【经典面试题】C++实现String类

Myth丶恋晨 2021-10-25 13:42 444阅读 0赞

String的默认构造函数，构造函数，拷贝构造函数，析构函数，赋值构造函数

#ifndef STRING_H_
    #define STRING_H_
    
    class String
    {
        public:
            String();
            String(const char *str);
            String(const String &rhs);
            ~String();
    
            String &operator=(const String &rhs);
            String operator+(const String &rhs);
            char operator[](const unsigned int index);
            bool operator==(const String &rhs);
            friend std::ostream &operator<<(std::ostream &out, const String &rhs);
        private:
            char *m_data;
    };
    
    
    
    #endif

#include<iostream>
    #include<cstring>
    
    
    String::String()
    {
        std::cout << "default constructor" << std::endl;
        m_data = new char[1];
        m_data[0] = '\0';
    }
    
    String::String(const char *str)
    {
        std::cout << "non-default constructor" << std::endl;
        if (NULL == str)
        {
            m_data = new char[1];
            m_data[0] = '\0';
        }
        else
        {
            m_data = new char[strlen(str)+1];
            strcpy(m_data, str);
        }
    }
    
    String::String(const String &another)
    {
        std::cout << "copy constructor" << std::endl;
        m_data = new char[strlen(another.m_data)+1];
        strcpy(m_data, another.m_data);
    }
    
    bool String::operator==(const String &rhs)
    {
        std::cout << "bool == " << std::endl;
        int result = strcmp(m_data, rhs.m_data);
        if (0 == result)
            return true;
        else
            return false;
    }
    
    String &String::operator=(const String &rhs)
    {
        std::cout << "assign constructor" << std::endl;
        if (this == &rhs)
            return *this;
        delete []m_data;
        m_data = new char[strlen(rhs.m_data)+1];
        strcpy(m_data, rhs.m_data);
        return *this;
    }
    
    String String::operator+(const String &rhs)
    {
        std::cout << "+" << std::endl;
        String newString;
        if (NULL == rhs.m_data)
            newString = *this;
        else if(NULL == m_data)
            newString = rhs;
        else
        {
            newString.m_data = new char[strlen(rhs.m_data)+strlen(m_data)+1];
            strcpy(newString.m_data, m_data);
            strcat(newString.m_data, rhs.m_data);
        }
        return newString;
    }
    
    char String::operator[](const unsigned int index)
    {
        std::cout << "[]" << std::endl;
        return m_data[index];
    }
    
    std::ostream &operator<<(std::ostream &out, const String &rhs)
    {
        out << rhs.m_data;
        return out;
    }
    
    String::~String()
    {
        std::cout << "destructor" << std::endl;
        delete []m_data;
    }
    
    int main(void)
    {
        const char *p = "hello, world";
        String s = "hello, world"; // 构造函数隐式转换 调用非默认构造函数
        String s1(p); // 调用非默认构造函数
        String s2 = s1; // 调用非默认构造函数
        String s3; // 调用默认构造函数
        s3 = s1; // 调用赋值构造函数
        String s4 = s3 + s1; // 调用+运算符，同时调用默认构造函数
        bool flag(s1 == s2); // 调用==运算符
        std::cout << s << std::endl;
        std::cout << s1 << std::endl;
        std::cout << s2 << std::endl;
        std::cout << s3 << std::endl;
        std::cout << flag << std::endl;
        char result = s3[1]; // 调用[]运算符
        std::cout << result << std::endl;
        std::cout << s4 << std::endl;
    
        return 0;
    }