C++17 Is Ready

At its winter meeting in Kona, HI, USA, the ISO C++ committee has finalized work on C++17, writes Herb Sutter. Here is a short summary of its main features.

The Kona meeting was mostly aimed to address review comments from national bodies and to fix bugs, so it did not bring many new features. In particular, a new std::byte type has been added at Kona to streamline the way programs deal with byte-oriented access to memory. This is currently done using either the char, signed char, or unsigned char types, which is less than ideal since those types support char and arithmetic operations. On the other hand, std::byte will be a separate, dedicated type.

Other significant new features in C++17 include:

• Lambdas can be used in constexprs, thus removing a preexisting limitation. constexpr allows to declare objects or functions that can be evaluated, e.g., at compile time, making it possible to provide advanced template parameters or array-size specifiers. The compiler will ensure that a constexpr object or function complies with given requirements to make possible its use as described.

• Capturing *this as a value in lambdas is now allowed, which makes a big difference in parallel code or with std::future, since there is no guarantee that this is still valid when the lambda executes.

• Optionals are available through std::optional to handle the representation of a value that can be defined or not. In C++ parlance, this means that an optional is a wrapper that manages the initialization state of an object.

• Union types are available through std::variant, which allow to define a type like “float or long integer”.

• Constructors can deduce the template arguments of the class, making it possible to just write:

  auto lock = std::lock_guard(mut_, r1);
  

  instead of:

  std::lock_guard<std::shared_timed_mutex, std::shared_lock<std::shared_timed_mutex>> lck(mut_, r1);
  

• Templates can infer their non-type parameters, e.g.:

  template <auto v> struct S;  // type of v is deduced`
  

The list of new features in C++17 is sufficiently long to not be possible to list all of them here. This answer on Stack Overflow provides a succinct summary, complete up to the latest meeting in Kona, including pointers to the official committee documents.