string类的实现

#include<stdio.h>
#include <iostream>
#include<string.h>
using namespace std;

class string_my
{
public:
    string_my(const char *str=NULL);//普通构造函数
    string_my(const string_my &other);//拷贝构造函数
    ~ string_my(void);//析构函数
    string_my&  operator=(const string_my &other);//赋值函数
    friend ostream & operator<<(ostream& o,const string_my &str);



private:
    char *m_data;
};
ostream & operator<<(ostream &o,const string_my &str)
{
    o<<str.m_data;
    return o;

}
string_my::string_my(const char *str)
{
    if(str==NULL)
    {
        m_data=new char[1];
        *m_data='\0';
    }
    else
    {
        int length=strlen(str);
        m_data=new char[length+1];
        strcpy(m_data,str);
    }
}

string_my::~string_my(void)
{
    delete []m_data;
}
string_my::string_my(const string_my &other)
{
    int length=strlen(other.m_data);
    m_data=new char[length+1];
    strcpy(m_data,other.m_data);
}


string_my& string_my::operator=(const string_my &other)
{
    if(this==&other)
        return *this;
    delete []m_data;
    int length=strlen(other.m_data);
    m_data=new char[length+1];
    strcpy(m_data,other.m_data);
    return *this;
}
int main()
{
    string_my s("sasa");
    cout<<s<<endl;
    string_my q("qqqqq");
    s=q;
    cout<<s<<endl;
    s="12123343";
    cout<<s<<endl;

}