重载运算符函数及其注意事项

#include "stdafx.h"
#include <string>

class CMyString
{
public:
    CMyString(char* pData = NULL);
    CMyString(const CMyString& str);
    ~CMyString(void);

    CMyString& operator = (const CMyString& str);

    void Print();
      
private:
    char* m_pData;
};

CMyString::CMyString(char *pData)
{
    if(pData == NULL)
    {
        m_pData = new char[1];
        m_pData[0] = '';
    }
    else
    {
        int length = strlen(pData);
        m_pData = new char[length + 1];
        strcpy(m_pData, pData);
    }
}

CMyString::CMyString(const CMyString &str)
{
    int length = strlen(str.m_pData);
    m_pData = new char[length + 1];
    strcpy(m_pData, str.m_pData);
}

CMyString::~CMyString()
{
    delete[] m_pData;
}

CMyString& CMyString::operator = (const CMyString& str)
{
    if(this == &str)
        return *this;

    delete []m_pData;
    m_pData = NULL;

    m_pData = new char[strlen(str.m_pData) + 1];
    strcpy(m_pData, str.m_pData);

    return *this;
}

// ====================测试代码====================
void CMyString::Print()
{
    printf("%s", m_pData);
}

void Test1()
{
    printf("Test1 begins:
");

    char* text = "Hello world";

    CMyString str1(text);
    CMyString str2;
    str2 = str1;

    printf("The expected result is: %s.
", text);

    printf("The actual result is: ");
    str2.Print();
    printf(".
");
}

// 赋值给自己
void Test2()
{
    printf("Test2 begins:
");

    char* text = "Hello world";

    CMyString str1(text);
    str1 = str1;

    printf("The expected result is: %s.
", text);

    printf("The actual result is: ");
    str1.Print();
    printf(".
");
}

// 连续赋值
void Test3()
{
    printf("Test3 begins:
");

    char* text = "Hello world";

    CMyString str1(text);
    CMyString str2, str3;
    str3 = str2 = str1;

    printf("The expected result is: %s.
", text);

    printf("The actual result is: ");
    str2.Print();
    printf(".
");

    printf("The expected result is: %s.
", text);

    printf("The actual result is: ");
    str3.Print();
    printf(".
");
}

int _tmain(int argc, _TCHAR* argv[])
{
    Test1();
    Test2();
    Test3();

    return 0;
}

剑指offer里面提到，如果让写一个重载运算符函数需要注意的4个事项

1：返回值一定要是引用，也就是自身*this，这样才能保证连续赋值

2:：形参要是引用不能是实例，否则会自身调用复制构造函数，造成浪费，减慢效率，同时为了不修改，应该const 修饰

3：在分配内存之前要释放以前的内存

4：最初一定要判断*this和传入的是否是同一个，如果是同一个，那么直接返回，否则，会delete全部删除