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;". And
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+=(const

     &);
    ~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;
}