[boost] [system][filesystem v3] Question about error_code arguments

Subject: [boost] [system][filesystem v3] Question about error_code arguments
From: Beman Dawes (bdawes_at_[hidden])
Date: 2009-10-20 11:15:37

A POSIX sub-group is looking at C++ bindings for POSIX.

They are talking about a filesystem library binding at least loosely
based on Boost.System, Boost.Filesystem, and C++ TR2.

In looking at http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2838.html,
"Library Support for Hybrid Error Handling (Rev 2)", they are
concerned about specifying hybrid error handling like this:

     void f(error_code& ec=throws());

They would rather see it specified like this:

     void f(error_code* ec=0);

One particular concern is that as currently worded, the N2838 proposal
requires throws() return a null reference, which they see as relying
on undefined behavior. I wasn't at the meeting where the LWG asked
that a null reference be mandated; my reading of the standard is that
it is unclear if returning a null reference is or isn't undefined
behavior. Some of the core folks seem to think it is undefined, but
compilers accept it.

Although the reference form is usual for C++, the use of anything
other than the default is limited, and usually in low-level code at
that. Although I could personally live with either form, I'm leaning
slightly toward the pointer form.

Regardless, I'd like to hear Boosters views on the two interfaces
above. Which do you prefer? Why?

--Beman