vs 2017 15.3 预览版c++17 特性
[原文发表地址] vs 2017 15.3 预览版c++17 特性
[原文发表时间] 2017/5/10
vs 2017 15.3 预览版 现在已经上线， 它涵盖了更新之后的C++ toolset (例如， 编译器，连接器以及库)， 和之前vs 2015 的更新一样， 我们基本以类似的发布频率在VS 2017 的改动中添加了C++17 特性。 然而和vs2015的改动（包含了IDE和toolset 的改变）不同的是， vs2017 IDE 的更新更加频繁（我们称之为“小步快跑”）。这也是为什么这是第一次toolset 的更新 (称之为“ 基础更新“)。
下表所示是关于编译器模块。虽然你可以下载vs 2017 15.3 预览版1， 但是我们最新提交的一些新增模块只能在预览版2中生效了， 当然这个预览版2也将很快被发布。为了明确指出这些模块的集合，因此在预览版2(当下还没有发布) 中生效的这些模块将被标注为[P2]。
| ++03/11 Core Language Features | Status | Paper | Notes |
| [Everything else] | VS 2015 | [A] | |
| Two-phase name lookup | Partial | [twoPhase] | |
| Expression SFINAE | Partial | N2634 | [B] |
| C99 preprocessor | Partial | N1653 | [C] |
| Extended integer types | N/A | N1988 | [D] |
| C++14 Core Language Features | Status | Paper | Notes |
| Avoiding/fusing allocations | N/A | N3664 | [E] |
| Tweaked wording for contextual conversions | VS 2013 | N3323 | |
| Binary literals | VS 2015 | N3472 | |
| auto and decltype(auto) return types | VS 2015 | N3638 | |
| init-captures | VS 2015 | N3648 | |
| Generic lambdas | VS 2015 | N3649 | |
| Variable templates | VS 2015.2 | N3651 | |
| Extended constexpr | VS 2017 | N3652 | |
| NSDMIs for aggregates | VS 2017 | N3653 | |
| [[deprecated]] attribute | VS 2015 | N3760 | |
| Sized deallocation | VS 2015 | N3778 | |
| Digit separators | VS 2015 | N3781 | |
| C++17 Core Language Features | Status | Paper | Notes |
| New rules for auto with braced-init-lists | VS 2015 | N3922 | [14] |
| Terse static_assert | VS 2017 | N3928 | |
| typename in template template-parameters | VS 2015 | N4051 | [14] |
| Removing trigraphs | VS 2010 | N4086 | [14] |
| Nested namespace definitions | VS 2015.3 | N4230 | |
| Fixing qualification conversions | No | N4261 | |
| Attributes for namespaces and enumerators | VS 2015 | N4266 | [14] |
| u8 character literals | VS 2015 | N4267 | [14] |
| Allowing more non-type template args | No | N4268 | |
| Fold expressions | No | N4295 | |
| Removing some empty unary folds | No | P0036R0 | |
| Removing the register keyword | VS 2017.3 | P0001R1 | |
| Removing operator++ for bool | VS 2017.3 | P0002R1 | |
| Adding noexcept to the type system | No | P0012R1 | |
| Extended aggregate initialization | No | P0017R1 | |
| Capturing *this by value | VS 2017.3 | P0018R3 | |
| __has_include | VS 2017.3 [P2] | P0061R1 | [14] |
| Rewording inheriting constructors | No | P0136R1 | |
| Direct-list-init of fixed enums from integers | VS 2017.3 | P0138R2 | |
| constexpr lambdas | VS 2017.3 | P0170R1 | |
| Generalized range-based for-loops | VS 2017 | P0184R0 | [14] |
| [[fallthrough]] attribute | VS 2017 | P0188R1 | |
| [[nodiscard]] attribute | VS 2017.3 | P0189R1 | |
| [[maybe_unused]] attribute | VS 2017.3 | P0212R1 | |
| Hexfloat literals | No | P0245R1 | |
| Using attribute namespaces without repetition | VS 2017.3 | P0028R4 | |
| Over-aligned dynamic memory allocation | No | P0035R4 | |
| Template argument deduction for class templates | No | P0091R3 | |
| Declaring non-type template parameters with auto | No | P0127R2 | |
| Guaranteed copy elision | VS 2017.3 | P0135R1 | |
| Refining expression evaluation order | No | P0145R3 | |
| Structured bindings | VS 2017.3 | P0217R3 | |
| Ignoring unrecognized attributes | No | P0283R2 | |
| constexpr if-statements | VS 2017.3 [P2] | P0292R2 | [F] |
| Selection statements with initializers | VS 2017.3 [P2] | P0305R1 | |
| Inline variables | No | P0386R2 | |
| Matching template template-parameters to compatible arguments | No | P0522R0 | |
| Removing dynamic-exception-specifications | No | P0003R5 | |
| Pack expansions in using-declarations | No | P0195R2 |
[A]除了动态异常说明之外， C++11 中被淘汰的那些模块大部分已经在C++17 中被移除。 注意到 c++17 18.4 [except.spec]/提到：” 非异常说明符throw() 已经被被淘汰（D,3） ”，也就说说非异常说明符noexcept(true)。” 但是vs2017 15.3 预览版目前考虑将throw() 替换为__declspec(nothrow)。
[twoPhase] 尽管vs2017 15.3 版本不支持two-Phase 字段查找， 但是编译器团队仍然在基于后台正在完成一些主要的功能。 对预览版2 而言， /permissive- 这个编译选项将会激活部分支持two-phase 字段的查找（这里部分粗略计算有60%）。 我们也会在后期发布一个详细的博客来阐述具体的细节。
[B] 尽管SFINAE 表达式是部分支持， 它基本已经覆盖完全了。在vs 2015.2 中，对于STL 后台而言是充分支持的， 而且SFINAE 表达式对于像boost之类的第三方库已经进行了进一步的缺陷修复。 目前存在的最大缺陷是: 当实现void_t/decltype SFINAE 表达式时， 需要使用特定的标签类型。
[C] 就可变参数宏的的工作范畴而言，C99 预处理目前仍是部分支持。 我们正在计划在标注为完成之前，检修预处理器。
[D] 允许拓展整型类型但是不要求这样做。对于Clang 和Gcc 编译器而言，我们将不会实现这项功能。
[E] allocations 的使用并没有进行严格的界限，当下，我们仍然选择暂不实现。
[F]支持 /std:C++14 警告C4984: ‘if constexpr’ is a C++17 language extension。这将会允许STL在函数实现内部 (将来)去使用 “if constexpr”。
[14] 这些特性已经是可用的，当然也支持 /std:C++14。
[P2] 这些特性已经被提交， 在vs2017.3预览版2 中将会可用。
下面是STL 模块的更新列表：
| Status | Std | Paper | Title | Notes |
| missing | C++17 | P0604R0 | Changing is_callable/result_of To is_invocable/invoke_result (Options A and B) | |
| missing | C++17 | P0005R4 | not_fn() | |
| patch | C++17 | P0358R1 | Fixes For not_fn() | |
| missing | C++17 | P0618R0 | Deprecating <codecvt> | |
| missing | C++17 | P0521R0 | Deprecating shared_ptr::unique() | |
| missing | C++17 | P0174R2 | Deprecating Vestigial Library Parts | |
| missing | C++17 | P0003R5 | Removing Dynamic Exception Specifications | |
| missing | C++17 | P0302R1 | Removing Allocator Support In std::function | |
| missing | C++17 | P0433R2 | Deduction Guides For The STL | |
| missing | C++17 | P0607R0 | Inline Variables For The STL (Options A and B2) | |
| missing | C++17 | P0258R2 | has_unique_object_representations | |
| missing | C++17 | P0426R1 | constexpr For char_traits | |
| missing | C++17 | P0063R3 | C11 Standard Library | |
| missing | C++17 | P0033R1 | Rewording enable_shared_from_this | |
| missing | C++17 | P0414R2 | shared_ptr<T[]>, shared_ptr<T[N]> | |
| patch | C++17 | P0497R0 | Fixing shared_ptr For Arrays | |
| missing | C++17 | P0083R3 | Splicing Maps And Sets | |
| patch | C++17 | P0508R0 | Clarifying insert_return_type | |
| missing | C++17 | P0067R5 | Elementary String Conversions | |
| … | C++17 | P0220R1 | Library Fundamentals V1 | |
| missing | C++17 | … | <memory_resource> | |
| patch | C++17 | P0337R0 | Deleting polymorphic_allocator Assignment | |
| missing | C++17 | P0030R1 | hypot(x, y, z) | |
| missing | C++17 | P0226R1 | Mathematical Special Functions | |
| missing | C++17 | P0024R2 | Parallel Algorithms | [G] |
| patch | C++17 | P0336R1 | Renaming Parallel Execution Policies | |
| patch | C++17 | P0394R4 | Parallel Algorithms Should terminate() For Exceptions | |
| patch | C++17 | P0452R1 | Unifying <numeric> Parallel Algorithms | |
| missing | C++17 | P0218R1 | <filesystem> | |
| patch | C++17 | P0219R1 | Relative Paths For Filesystem | |
| patch | C++17 | P0317R1 | Directory Entry Caching For Filesystem | |
| patch | C++17 | P0392R0 | Supporting string_view In Filesystem Paths | |
| patch | C++17 | P0430R2 | Supporting Non-POSIX Filesystems | |
| patch | C++17 | P0492R2 | Resolving NB Comments For Filesystem | |
| VS 2017.3 [P2] | C++17 | P0031R0 | constexpr For <array> (Again) And <iterator> | |
| VS 2017.3 [P2] | C++17 | P0084R2 | Emplace Return Type | |
| VS 2017.3 [P2] | C++17 | P0295R0 | gcd(), lcm() | |
| VS 2017.3 [P2] | C++17 | P0298R3 | std::byte | [H] |
| VS 2017.3 [P2] | C++17 | P0435R1 | Overhauling common_type | [14] |
| VS 2017.3 [P2] | C++17 | P0505R0 | constexpr For <chrono> (Again) | |
| VS 2017.3 [P2] | C++17 | P0548R1 | Tweaking common_type And duration | [14] |
| VS 2017.3 | C++17 | … | Boyer-Moore search() | |
| VS 2017.3 | C++17 | P0040R3 | Extending Memory Management Tools | |
| VS 2017.3 | C++17 | P0152R1 | atomic::is_always_lock_free | |
| VS 2017.3 | C++17 | P0154R1 | hardware_destructive_interference_size, etc. | |
| VS 2017.3 | C++17 | P0156R2 | Renaming Variadic lock_guard To scoped_lock | |
| VS 2017.3 | C++17 | P0253R1 | Fixing Searcher Return Types | |
| VS 2017.3 | C++17 | P0403R1 | UDLs For <string_view> (“meow”sv, etc.) | |
| VS 2017.3 | C++17 | P0418R2 | atomic compare_exchange memory_order Requirements | [14] |
| VS 2017.3 | C++17 | P0513R0 | Poisoning hash | [14] |
| VS 2017.3 | C++17 | P0516R0 | Marking shared_future Copying As noexcept | [14] |
| VS 2017.3 | C++17 | P0517R0 | Constructing future_error From future_errc | [14] |
| VS 2017.3 | C++17 | P0558R1 | Resolving atomic<T> Named Base Class Inconsistencies | [I] [14] |
| VS 2017.3 | C++17 | P0599R1 | noexcept hash | [14] |
| VS 2017 | C++17 | … | <algorithm> sample() | |
| VS 2017 | C++17 | … | <any> | |
| VS 2017 | C++17 | … | <optional> | |
| VS 2017 | C++17 | … | <string_view> | |
| VS 2017 | C++17 | … | <tuple> apply() | |
| VS 2017 | C++17 | P0032R3 | Homogeneous Interface For variant/any/optional | |
| VS 2017 | C++17 | P0077R2 | is_callable, is_nothrow_callable | |
| VS 2017 | C++17 | P0088R3 | <variant> | |
| VS 2017 | C++17 | P0163R0 | shared_ptr::weak_type | |
| VS 2017 | C++17 | P0209R2 | make_from_tuple() | |
| VS 2017 | C++17 | P0254R2 | Integrating string_view And std::string | |
| VS 2017 | C++17 | P0307R2 | Making Optional Greater Equal Again | |
| VS 2017 | C++17 | P0393R3 | Making Variant Greater Equal | |
| VS 2017 | C++17 | P0504R0 | Revisiting in_place_t/in_place_type_t<T>/in_place_index_t<I> | |
| VS 2017 | C++17 | P0510R0 | Rejecting variants Of Nothing, Arrays, References, And Incomplete Types | |
| VS 2015.3 | C++17 | P0025R1 | clamp() | |
| VS 2015.3 | C++17 | P0185R1 | is_swappable, is_nothrow_swappable | |
| VS 2015.3 | C++17 | P0272R1 | Non-const basic_string::data() | |
| VS 2015.2 | C++17 | N4387 | Improving pair And tuple | [14] |
| VS 2015.2 | C++17 | N4508 | shared_mutex (Untimed) | [14] |
| VS 2015.2 | C++17 | P0004R1 | Removing Deprecated Iostreams Aliases | [J] |
| VS 2015.2 | C++17 | P0006R0 | Variable Templates For Type Traits (is_same_v, etc.) | [14] |
| VS 2015.2 | C++17 | P0007R1 | as_const() | [14] |
| VS 2015.2 | C++17 | P0013R1 | Logical Operator Type Traits (conjunction, etc.) | [14] |
| VS 2015.2 | C++17 | P0074R0 | owner_less<> | [14] |
| VS 2015.2 | C++17 | P0092R1 | <chrono> floor(), ceil(), round(), abs() | [14] |
| VS 2015.2 | C++17 | P0156R0 | Variadic lock_guard | [14] |
| VS 2015 | C++17 | N3911 | void_t | [14] |
| VS 2015 | C++17 | N4089 | Safe Conversions In unique_ptr<T[]> | [14] |
| VS 2015 | C++17 | N4169 | invoke() | [14] |
| VS 2015 | C++17 | N4190 | Removing auto_ptr, random_shuffle(), And Old <functional> Stuff | [K] |
| VS 2015 | C++17 | N4258 | noexcept Cleanups | [14] |
| VS 2015 | C++17 | N4259 | uncaught_exceptions() | [14] |
| VS 2015 | C++17 | N4277 | Trivially Copyable reference_wrapper | [14] |
| VS 2015 | C++17 | N4279 | insert_or_assign()/try_emplace() For map/unordered_map | [14] |
| VS 2015 | C++17 | N4280 | size(), empty(), data() | [14] |
| VS 2015 | C++17 | N4366 | Precisely Constraining unique_ptr Assignment | [14] |
| VS 2015 | C++17 | N4389 | bool_constant | [14] |
| VS 2013 | C++17 | N4510 | Supporting Incomplete Types In vector/list/forward_list | [14] |
| N/A | C++17 | N4284 | Contiguous Iterators | |
| N/A | C++17 | P0175R1 | Synopses For The C Library | |
| N/A | C++17 | P0180R2 | Reserving Namespaces For Future Standardization | |
| N/A | C++17 | P0346R1 | A <random> Nomenclature Tweak | |
| N/A | C++17 | P0371R1 | Discouraging memory_order_consume | |
| N/A | C++17 | P0467R2 | Requiring Forward Iterators In Parallel Algorithms | |
| N/A | C++17 | P0502R0 | Parallel Algorithms Should terminate() For Exceptions, Usually | |
| N/A | C++17 | P0503R0 | Correcting Library Usage Of “literal type” | |
| N/A | C++17 | P0509R1 | Updating “Restrictions on exception handling” | |
| N/A | C++17 | P0518R1 | Copying Trivially Copy Constructible Elements In Parallel Algorithms | |
| N/A | C++17 | P0523R1 | Relaxing Complexity Requirements Of Parallel Algorithms (General) | |
| N/A | C++17 | P0574R1 | Relaxing Complexity Requirements Of Parallel Algorithms (Specific) | |
| N/A | C++17 | P0623R0 | Final C++17 Parallel Algorithms Fixes | |
“missing” 和“patch”已经分组管理，这一份将不需加以说明。库V1版本的 基本问题已经被分割为独立的模块，标注为“…” 。 最后， “N/A“意味着不再进行句法规范化的修改， 对用户而言也不会衍生出新的模块。 例如，当我们实现了部分算法的时候， 我们将会遵循当下的规范。 为了进一步跟踪，P0394R4 （并行算法应该以抛出异常终止）对用户而言是非常重要的， 而且也是实施者需要去了解的。 然而P0502R0(并行算法应该以抛出异常终止，经常 )却不是这样的。（允许非标准的执行策略产生不同的行为）。
[G] for_each_n() 系列将会在预览版2中生效。
[H]/std:c++17 和/std:c++latest 支持std::byte, 它可以通过定义_HAS_STD_BYTE 为0 来关闭
[I] 除了atomic_meow() 和atomic_meow_explicit()的命名问题之外， 这个问题可以通过替换命名或者提供显式模板参数来修正。我们已经实现了P0558R1 中用户普遍使用的部分（解决了atomic<T> j基类明名不一致问题）
[J] /std:c++17 和/std”c++latest 移除了旧的输入输出流成员函数， 它们也可以通过定义_HAS_OLD_IOSTREAMS_MEMBERS 为1 来恢复。
[K] /std:c++17 和 /std:c++latest 移除了auto_ptr, random_shuffle() 以及一些旧的<functional>成员。 它们也可以通过定义_HAS_AUTO_PTR_ETC 为1 来恢复。
我们已经完成了29个LWG 问题的修复，并且做了一些后台场景的改进，我们这儿提到的将会更接近于vs 2017.3 的最终发布版本。
请尝试vs 2017 15.3 预览版, 当我们知道您的想法。 (请记住：您需要用/std:c++17 或者/std:c++latest 来编译才能激活C++17 的特性， 除过上面表格中标注[14]的之外 – 请记住[P2]只有在预览版2 之后才生效。) 你可以使用IDE 的报告一个问题来提交问题。 关于编译器和库函数的问题， 包含一个完整的测试用例是重要的。