m_data) > 0;
    }
    bool operator>=(const String& other) const {
        return std::strcmp(m_data, other.m_data) >= 0;
    }
    char operator[](int index) const {
        return m_data[index];
    }
    char& operator[](int index) {
        return m_data[index];
    }
    friend std::ostream& operator<<(std::ostream& os, const String& str);
    friend std::istream& operator>>(std::istream& is, String& str);
private:
    char* m_data;
    int m_size;
};

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

std::istream& operator>>(std::istream& is, String& str) {
    char buffer[1024];
    is >> buffer;
    str = buffer;
    return is;
}

int main() {
    String str1("Hello");
    String str2("World");
    String str3 = str1 + str2;
    std::cout << str3 << std::endl;
    str3 += "!";
    std::cout << str3 << std::endl;
    std::cout << (str1 == str2) << std::endl;
    std::cout << (str1 != str2) << std::endl;
    std::cout << (str1 < str2) << std::endl;
    std::cout << (str1 <= str2) << std::endl;
    std::cout << (str1 > str2) << std::endl;
    std::cout << (str1 >= str2) << std::endl;
    std::cout << str1[0] << std::endl;
    std::cout << str2[1] << std::endl;
    String str4;
    std::cin >> str4;
    std::cout << str4 << std::endl;
    return 0;
}

注意，这只是一个简单的实现，还有很多细节问题需要考虑，比如内存泄漏、异常处理、Unicode编码等等。