string 模拟之-big three

Big-three是指拷贝构造，赋值和析构这三个函数一般一起出现或都不出现

下面是一个模拟string的类。这个类的写法有以下几点需要注意：

1 赋值函数的写法，尤其是参数的写法 MyString& operator= (const MyString& str)

2 在new之后，不用去check 指针是否为null，因为new如果不成功会有exception抛出

3 在赋值函数中，没有去比较是否为自己给自己赋值，这样是因为这种情况很少，增加了程序执行的效率；

    用了一个临时变量，这样在new的过程中即使出现exception，也不会影响原来的值

#include <cstring>#include <iostream>#include <vector>using namespace std;class MyString{    public:        MyString(const char* str);        MyString();        //copy constructor        MyString(const MyString& lval);        //assignment operator        MyString& operator= (const MyString& rval);        //deconstructor        ~MyString();    private:        char* m_data;};MyString::MyString():m_data(new char[1]){    cout << "default constructor called" << endl;    m_data[0] = '\0';}MyString::MyString(const MyString& lval){    std::cout << "copy constructor called" <<  endl;    m_data = new char[strlen(lval.m_data) + 1];    strcpy(m_data, lval.m_data);}MyString::MyString(const char* str){    std::cout << "constructor1 called" << endl;    m_data = new char[strlen(str) + 1];    strcpy(m_data, str);}MyString::~MyString(){    cout << "deconstructor called" << endl;    delete[] m_data;}MyString& MyString::operator= (const MyString& rval){    std::cout << "Assignment called" << endl;    char* tmp = new char[strlen(rval.m_data) + 1];    strcpy(tmp, rval.m_data);    delete[] m_data;    m_data = tmp;    return *this;}int main(){    cout << "string 1" << endl;    MyString str1 = "copy";    cout << "string 2" << endl;    MyString str2 = str1;    cout << "string 3" << endl;    MyString str3;    str3 = str1;}

The output is:

string 1
constructor1 called
string 2
copy constructor called
string 3
default constructor called
Assignment called
deconstructor called
deconstructor called
deconstructor called