C/C++面试题—实现MyString类

柔光的暖阳◎ 2022-05-24 03:35 250阅读 0赞

# 题目介绍 #

C++岗位面试题：实现自定义MyString类，实现拷贝构造函数和 \[\]、=、+、== 、!=、<< 、>> 运算符重载函数。  
自定义MyString估计是C++岗位常考的题目吧，能够较好的验证面试者对C++基础知识掌握的是否牢固。值得每个C++程序员注意的时，如果参数有传递引用，一定要有非空验证，因为引用帮我们封装了指针，MyString \*pStr = nullptr; 将\*pstr传递给引用参数是不会报错的，但是程序运行会崩溃的！  
下面是笔者的写法。

# MyString.h #

#pragma once
    #include <cstdlib>
    #include <iostream>
    using namespace std;
    class MyString
    {
    public:
        MyString(const char* pData = nullptr);
        MyString(const MyString& otherStr);
        ~MyString();
    
        MyString& operator=(const MyString& otherStr);
        MyString& operator=(const char* pData);
    
        char& operator[](unsigned int index);
    
        MyString operator+(const MyString &str);
        MyString operator+(const char* pData);
    
        bool operator==(const MyString &str);
        bool operator==(const char* pData);
    
        bool operator!=(const MyString &str);
        bool operator!=(const char* pData);
    
        friend ostream& operator<<(ostream& cout, const MyString& str);
        friend istream& operator>>(istream& cin, MyString& str);
    
    
    private:
        char* m_pData;
        int m_size;
        int m_capacity;
    };
    ostream& operator<<(ostream& cout, const MyString& str);
    
    istream & operator >> (istream & cin, MyString & str);

# MyString.cpp #

#include "01MyString.h"
    
    /* 普通构造函数 */
    MyString::MyString(const char * pData)
    {
        if (nullptr == pData)
        {
            m_size = 0;
            m_capacity = 100;
            m_pData = new char[m_capacity]{ 0 };
            return;
        }
        m_size = strlen(pData);
        m_capacity = 2 * m_size + 1;    //开辟2倍大空间，方便后续存储
        m_pData = new char[m_capacity]{ 0 };
        strcpy(m_pData, pData);
    }
    /* 拷贝构造函数 */
    MyString::MyString(const MyString & otherStr)   
    {
        if (nullptr == &otherStr)   //参数传递引用时一定要做非空验证
        {
            m_size = 0;
            m_capacity = 100;
            m_pData = new char[m_capacity] { 0 };
            return;
        }
        m_size = otherStr.m_size;
        m_capacity = otherStr.m_capacity;
        m_pData = new char[m_capacity]{ 0 };
        strcpy(m_pData, otherStr.m_pData);
    }
    
    /* 析构函数 */
    MyString::~MyString()
    {
        if(nullptr != m_pData)
            delete[] m_pData;
    }
    
    MyString & MyString::operator=(const MyString & otherStr)
    {
        if (this != &otherStr)          //剑指offer写法
        {
            MyString tmpStr(otherStr);  //调用拷贝构造，交换字符串指针
            char* p = tmpStr.m_pData;
            tmpStr.m_pData = m_pData;
            m_pData = p;
        }
        return *this;
    }
    
    MyString & MyString::operator=(const char * pData)
    {
        MyString tmpStr(pData); //调用构造函数
        char* p = tmpStr.m_pData;
        tmpStr.m_pData = m_pData;
        m_pData = p;
        return *this;
    }
    
    /* 返回新对象 */
    MyString MyString::operator+(const MyString & str)
    {
        if (nullptr == &str)
        {
            return MyString(*this);
        }
        int newSize = m_size + str.m_size + 1;
        char* temp = new char[newSize] {
       0};
        strcat(temp, m_pData);
        strcat(temp, str.m_pData);
        MyString res(temp);
        delete[] temp;
    
        return res;
    }
    /* 返回新对象 */
    MyString MyString::operator+(const char * pData)
    {
        if (nullptr == pData)
        {
            return MyString(pData);
        }
        int newSize = m_size + strlen(pData) + 1;
        char* temp = new char[newSize] {
       0};
        strcat(temp, m_pData);
        strcat(temp, pData);
        MyString res(temp);
        delete[] temp;
    
        return res;
    }
    
    char & MyString::operator[](unsigned int index)
    {
        return m_pData[index];
    }
    
    bool MyString::operator==(const MyString & str)
    {
        bool res = false;
        if (nullptr == &str)
            res = false;
        if (this == &str)
            res = true;
        if (strcmp(m_pData, str.m_pData) == 0)
            res = true;
        return res;
    }
    
    bool MyString::operator==(const char * pData)
    {
        bool res = false;
        if (nullptr == pData)
            res = false;
        if (strcmp(m_pData, pData) == 0)
            res = true;
        return res;
    }
    
    bool MyString::operator!=(const MyString & str)
    {
        return !operator==(str);
    }
    
    bool MyString::operator!=(const char * pData)
    {
        return !operator==(pData);
    }
    
    ostream& operator<<(ostream& cout, const MyString& str)
    {
        if (nullptr == &str)
            return cout;
        cout << str.m_pData;
        return cout;
    }
    
    inline istream & operator >> (istream & cin, MyString & str)
    {
        if (nullptr == &str)
            return cin;
        memset(str.m_pData, 0, str.m_capacity); //清空数据
        cin >> str.m_pData;
        return cin;
    }
    
    int main(int argc, char *argv[])
    {
        //测试 构造函数 拷贝构造函数
        MyString name = "Laymond";
        MyString copyName = name;
        cout << name << " " << copyName << endl;
        //测试 [] 运算符
        copyName[0] = 'A';
        cout << copyName << endl;
        //测试 + = 运算符
        copyName = name + " is " + "good !";
        cout << copyName << endl;
    
        //测试<< >> == != 运算符
        MyString str1, str2;
        cin >> str1 >> str2;
        cout << "str1:"<<str1 << endl;
        cout << "str2:"<<str2 << endl;
        cout << (MyString("hello") == "hello") << endl;
        cout << (MyString("hello") != MyString("hello")) << endl;
        cout << (MyString("hello") == MyString("hello")) << endl;
    
        //在参数中引用的地方，一定要做非空验证，比如说如下，如果拷贝构造函数 没有做非空验证 程序会崩溃
        MyString *pStr = NULL;
        MyString str(*pStr);
    
        return 0;
    }

# 运行检测 #

![这里写图片描述][70]

[70]: /images/20220524/9cce484db4b24dff890222372acbd0e8.png