AD+EXCHANGE邮件服务器的迁移实战2

class String
{
public:
String(char* str="")
{
_str = new char[strlen(str) + 1];
strcpy(_str, str);
}

String(const String& str)
{
_str = new char[strlen(str._str) + 1];
strcpy(_str, str._str);
}

~String()
{
delete[] _str;
}

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

delete[] _str;

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

return *this;
}

bool operator==(const String& str)
{
return strcmp(_str, str._str) == 0;
}

friend ostream& operator<<(ostream& o,String& str)
{
o << str._str;
return o;
}
private:
char* _str;
};

String s1 = "hello";
String s2 = "world";
String s3 = "o";

cout << "s1=" << s1 << endl;
cout << "s2=" << s2 << endl;
cout << "s3=" << s3 << endl;
cout << endl;

String s4(s1);
String s5(s1);
String s6(s1);

s5 = s2;
cout << "s4=" << s4 << endl;
cout << "s5=" << s5 << endl;
cout << (s4 == s5) << endl;
cout << (s4 == s6) << endl;