What are the new features in C++17?

Question

C++17 is now feature complete, so unlikely to experience large changes. Hundreds of proposals were put forward for C++17.

Which of those features were added to C++ in C++17?

When using a C++ compiler that supports "C++1z", which of those features are going to be available when the compiler updates to C++17?

Solution

Language features:

Templates and Generic Code

• Template argument deduction for class templates

  • Like how functions deduce template arguments, now constructors can deduce the template arguments of the class
  • http://wg21.link/p0433r2 http://wg21.link/p0620r0 http://wg21.link/p0512r0

• template <auto>

  • Represents a value of any (non-type template argument) type.

• Non-type template arguments fixes

• template<template<class...>typename bob> struct foo {}

• ( Folding + ... + expressions ) and Revisions

• auto x{8}; is an int

• modernizing using with ... and lists

Lambda

• constexpr lambdas

  • Lambdas are implicitly constexpr if they qualify

• Capturing *this in lambdas

  • [*this]{ std::cout << could << " be " << useful << '\n'; }

Attributes

• [[fallthrough]], [[nodiscard]], [[maybe_unused]] attributes

• [[attributes]] on namespaces and enum { erator[[s]] }

• using in attributes to avoid having to repeat an attribute namespace.

• Compilers are now required to ignore non-standard attributes they don't recognize.

  • The C++14 wording allowed compilers to reject unknown scoped attributes.

Syntax cleanup

• Inline variables

• Like inline functions

• Compiler picks where the instance is instantiated

• Deprecate static constexpr redeclaration, now implicitly inline.

• namespace A::B

• Simple static_assert(expression); with no string

• no throw unless throw(), and throw() is noexcept(true).

Cleaner multi-return and flow control

• Structured bindings

• Basically, first-class std::tie with auto

• Example: * const auto [it, inserted] = map.insert( {"foo", bar} ); * Creates variables it and inserted with deduced type from the pair that map::insert returns.

• Works with tuple/pair-likes & std::arrays and relatively flat structs

• Actually named structured bindings in standard

• if (init; condition) and switch (init; condition)

• if (const auto [it, inserted] = map.insert( {"foo", bar} ); inserted)

• Extends the if(decl) to cases where decl isn't convertible-to-bool sensibly.

• Generalizing range-based for loops

  • Appears to be mostly support for sentinels, or end iterators that are not the same type as begin iterators, which helps with null-terminated loops and the like.

• if constexpr

• Much requested feature to simplify almost-generic code.

Misc

• Hexadecimal float point literals

• Dynamic memory allocation for over-aligned data

• Guaranteed copy elision

  • Finally!
  • Not in all cases, but distinguishes syntax where you are "just creating something" that was called elision, from "genuine elision".

• Fixed order-of-evaluation for (some) expressions with some modifications

  • Not including function arguments, but function argument evaluation interleaving now banned
  • Makes a bunch of broken code work mostly, and makes .then on future work.

• Direct list-initialization of enums

• Forward progress guarantees (FPG) (also, FPGs for parallel algorithms)

• I think this is saying "the implementation may not stall threads forever"?

• u8'U', u8'T', u8'F', u8'8' character literals (string already existed)

• "noexcept" in the type system

• __has_include

  • Test if a header file include would be an error
  • makes migrating from experimental to std almost seamless

• Arrays of pointer conversion fixes

• inherited constructors fixes to some corner cases (see P0136R0 for examples of behavior changes)

• aggregate initialization with inheritance.

• std::launder, type punning, etc

Library additions:

Data types

• std::variant<Ts...>

• Almost-always non-empty last I checked?

• Tagged union type

• {awesome|useful}

• std::optional

  • Maybe holds one of something
  • Ridiculously useful

• std::any

  • Holds one of anything (that is copyable)

• std::string_view

  • std::string like reference-to-character-array or substring
  • Never take a string const& again. Also can make parsing a bajillion times faster.
  • "hello world"sv
  • constexpr char_traits

• std::byte off more than they could chew.

  • Neither an integer nor a character, just data

Invoke stuff

• std::invoke

  • Call any callable (function pointer, function, member pointer) with one syntax. From the standard INVOKE concept.

• std::apply

  • Takes a function-like and a tuple, and unpacks the tuple into the call.

• std::make_from_tuple, std::apply applied to object construction

• is_invocable, is_invocable_r, invoke_result

• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0077r2.html

• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0604r0.html

• Deprecates result_of

• is_invocable<Foo(Args...), R> is "can you call Foo with Args... and get something compatible with R", where R=void is default.

• invoke_result<Foo, Args...> is std::result_of_t<Foo(Args...)> but apparently less confusing?

File System TS v1

• [class.path]

• [class.filesystem.error]

• [class.file_status]

• [class.directory_entry]

• [class.directory_iterator] and [class.recursive_directory_iterator]

• [fs.ops.funcs]

• fstreams can be opened with paths, as well as with const path::value_type* strings.

New algorithms

• for_each_n

• reduce

• transform_reduce

• exclusive_scan

• inclusive_scan

• transform_exclusive_scan

• transform_inclusive_scan

• Added for threading purposes, exposed even if you aren't using them threaded

Threading

• std::shared_mutex

  • Untimed, which can be more efficient if you don't need it.

• atomic<T>::is_always_lockfree

• scoped_lock<Mutexes...>

  • Saves some std::lock pain when locking more than one mutex at a time.

• Parallelism TS v1

  • The linked paper from 2014, may be out of date
  • Parallel versions of std algorithms, and related machinery

• hardware_*_interference_size

(parts of) Library Fundamentals TS v1 not covered above or below

• [func.searchers] and [alg.search]

  • A searching algorithm and techniques

• [pmr]

  • Polymorphic allocator, like std::function for allocators
  • And some standard memory resources to go with it.
  • http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0358r1.html

• std::sample, sampling from a range?

Container Improvements

• try_emplace and insert_or_assign

  • gives better guarantees in some cases where spurious move/copy would be bad

• Splicing for map<>, unordered_map<>, set<>, and unordered_set<>

  • Move nodes between containers cheaply.
  • Merge whole containers cheaply.

• non-const .data() for string.

• non-member std::size, std::empty, std::data

  • like std::begin/end

• Minimal incomplete type support in containers

• Contiguous iterator "concept"

• constexpr iterators

• The emplace family of functions now returns a reference to the created object.

Smart pointer changes

• unique_ptr<T[]> fixes and other unique_ptr tweaks.
• weak_from_this and some fixed to shared from this

Other std datatype improvements:

• {} construction of std::tuple and other improvements
• TriviallyCopyable reference_wrapper, can be performance boost

Misc

• C++17 library is based on C11 instead of C99

• Reserved std[0-9]+ for future standard libraries

• destroy(_at|_n), uninitialized_move(_n), uninitialized_value_construct(_n), uninitialized_default_construct(_n)

• utility code already in most std implementations exposed

• Special math functions

• scientists may like them

• std::clamp()

  • std::clamp( a, b, c ) == std::max( b, std::min( a, c ) ) roughly

• gcd and lcm

• std::uncaught_exceptions

  • Required if you want to only throw if safe from destructors

• std::as_const

• std::bool_constant

• A whole bunch of _v template variables

• std::void_t<T>

  • Surprisingly useful when writing templates

• std::owner_less<void>

  • like std::less<void>, but for smart pointers to sort based on contents

• std::chrono polish

• std::conjunction, std::disjunction, std::negation exposed

• std::not_fn

• http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0358r1.html

• Rules for noexcept within std

• std::is_contiguous_layout, useful for efficient hashing

• std::to_chars/std::from_chars, high performance, locale agnostic number conversion; finally a way to serialize/deserialize to human readable formats (JSON & co)

• std::default_order, indirection over std::less. (breaks ABI of some compilers due to name mangling, removed.)

• memory_order_consume, added language to prefer use of memory_order_acquire

Traits

• swap
• is_aggregate
• has_unique_object_representations

Deprecated

• Some C libraries,
• <codecvt>
• result_of, replaced with invoke_result
• shared_ptr::unique, it isn't very threadsafe

Isocpp.org has has an independent list of changes since C++14; it has been partly pillaged.

Naturally TS work continues in parallel, so there are some TS that are not-quite-ripe that will have to wait for the next iteration. The target for the next iteration is C++20 as previously planned, not C++19 as some rumors implied. C++1O has been avoided.

Initial list taken from this reddit post and this reddit post, with links added via googling or from the above isocpp.org page.

Additional entries pillaged from SD-6 feature-test list.

clang's feature list and library feature list are next to be pillaged. This doesn't seem to be reliable, as it is C++1z, not C++17.

these slides had some features missing elsewhere.

While "what was removed" was not asked, here is a short list of a few things ((mostly?) previous deprecated) that are removed in C++17 from C++:

Removed:

• register, keyword reserved for future use
• bool b; ++b;
• trigraphs
  • if you still need them, they are now part of your source file encoding, not part of language
• ios aliases
• auto_ptr, old <functional> stuff, random_shuffle
• allocators in std::function

There were rewordings. I am unsure if these have any impact on code, or if they are just cleanups in the standard:

Papers not yet integrated into above:

• P0505R0 (constexpr chrono)

• P0418R2 (atomic tweaks)

• P0512R0 (template argument deduction tweaks)

• P0490R0 (structured binding tweaks)

• P0513R0 (changes to std::hash)

• P0502R0 (parallel exceptions)

• P0509R1 (updating restrictions on exception handling)

• P0012R1 (make exception specifications be part of the type system)

• P0510R0 (restrictions on variants)

• P0504R0 (tags for optional/variant/any)

• P0497R0 (shared ptr tweaks)

• P0508R0 (structured bindings node handles)

• P0521R0 (shared pointer use count and unique changes?)

Spec changes:

• exception specs and throw expressions

Further reference:

• papers grouped by year; not all accepted

• https://isocpp.org/files/papers/p0636r0.html

  • Should be updated to "Modifications to existing features" here.

Comments

[L. F.]

Um, memory_order_consume does not seem to be officially deprecated. It is just discouraged in the note. Maybe it makes sense to mention this (with a trailing parenthesis, for example)?