Re: [boost] manipulators for filtering_stream s

Subject: Re: [boost] manipulators for filtering_stream s
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-01-21 18:26:18

On 03/01/08 04:58, Roland Schwarz wrote:
 >
 > Larry Evans wrote:
 >> A long time ago, I had one and was intending *eventually* to add it to
 >> iostreams, but never got around to it.

[snip]

 > I am not sure however you noticed the difference to my implementation:
 >
 > I am storing state _in_ the stream, i.e. in ios base member and thus
 > allowing
 > out << indent_in << "blah blah\n";
 > out << "still indented\n";
 >
 > And a function
 > void foo(ostream& os)
 >
 > that gets passed down my indent_filter'ed stream still knowns
 > the indentation status.
 >
 > The margin_ostream implements indent operation only the stream object.
 > Does is also have manipulators?

No.

 >
 > Will margin_ostream also work when downcasted to a plain ostream?
 >

No.

However, I've looked further at indent.hpp and the storing of the
pointer to the filter in the ios_base using ios_base::pword.  What's
the advantage of using ios_base::pword to store the pointer to
something that's already available via basic_ios<...>::rd_buf and then
filtering_streambuf<...>::component<T>(int):

 
http://www.boost.org/doc/libs/1_37_0/libs/iostreams/doc/classes/filtering_streambuf.html

This design is implemented in boost vault under the "Input - Output"
directory in indent_scoped_ostreambuf.zip.