Thursday, 21 October 2010
The October 2010 mailing for the C++ Standards Committee was published earlier this week. This is the pre-meeting mailing for the November 2010 committee meeting.
As well as the usual core and library issues lists, this mailing also includes a Summary of the status of the FCD comments, along with a whole host of papers attempting to address some the remaining FCD comments.
To move or not to move
The big issue of the upcoming meeting is looking to be whether or not the compiler should implicitly generate move constructors and move assignment operators akin to the copy constructors and copy assignment operators that are currently auto generated. The wording in the FCD requires this, but people are concerned that this will break existing code when people start using their code with a C++0x compiler and library. There are two papers on the subject in the mailing: N3153: Implicit Move Must Go by Dave Abrahams, and N3174: To move or not to move by Bjarne Stroustrup.
There seems to be consensus among committee members that the FCD requires compilers to generate the move constructor and move assignment operator in cases that will break existing code. The key question is whether the breakage can be limited by restricting the cases in which the move members are implicitly generated, or whether implicit generation should be abandoned altogether. The various options are explained very clearly in the papers.
Exceptions and Destructors
N3166: Destructors default to noexcept is another potentially controversial issue. It is generally acknowledged that throwing exceptions from destructors is a bad idea, not least because this leads to termination if the destructor is invoked whilst the stack is being unwound due to another exception. Herb Sutter wrote about this way back in 1998 when the original C++ standard was hot off the presses, in GotW #47: Uncaught Exceptions.
The proposal in the paper comes from a Finnish comment on the FCD,
and is quite simple: by default all destructors are assumed to be
noexcept(true) (which is the new way of saying
they cannot throw an exception, similar to an exception specification
throw()), unless they explicitly have a non-empty
exception specification or are marked
Since it is generally good practice not to throw from a destructor,
you'd think this would be uncontroversial. Unfortunately it is not the
case — there are currently situations where throwing from a
destructor has defined behaviour, and even does exactly what people
want. The example most frequently cited is the SOCI project for accessing
databases from C++. This library provides an easy syntax for
constructing SQL queries using the
<< operator. The
operator builds a temporary object which executes the SQL in the
destructor. If the SQL is invalid, or executing it causes an exception
for any other reason then the destructor throws. Changing destructors
noexcept(true) by default will make such code
terminate on a database error unless the destructor is updated to
declare that it can throw exceptions. Working code with defined
behaviour is thus broken when recompiled with a C++0x compiler.
There are 3 concurrency-related papers in this mailing, which I've summarised below.
- N3152: Progress guarantees for C++0x (US 3 and US 186)
The FCD does not make any progress guarantees when multiple threads are used. In particular, writes made by one thread do not ever have to become visible to other threads, and threads aren't guaranteed ever to actually run at all. This paper looks at the issues and provides wording for minimal guarantees.
- N3164: Adjusting C++ Atomics for C Compatibility
- N3170: Clarifying C++ Futures
There were a few FCD comments from the US about the use of futures; this paper outlines all the issues and potential solutions. The proposed changes are actually fairly minor though:
share()member function for easy conversion to the corresponding
- Accessing a
falseis now required to throw an exception rather than be undefined behaviour;
atomic_futureis to be removed;
A few minor changes have also been made to the wording to make things clearer.
If you have any opinions on any of the papers listed here, or the resolution of any NB comments, please add them to the comments for this post.