[Boost-users] boost::iostreams::copy() closes the sink and source?

From: Melville, Ryan (Ryan_Melville_at_[hidden])
Date: 2008-06-24 09:11:10

Hi,

 

While upgrading from boost 1.34.1 to boost 1.35.0 we encountered an
issue with boost::iostreams.

 

It seems that boost::iostreams::copy() closes its Source and Sink input
parameters once it completes its copy.  This seems to be a problem for
us because copy() is not the last thing we want to do with our Sinks; we
copy() into them but then also put more stuff in them (which now fails).
This seems like a natural use case to me...

 

Does anyone know why boost::iostreams::copy() feels the need to close
the Source and Sink passed to it?  It seems to me that it should not.
The caller may or may not want them closed after the copy() and this
behavior precludes using copy() as anything other than the last thing
you want to do with a Sink.

 

BTW, the apparent intended behavior of copy() is not different in this
regard between 1.34.1 and 1.35.0.  Rather, the trigger here was that our
boost::iostreams::filtering_ostream derived classes are now "closable"
by virtue of the "closable_tag" being added to the
boost::iostreams::filtering_ostream base class' "categories".  Once they
became "closeable", copy()'s invocation of close() actually resulted in
a close() on them instead of just a flush().

 

Any insight would be greatly appreciated.  Unless I hear some rational
for the current behavior I'm inclined to change copy() to not close its
Source and Sink.  For backwards compatibility, I suppose a "bool close =
true" input parameter could be added.  That would allow one to override
the existing behavior without changing the current behavior on existing
code.  Although, it really doesn't seem natural to have something called
"copy()" close its input parameters and I'm sure the unwary will get
burned like us when subsequent uses of the Sink fail unexpectedly.

 

Also, I'm not sure how this interacts with "const" inputs.  The
documentation implies that copy() also works with a const Source or
const Sink, but I haven't tried that.  It could not close a const Source
or Sink, right?  And it could not even write into a const Sink so I'm
not sure what the purpose of that input combination would be.

 

Regards,

 

Ryan

 

 

 

Ryan Melville 

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