Re: iomanip to escape xml

James Kanze <>
Wed, 20 Jan 2010 12:32:48 -0800 (PST)
On Jan 19, 9:02 am, shaun <> wrote:

My other (and currently used) solution is to define a new
ostream with a new unbuffered stream class which overrides the
'overflow' method so that xml-escape characters become
entities; so my final code looks like
MyXmlStreamBuffer xbufstream;
std::ostream xstr(&xbufstream);
Is there a solution closer to my original intention?

What's wrong with this solution? It's the more or less standard
idiom in such cases.

e.g. is it possible to use the iword/pword information in
combination with some method I am unaware of which can change
the characters on output?

You can't modify the formatting of e.g. a string to ostream. It
is possible to define a special manipulator which returns
something other than an ostream, then define some different
formatting on that type, but if I understand you correctly, what
you really want is to translate specific characters, regardless
of what formatting has produced them. And that's the role of
the streambuf.

Thanks for the reply. I should like to be able to turn this on and off
and mix it with cout as well; my application is outputting xml tags
(which have to retain the unchanged '<' and '>') followed by text
which has to be escaped.

You can insert and remove filtering streambuf's on the fly,
provided you ensure proper synchronization -- for output, just
not buffering in the filtering streambuf is sufficient.

At the moment I use cout to output the tags and then my xstr
instance to output the xml which must be escaped. Because my
xstr is unbuffered, I have to be sure to flush cout before
sending things to xstr. Possibly there is a way to not only
buffer xstr but also to synchronize the buffer with the cout

Have it forward to the cout buffer, and don't buffer in it.

Or some way to use my custom buffer but turn on/off
the conversion with a manipulator (so I dont have to use cout
at all)?

That's also possible, but a little more awkward. Basically, you
can arrange for the manipulator to insert or remove the
streambuf on the fly. But you'll probably need some way of
keeping track of the state (using pword), and you may need to
catch events from the stream in order to ensure clean up.

I'll think some more; right now I consider the solution
adequate (for my current use) but unsatisfying (I'd like to
re-use it in other contexts).

For starters, check out the literature on filtering streambufs
(sort of a bit of self-publicing there:-)). And check out
Boost's iostream---it contains a very well done implementation
of filtering streambufs. And if you want a manipulator, check
out ios::xalloc and ios::pword, and the possibility of
registering for events from the stream. (One of the events, for
example, is that the stream is being destructed, so if you
allocated memory somewhere, you can free it.)

James Kanze

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