Trip Report: October 2007 ISO C++ Standards Meeting

The ISO C++ committee met in Kona on October 1-6. Here’s a quick summary of what we did, and information about upcoming meetings.

What’s in C++0x, and When?

As I’ve blogged before (most recently here and here), the ISO C++ committee is working on the first major revision to the C++ standard, informally called C++0x, which will include a number of welcome changes and enhancements to the language itself as well as to the standard library.

The committee is determined to finish technical work on this standard in 2009. To that end, we plan to publish a feature-complete draft in September 2008, and then spend the next year fine-tuning it to address public feedback and editorial changes.

Note that this represents essentially a one-year slip from the schedule I blogged about at the beginning of the year. Why the slip? There are a few major features that haven’t yet been "checked in" to the working paper and that are long poles for this release. Here are three notable ones:

• concepts (a way to write constraints for templates that lets us get way better error messages, overloading, and general template usability)
• advanced concurrency libraries (e.g., thread pools and reader-writer locks)
• garbage collection

Probably the biggest thing we did at this meeting was to choose time over scope: We decided that we can’t ship C++0x without concepts, but we can and will ship without some or all of the other two.

Concurrency: This was a pivotal meeting for concurrency. We voted a slew of concurrency extensions into the working draft at this meeting, as they happily all got to the ready point at the same time (see below for details):

• memory model
• atomics library
• basic threads, locks, and condition variables

And we decided to essentially stop there; we still plan to add an asynchronous future<T> type in C++0x, but features like thread pools are being deferred until after this standard.

Garbage collection: For C++0x, we’re not going to add explicit support for garbage collection, and only intend to find ways to remove blocking issues like pointer hiding that make it difficult to add garbage collection in a C++ implementation. In particular, the scope of this feature is expected to be constrained as follows:

• C++0x will include making some uses of disguised pointers undefined, and providing a small set of functions to exempt specific objects from this restriction and to designate pointer-free regions of memory (where these functions would have trivial implementations in a non-collected conforming implementation).
• C++0x will not include explicit syntax or functions for garbage collection or related features such as finalization. These could well be considered again after C++0x ships.

What we voted into draft C++0x

Here is a list of the main features we voted into the C++0x working draft at this meeting, with links to the relevant papers to read for more information.

nullptr (N2431)

This is an extension from C++/CLI that allows explicitly writing nullptr to designate a null pointer, instead of using the unfortunately-overloaded literal 0 (including its macro spelling of NULL) which makes it hard to distinguish between an null and a zero integer. The proposal was written by myself and Bjarne.

Explicit conversion operators (N2437 and N2434)

You know how in C++ you can make converting constructors only fire when invoked explicitly?

class C {
public:
  C( int );
explicit C( string );
};

void f( C );

f( 1 ); // ok, implicit conversion from int to C
f( "xyzzy" ); // error, no implicit from string literal to C
f( C("xyzzy") ); // ok, explicit conversion to C

But C++ has two ways to write an implicit conversion — using a one-argument constructor as above to convert "from" some other type, or as a conversion operator "to" some other type as shown below, and we now allow explicit also on this second form:

class C {
public:
  operator int();
explicit operator string(); // <– the new feature
};

void f( int );
void g( string );

C c;
f( c ); // ok, implicit conversion from C to int
g( c ); // error, no implicit from C to string
g( string(c) ); // ok, explicit conversion to C

Now the feature is symmetric, which is cool. See paper N2434 for how this feature is being used within the C++ standard library itself.

Concurrency memory model (N2429)

As I wrote in "The Free Lunch Is Over", chip designers and compiler writers "are under so much pressure to deliver ever-faster CPUs that they’ll risk changing the meaning of your program, and possibly break it, in order to make it run faster." This only gets worse in the presence of multiple cores and processors.

A memory model is probably of the lowest-level treaty between programmers and would-be optimizers, and fundamental for any higher-level concurrency work. Quoting from my memory model paper: "A memory model describes (a) how memory reads and writes may be executed by a processor relative to their program order, and (b) how writes by one processor may become visible to other processors. Both aspects affect the valid optimizations that can be performed by compilers, physical processors, and caches, and therefore a key role of the memory model is to define the tradeoff between programmability (stronger guarantees for programmers) and performance (greater flexibility for reordering program memory operations)."

Atomic types (N2427)

Closely related to the memory model is the feature of atomic types that are safe to use concurrently without locks. In C++0x, they will be spelled "atomic<T>". Here’s a sample of how to use them for correct (yes, correct!) Double-Checked Locking in the implementation of a singleton Widget:

atomic<Widget*> Widget::pInstance = 0;

Widget* Widget::Instance() {
  if( pInstance == 0 ) { // 1: first check
    lock<mutex> hold( mutW ); // 2: acquire lock (crit sec enter)
    if( pInstance == 0 ) { // 3: second check
      pInstance = new Widget(); // 4: create and assign
    }
  } // 5: release lock
  return pInstance; // 6: return pointer
}

Threading library (N2447)

You might have noticed that the above example used a lock<mutex>. Those are now in the draft standard too. C++0x now includes support for threads, various flavors of mutexes and locks, and condition variables, along with s
ome other useful concurrency helpers like an efficient and portable std::call_once.

Some other approved features

• N2170 "Universal Character Names in Literals"
• N2442 "Raw and Unicode String Literals; Unified Proposal (Rev. 2)"
• N2439 "Extending move semantics to *this (revised wording)"
• N2071 "Iostream manipulators for convenient extraction and insertion of struct tm objects"
• N2401 "Code Conversion Facets for the Standard C++ Library"
• N2440 "Abandoning a Process"
• N2436 "Small Allocator Fix-ups"
• N2408 "Simple Numeric Access Revision 2"

Next Meetings

Here are the next meetings of the ISO C++ standards committee, with links to meeting information where available.

• February 24 – March 1, 2008: Bellevue, Washington, USA (N2465)
• June 8-14, 2008: Sophia Antipolis, France
• September 14-20, 2008: San Francisco Bay area, California, USA (this is the anticipated date)

The meetings are public, and if you’re in the area please feel free to drop by.