PR 3983, basic_istringstream<unsigned char> fails

[Nathan Myers]

On Mon, Feb 25, 2002 at 11:10:47AM -0800, Benjamin Kosnik wrote:
> 
> This issue keeps resurfacing in various forms. I should probably write 
> something up and put it in the 27_io FAQ. 
> 
> Here's the first discussion:
> 2001-12-03
> 	_M_fill initialization in basic_ios::init
> 	http://gcc.gnu.org/ml/libstdc++/2001-12/msg00021.html
> 	http://gcc.gnu.org/ml/libstdc++/2001-12/msg00106.html
> 	http://gcc.gnu.org/ml/libstdc++/2001-12/msg00113.html
> 	http://gcc.gnu.org/ml/libstdc++/2001-12/msg00120.html
> ... 
>       // NB: The 27.4.4.1 Postconditions Table only specifies
>       // requirements after basic_ios::init() has been called. As part
>       // of this, fill() must return widen(' '), which needs an imbued
>       // ctype facet of char_type to return without throwing an
>       // exception. This is not a required facet, so streams with
>       // char_type != [char, wchar_t] will not have it by
>       // default. However, because fill()'s signature is const, this
>       // data member cannot be lazily initialized.  Thus, thoughts of
>       // using a non-const helper function in ostream inserters is
>       // really besides the point.
>       _M_fill = this->widen(' ');

I don't understand this.  It doesn't matter that fill() is const.  
The _M_fill member (as any other data member) can be mutable if
declaring it so enables an optimization.

Nathan Myers
ncm at cantrip dot org