Red Hat at the ISO C++ Standards Meeting, Bristol, UK

Red Hat has actively participated in the ISO group defining the C++ standard for many years, and continues to make a significant contribution. The Red Hat toolchain team was well-represented at the spring meeting of the standardization committee (technically JTC1/SC22/WG21) in Bristol, UK, last month: we had three people there for the full week, with one other visiting a couple of times during the week. In this article, Jason Merrill summarizes the main highlights and developments of interest to Red Hat's customers and partners:

Since the publication of the 2011 standard, (you can buy a copy of the full standard from ansi.org here) the pace of development of the language has picked up significantly, and the committee is aiming for an updated standard in 2014, with a few additional features relative to C++11.  If we're going to keep to that schedule, this was the last meeting for accepting any new features into the working draft.

The new language features we accepted for 2014 are:

• Binary literals

   

  unsigned mask = 0b101001

  These have been accepted by GCC since 4.3 as a GNU extension.

• Return type deduction for normal functions:

  auto f() { return 42; }

  This addresses a hole noticed by many C++ users introduced to C++11 auto and lambda return type deduction, by extending the same return type deduction semantics to normal functions.  This proposal and its initial implementation were developed in GCC by Red Hat's Jason Merrill.

• Arrays of runtime bound (VLAs):

  void f(int n) { int array[n]; ... }

  Variable length arrays have been part of GCC for many years, and were added to C in the C99 standard; with the acceptance of this proposal, they are now part of C++ as well.  But there are some significant restrictions relative to the C/GCC feature: C++ VLA types can only be used to declare array variables, and trying to create a pointer or reference to such an array, or take its sizeof, is ill-formed.  For more flexible arrays of dynamic size, see std::dynarray below.

• Lambda capture initializers:
   

  [x = 42]{ return x; }

  This was part of the original lambda proposal for C++11, and has been supported by GCC since lambdas were added in GCC 4.5.

• Polymorphic/generic lambdas:
   

  [](auto x){ return x+1; }
  

  Several people at the meeting said that this is the feature they've had the most users asking for to complete C++11.  The syntax causes the lambda function-call operator to be a member template rather than a non-template function.

• Variable templates:

  template <class T> constexpr T pi = 3.14159...

  In C++11, programmers who want to write a simple parameterized constant need to wrap their variable in a class template; the next standard will allow them to make the variable itself a template.  This feature has been developed in G++.

• Extended constexpr:
   

  constexpr int fact(int i) {
    int product = 0;
    for (int j = 1; j < i; ++j)
      product *= j;
      return j;
    }

  This proposal removes many of the C++11 restrictions on operations within a constexpr function: most notably, a constant expression can now modify a variable with a lifetime that is limited to the expression, and loops can be used. 

We also expect to publish at around the same time a separate technical specification describing the "concepts lite" feature (http://concepts.axiomatics.org/~ans/), which has been developed in GCC and currently seems likely to be part of C++17 if it is not supplanted by a different concepts design. 

Notable new library functionality accepted for the next standard includes:

• Compile-time integer sequences:

   

  template <size_t... I>
  void f(std::index_sequence<I...>)
  { dump(db.get(I)...); }

• Addressing tuples by type:

  tuple<string, string, int> t("foo", "bar", 7);
  int i = get<int>(t); // i == 7

• std::exchange, to provide usage similar to atomic exchange:

  if (pointer old = std::exchange(ptr_, p))
  deleter_(old);

• std::make_unique:

  // s is a unique_pointer<string>
  auto s = make_unique<string>("foo"); 

• Quoted strings:

  // outputs: "She said "Hi!""
  std::cout << quoted("She said "Hi!""); 

• User-defined literal operators for string and chrono:

  "foo"s // Has type string
  12h    // Has type chrono::hours 

• Improved transformation traits:

  // equivalent to typename remove_reference<T>::type
  remove_reference_t<T> 

• Optional objects:

  optional<int> oi;
  ...
  if (oi) digest(*oi);

•  std::dynarray: Like std::array to constant-size arrays:

  dynarray<int>{1,2,3,4};

• std::shared_mutex: Locks that can be held in shared mode or upgraded to being held exclusively.

We also expect to publish technical specifications for filesystems and networking libraries in the same time frame as the standard.

A significant amount of this new functionality will be available in GCC 4.9, and we are working to implement the rest.

We hope you see something in this preview that looks interesting to you; leave a comment and let us know what you think!

Thanks to Jason Merrill of Red Hat's toolchain team for this summary.