Re: operator=() issue

[David Fang <fang at csl dot cornell dot edu>]

> 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

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.

HTH,

Fang


David Fang
http://www.csl.cornell.edu/~fang/
http://www.achronix.com/