operator=() issue

[Arthur Schwarz]

operator=() is private in ios_base. Using private inheritance of ios_base the 
program below fails in the constructor when '=' is used (but not during memory 
initialization). I don't understand why assignment is prohibited.

art

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


Program 2 succeeds

# include <ostream>

using namespace std;

class thing : private ios_base {
    ostream*  yo;
public:
    thing(ostream& y)         { yo = &y; }
    thing(ostream  y)         { yo = &y; }
};

Program 4 fails

# include <ostream>

using namespace std;

class thing : private ios_base {
    ios_base&  xo;
public:
    thing(ios_base& y) { xo = y; }
};

gcc.3.4.4 messaging
x.cpp: In constructor `thing::thing(std::ios_base&)':
x.cpp:9: error: uninitialized reference member `thing::xo'
/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