string类实现

#include <iostream>
#include<string.h>
using namespace std;
class String
{
public:
    String(const char* str="");//通用构造函数
    String(const String& another);//拷贝构造函数
    ~String();//析构函数
    String& operator =(const String& rhs);//赋值函数
    bool operator==(const String& str);//字符串相等判断
	friend ostream& operator<<(ostream& o,const String& str);//<<重载
private:
    char* m_data;//用于保存字符串
};
String::String(const char* str)
{
    if(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)
{
    m_data=new char[strlen(another.m_data)+1];
    strcpy(m_data,another.m_data);
}
String::~String()
{
    delete[] m_data;
}
String& String::operator=(const String& rhs)
{
    if(this==&rhs)
        return *this;
    delete[] m_data;//释放原来的内存，新开一块内存
    m_data=new char[strlen(rhs.m_data)+1];
    strcpy(m_data,rhs.m_data);
    return *this;
}
bool String::operator==(const String& str)
{
    return 0==strcmp(m_data,str.m_data);
}
ostream& operator<<(ostream& o,const String& str)
{
    o<<str.m_data;
    return o;
}
int main()
{
    String s;
    String sc(s);
    string st;
    string stc(st);
    cout<<s<<" "<<sc<<endl;
    cout<<st<<" "<<stc<<endl;

    //char *ch=NULL;
    //String sc1(ch); //Interrupt
    //string stc1(ch);//Interrupt

    String s1;
    String s2("s2 copy s1");
    s1=s2;
    string st1;
    string st2("st2 copy st1");
    st1=st2;
    cout<<s2<<endl;
    cout<<st2<<endl;
    cout<<(s1==s2)<<endl;
    cout<<(st1==st2)<<endl;
    return 0;
}

const char* str=""

const char* str=NULL

char *ch=NULL;
String sc1(ch);
string stc1(ch);