C++ 17中有哪些新功能？

Question

C++ 17现在功能齐全,因此不太可能经历大的变化.为C++ 17提出了数百个提案.

在C++ 17中,哪些特性被添加到C++中？

当使用支持"C++ 1z"的C++编译器时,当编译器更新到C++ 17时,哪些功能可用？

Answer 1

语言特色:

模板和通用代码

• 类模板的模板参数推导

  • 就像函数如何推导模板参数一样,现在构造函数可以推导出类的模板参数
  • http://wg21.link/p0433r2 http://wg21.link/p0620r0 http://wg21.link/p0512r0

• template <auto>

  • 表示任何(非类型模板参数)类型的值.

• 非类型模板参数修复

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

• (折叠+ ... +表达式)和修订版

• auto x{8}; 是一个 int

• 现代化using与...和列表

LAMBDA

• constexpr lambdas

  • Lambdas如果符合条件,就会隐含着constexpr

• 捕捉*thislambdas

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

属性

• [[fallthrough]],[[nodiscard]],[[maybe_unused]]属性

• [[attributes]]在namespaces和enum { erator[[s]] }

• using在属性中避免重复属性命名空间.

• 编译器现在需要忽略它们无法识别的非标准属性.

  • C++ 14的措辞允许编译器拒绝未知的范围属性.

语法清理

• 内联变量

  • 像内联函数
  • 编译器选择实例实例化的位置
  • 弃用静态constexpr重新声明,现在隐式内联.

• namespace A::B

• 简单static_assert(expression);,没有字符串

• 不,throw除非throw(),throw()是noexcept(true).

更清洁的多回程和流量控制

• 结构化绑定

  • 基本上,一流std::tie的auto
  • 例:
    • const auto [it, inserted] = map.insert( {"foo", bar} );
    • 创建变量it并inserted从与推导的类型pair是map::insert返回.
  • 适用于元组/对喜欢std::array和相对扁平的结构
  • 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
• 嗯，“memory_order_consume”似乎并未被正式弃用。只是在注释中不鼓励这样做。也许提到这一点是有意义的（例如，带有尾随括号）？ (3认同)