Re: operator=() issue

[Arthur Schwarz <aschwarz1309 at verizon dot net>]

To all, I stand abashed - don't try this without a trained instructor. I misread the Schildt quote and (I think) wasted your time(s). 

Thank you
art

--- On Fri, 4/10/09, David Fang <fang@csl.cornell.edu> wrote:

> From: David Fang <fang@csl.cornell.edu>
> Subject: Re: operator=() issue
> To: "Arthur Schwarz" <aschwarz1309@verizon.net>
> Cc: gcc@gcc.gnu.org
> Date: Friday, April 10, 2009, 5:45 PM
> One more thing to add ...
> 
> >> Program 1 fails
> >> # include <ostream>
> >> 
> >> using namespace std;
> >> 
> >> class thing : private ios_base {
> >>? ? ostream& xo;
> >> public:
> >>? ? thing(ostream& y) : xo(y) { xo =
> y; }
> >> };
> >> 
> >> gcc.3.4.4 messaging
> >> x.cpp: In member function `std::basic_ios<char,
> std::char_traits<char> >&
> std::basic_ios<char, std::char_traits<char>
> >::operator=(const std::basic_ios<char,
> std::char_traits<char> >&)':
> >>
> /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ios_base.h:784:
> error: `std::ios_base& std::ios_base::operator=(const
> std::ios_base&)' is private
> >> x.cpp:9: error: within this context
> > 
> > std::ios_base was never meant to be
> copy-able/assign-able, this has nothing to do with
> public/private *inheritance*, since it is the members of the
> base that are private, and thus inaccessible to any derived
> classes.
> > 
> > In your thing::thing ctor:
> > 
> > "xo(y)" initializes the member *reference*
> (essentially taking the address of y), whereas "xo = y;" is
> assigning the *object* referenced by 'ox', which is not the
> same.? This is why you hit the inaccessible assignment
> error.
> 
> The real problem is that you are assigning an ostream to an
> ostream, which is not allowed *in any context* because
> ostream ultimately derives from ios_base, which privatizes
> assignment.? You are seeing this message about ios_base
> (misleading you to think it has to do with your thing class
> inheriting from ios_base) because the default assignment of
> class ostream (not explicitly defined) is invoking the
> default assignments of its base classes, including
> ios_base.? This is more an issue of mis-using the
> standard ostream class.
> 
> Fang
> 
> David Fang
> http://www.csl.cornell.edu/~fang/
> http://www.achronix.com/
> 
>