Re: What's wrong with the member function?

Hi,
I am learning C++ with C++ primer written by Lippman. The first output
line: cout << s1 << endl; works right. The second is wrong. I find the
problem is that s1 is destroyed in the call from "result += s;". An=

even though the "result" is correct in the routine:

String
String::operator+( const String &s ) const {
   String result = *this;
   result += s;
   return result;
}

It is wrong after callback. I am not sure where is the problem. Because
it is from the book example, maybe there is some little part I do not
notice. Thank you very much.

------------
#include <string.h>
#include <assert.h>

class String {
   friend ostream&
           operator<<( ostream&, String& );

public:
   String( const char*);
   String( int );
   String operator+(const String &) const; String& operator+=(cons=

   &);
   ~String();
private:
   int len;
   char *str;
};

String::String( const char *s)
{
   len = strlen( s );
   str = new char[ len + 1 ];
   assert( str != 0);

   strcpy(str, s );
}

String::~String()
{ delete str;
}

ostream& operator<<( ostream& os, String& str ) {
   char *s = str.str;
   while ( *s ) os.put( *s++ );
   return os;
}

String&
String::operator+=( const String &s ) {
   len += s.len;
   char *p = new char[len+1];
   assert( p != 0);
   strcpy(p,str);
   strcat(p,s.str);
   delete str;
   str=p;
   return *this;
}

String
String::operator+( const String &s ) const {
   String result = *this;
   result += s;
   return result;
}
-----------------------
#include <iostream.h>
#include "String.h"

main()
{
   String s1("gobbledy");
   String s2("gook");
   s1 += s2;
   cout << s1 << endl;

   String s3 = s1 + s2;
   cout << s3 << endl;

   return 0;
}