C++实现string类

#include <iostream>
#include <cstring>
#include <memory>
using namespace std;

class String
{
    friend ostream& operator<<(ostream&,String&);
    friend istream& operator>>(istream&, String&);
public:
    String();
    String(const char* src);
    String(String& tmpStr);
    String& operator=(String& tmpStr);
    String operator+(String& tmpStr);

    int size()
    {
        return strlen(str);
    }
    ~String()
    {
        delete[] str;
    }
private:
    char* str;
};
String::String()
{
    if (NULL == str)
    {
        str = new char[1];
        str = '\0';
    }
}
String::String(const char* src)
{
    if (NULL == src)
    {
        str = new char[1];
        str = '\0';
    }
    else
    {
        int len = strlen(src);
        str = new char[len+1];
        strncpy(str,src,len);
        str[len]='\0';
    }
}
String::String(String &tmpStr)
{
    if (tmpStr.str == NULL)
    {
        str = new char[1];
        str = '\0';
    }
    else
    {
        int len = strlen(tmpStr.str);
        str = new char[len+1];
        strncpy(str,tmpStr.str,len);
        str[len]='\0';
    }
}

String& String::operator =(String& tmpStr)
{
    if (this != &tmpStr)
    {
        delete[] str;
        if (tmpStr.str == NULL)
        {
            str = new char[1];
            str = '\0';
        }
        else
        {
            int len = strlen(tmpStr.str);
            str = new char[len+1];
            strncpy(str,tmpStr.str,len);
            str[len]='\0';
        }
    }
    return *this;
}
String String::operator +(String& tmpStr)
{
    String newStr;
    if (tmpStr.str == NULL)
    {
        newStr = *this;
    }
    else if (str == NULL)
    {
        newStr = tmpStr;
    }
    else
    {
        int len_str = strlen(str);
        int len_tmp = strlen(tmpStr.str);
        newStr.str = new char[len_str+len_tmp+1];
        strncpy(newStr.str,str,len_str);
        strncpy(newStr.str+len_str,tmpStr.str,len_tmp);
        newStr.str[len_str+len_tmp] = '\0';
    }
    return newStr;
}
ostream& operator<<(ostream& os,String& Str)
{
    os<<Str.str;
    return os;
}
istream& operator>>(istream& is, String& Str)
{
    is>>Str.str;
    return is;
}
void main()
{
    String str1="111111111";
    String str2="2222222222222222";
    String str3 = str1+str2;
    cout<<str3<<"\n"<<str3.size()<<endl;
}

注意：字符串最后的‘\0’.否则会出现乱码