C++17 Features And STL Fixes In VS 2017 15.3

Visual Studio 2017’s first toolset update, version 15.3, is currently in preview and will be released in its final form very soon. (The toolset consists of the compiler, linker, and libraries. After VS 2017 RTM, the 15.1 and 15.2 updates improved the IDE. The 15.3 update improves both the IDE and the toolset. In general, you should expect the IDE to be updated at a higher frequency than the toolset.)

As usual, we’ve maintained a detailed list of the STL fixes that are available in the 15.3 update. We also have newer feature tables for the STL and the compiler.

New Features (in addition to C++17 features):

* The STL no longer depends on Magic Statics, allowing clean use in code compiled with /Zc:threadSafeInit-.

* Implemented P0602R0 “variant and optional should propagate copy/move triviality”.

* The STL now officially tolerates dynamic RTTI being disabled via /GR-. dynamic_pointer_cast() and rethrow_if_nested() inherently require dynamic_cast, so the STL now marks them as =delete under /GR-.

* Even when dynamic RTTI has been disabled via /GR-, “static RTTI” (in the form of typeid(SomeType)) is still available and powers several STL components. The STL now supports disabling this too, via /D_HAS_STATIC_RTTI=0. Note that this will disable std::any, std::function’s target() and target_type(), and shared_ptr’s get_deleter().

Correctness Fixes:

* STL containers now clamp their max_size() to numeric_limits<difference_type>::max() rather than size_type’s max. This ensures that the result of distance() on iterators from that container is representable in the return type of distance().

* Fixed missing specialization auto_ptr<void>.

* The meow_n() algorithms previously failed to compile if the length argument was not an integral type; they now attempt to convert non-integral lengths to the iterators’ difference_type.

* normal_distribution<float> no longer emits warnings inside the STL about narrowing from double to float.

* Fixed some basic_string operations which were comparing with npos instead of max_size() when checking for maximum size overflow.

* condition_variable::wait_for(lock, relative_time, predicate) would wait for the entire relative time in the event of a spurious wake. Now, it will wait for only a single interval of the relative time.

* future::get() now invalidates the future, as the standard requires.

* iterator_traits<void *> used to be a hard error because it attempted to form void&; it now cleanly becomes an empty struct to allow use of iterator_traits in “is iterator” SFINAE conditions.

* Some warnings reported by Clang -Wsystem-headers were fixed.

* Also fixed “exception specification in declaration does not match previous declaration” reported by Clang -Wmicrosoft-exception-spec.

* Also fixed mem-initializer-list ordering warnings reported by Clang and C1XX.

* The unordered containers did not swap their hashers or predicates when the containers themselves were swapped. Now they do.

* Many container swap operations are now marked noexcept (as our STL never intends to throw an exception when detecting the non-propagate_on_container_swap non-equal-allocator undefined behavior condition).

* Many vector<bool> operations are now marked noexcept.

* The STL will now enforce matching allocator value_types (in C++17 mode) with an opt-out escape hatch.

* Fixed some conditions where self-range-insert into basic_strings would scramble the strings’ contents. (Note: self-range-insert into vectors is still prohibited by the Standard.)

* basic_string::shrink_to_fit() is no longer affected by the allocator’s propagate_on_container_swap.

* std::decay now handles abominable function types (i.e. function types that are cv-qualified and/or ref-qualified).

* Changed include directives to use proper case sensitivity and forward slashes, improving portability.

* Fixed warning C4061 “enumerator ‘Meow’ in switch of enum ‘Kitten’ is not explicitly handled by a case label”. This warning is off-by-default and was fixed as an exception to the STL’s general policy for warnings. (The STL is /W4 clean, but does not attempt to be /Wall clean. Many off-by-default warnings are extremely noisy and aren’t intended to be used on a regular basis.)

* Improved std::list’s debug checks. List iterators now check operator->(), and list::unique() now marks iterators as invalidated.

* Fixed uses-allocator metaprogramming in tuple.

Performance/Throughput Fixes:

* Worked around interactions with noexcept which prevented inlining std::atomic’s implementation into functions that use Structured Exception Handling (SEH).

* Changed the STL’s internal _Deallocate() function to optimize into smaller code, allowing it to be inlined into more places.

* Changed std::try_lock() to use pack expansion instead of recursion.

* Improved std::lock()’s deadlock avoidance algorithm to use lock() operations instead of spinning on all the locks’ try_lock()s.

* Enabled the Named Return Value Optimization in system_category::message().

* conjunction and disjunction now instantiate N + 1 types, instead of 2N + 2 types.

* std::function no longer instantiates allocator support machinery for each type-erased callable, improving throughput and reducing .obj size in programs that pass many distinct lambdas to std::function.

* allocator_traits<std::allocator> contains manually inlined std::allocator operations, reducing code size in code that interacts with std::allocator through allocator_traits only (i.e. most code).

* The C++11 minimal allocator interface is now handled by the STL calling allocator_traits directly, instead of wrapping the allocator in an internal class _Wrap_alloc. This reduces the code size generated for allocator support, improves the optimizer’s ability to reason about STL containers in some cases, and provides a better debugging experience (as now you see your allocator type, rather than _Wrap_alloc<your allocator type> in the debugger).

* Removed metaprogramming for customized allocator::reference, which allocators aren’t actually allowed to customize. (Allocators can make containers use fancy pointers but not fancy references.)

* The compiler front-end was taught to unwrap debug iterators in range-based for-loops, improving the performance of debug builds.

* basic_string’s internal shrink path for shrink_to_fit() and reserve() is no longer in the path of reallocating operations, reducing code size for all mutating members.

* basic_string’s internal grow path is no longer in the path of shrink_to_fit().

* basic_string’s mutating operations are now factored into non-allocating fast path and allocating slow path functions, making it more likely for the common no-reallocate case to be inlined into callers.

* basic_string’s mutating operations now construct reallocated buffers in the desired state rather than resizing in place. For example, inserting at the beginning of a string now moves the content after the insertion exactly once (either down or to the newly allocated buffer), instead of twice in the reallocating case (to the newly allocated buffer and then down).

* Operations calling the C standard library in <string> now cache errno’s address to remove repeated interaction with TLS.

* Simplified is_pointer’s implementation.

* Finished changing function-based Expression SFINAE to struct/void_t-based.

* STL algorithms now avoid postincrementing iterators.

* Fixed truncation warnings when using 32-bit allocators on 64-bit systems.

* std::vector move assignment is now more efficient in the non-POCMA non-equal-allocator case, by reusing the buffer when possible.

Readability And Other Improvements:

* The STL now uses C++14 constexpr unconditionally, instead of conditionally-defined macros.

* The STL now uses alias templates internally.

* The STL now uses nullptr internally, instead of nullptr_t{}. (Internal usage of NULL has been eradicated. Internal usage of 0-as-null is being cleaned up gradually.)

* The STL now uses std::move() internally, instead of stylistically misusing std::forward().

* Changed static_assert(false, “message”) to #error message. This improves compiler diagnostics because #error immediately stops compilation.

* The STL no longer marks functions as __declspec(dllimport). Modern linker technology no longer requires this.

* Extracted SFINAE to default template arguments, which reduces clutter compared to return types and function argument types.

* Debug checks in <random> now use the STL’s usual machinery, instead of the internal function _Rng_abort() which called fputs() to stderr. This function’s implementation is being retained for binary compatibility, but has been removed in the next binary-incompatible version of the STL.

STL Feature Status:

We’re going to continue to add new features to VS 2017 in toolset updates, and we’re working on the second toolset update right now. While we can’t reveal its version number or provide an ETA, we can show you which features have already been implemented (and this list will continue to grow). For now, we’ll refer to the second toolset update as “VS 2017 15.x” (please don’t try to guess what x is, you’ll just create confusion).

For clarity, the Library Fundamentals V1 paper has been decomposed into its individual features, marked by “…” here.

To give you a better idea of our status, unimplemented papers are marked “missing” for primary features, or “patch” for papers that merely fixed parts of a primary feature. We implement them together, so the large number of “patch” rows doesn’t really indicate a large amount of missing work.

[DR] These papers were voted into the Working Paper after C++17, but as Defect Reports, meaning that they retroactively apply to C++17 (as bugfixes).

[parallel] The Parallel Algorithms are being gradually implemented. Some are available, but we’re still working on them.

[rem] Feature removals are activated by /std:c++17 (or /std:c++latest), with opt-out macros. The macros are _HAS_AUTO_PTR_ETC, _HAS_FUNCTION_ALLOCATOR_SUPPORT, _HAS_OLD_IOSTREAMS_MEMBERS, and _HAS_UNEXPECTED.

[14] These C++17 features are implemented unconditionally, even in /std:c++14 mode (the default). For some features, this was because they predated the introduction of MSVC’s Standard mode options. For other features, conditional implementation would be nearly pointless or undesirably complicated.

[depr] VS 2017 15.x (the second toolset update) will warn about usage of all STL features that were deprecated in C++17 (with the exception of the <stdio.h> family of C headers). /std:c++14 will not emit these warnings, but /std:c++17 (and /std:c++latest) will. The warning messages will be highly detailed, and will mention both the coarse-grained and fine-grained escape hatch macros. In particular, note that while invoke_result was implemented in 15.3, result_of deprecation will appear in 15.x.

[obj_rep] has_unique_object_representations is powered by a compiler intrinsic. Although this has been implemented in EDG (powering Intellisense), we haven’t activated it for that compiler yet. Also, the intrinsic is not yet available in Clang at all.

[byte] std::byte is enabled by /std:c++17 (and /std:c++latest), but has a fine-grained opt-out macro (_HAS_STD_BYTE can be defined to be 0). This is because given certain patterns of using-directives, it can conflict with the Windows SDK’s headers. This has been reported to the SDK team and will be fixed, but in the meantime the escape hatch is available.

[atomic] This is almost completely implemented in VS 2017 15.3, and the remaining differences are difficult to observe (some signatures differ from the Standard, as observed by taking their address or providing explicit template arguments). The STL’s next major binary-incompatible version will fix the remaining differences.

[C11] First available in VS 2015, the Universal CRT implemented the parts of the C11 Standard Library that are required by C++17, with minor exceptions. Those exceptions (which are tracked by bugs) are: missing C99 strftime() E/O alternative conversion specifiers, missing C11 fopen() exclusive mode, and missing C11 aligned_alloc(). The strftime() and fopen() functionality will be implemented in the future. aligned_alloc() will probably never be implemented, as C11 specified it in a way that’s incompatible with our implementation (namely, that free() must be able to handle highly aligned allocations).

For clarity, this table has omitted a number of papers that are Not Applicable (nothing for implementers to do, or users to take advantage of), such as wording clarifications.

Finally, note that this table contains one change relative to VS 2017 15.3 Preview 2 – we implemented P0604R0 “invoke_result, is_invocable, is_nothrow_invocable”, permanently renaming P0077R2 “is_callable, is_nothrow_callable”.

Compiler Feature Status:

[A] While dynamic exception specifications remain unimplemented, they were mostly removed in C++17 by P0003R5. One vestige remains in C++17, where throw() is deprecated and required to behave as a synonym for noexcept(true). MSVC doesn’t implement that behavior for throw() (it is still treated as a synonym for __declspec(nothrow)), but you can simply avoid throw() and use noexcept instead.

[B] Two-phase name lookup is partially implemented in VS 2017 15.3, and a detailed blog post will be available very soon.

[C] Expression SFINAE is partially implemented in VS 2017 15.3. While many scenarios work (and it has been sufficiently solid for the STL’s purposes for quite a while), some parts are still missing and some workarounds are still required.

[D] Support for C99’s preprocessor rules is unchanged (considered partial due to support for variadic macros, although there are numerous bugs). The preprocessor will be overhauled as part of finishing C++17.

[E] Extended Integer Types are marked as Not Applicable because implementations are permitted, but not required, to provide such types. Like GCC and Clang, MSVC has chosen to not provide extended integer types.

[F] Similarly, the rules for avoiding/fusing allocations are marked as Not Applicable because this is an optimization that is permitted, but not required. We currently have no plans to implement this (as reports indicate that it isn’t an especially valuable optimization).

[14] Unconditionally available, even in /std:c++14 mode.

[G] “if constexpr” is supported in /std:c++14 with a warning that can be suppressed, delighting template metaprogramming library authors everywhere.

[H] Unfortunately, while Guaranteed Copy Elision was implemented in preview builds of VS 2017 15.3, it had to be reverted due to bugs that were discovered. These bugs will be fixed before the feature is restored.

[DR] Like the STL, the Core Language also had a paper that was voted in as a Defect Report, retroactively applying to C++17. Time is no obstacle to the C++ Standardization Committee.

Reporting Bugs

Please let us know what you think about VS 2017 15.3. You can use the IDE’s Report A Problem to report bugs. For compiler and library bugs, it’s important to provide self-contained test cases.

Billy Robert O’Neal III @MalwareMinigun
bion@microsoft.com

Casey Carter @CoderCasey
cacarter@microsoft.com

Stephan T. Lavavej @StephanTLavavej
stl@microsoft.com

Steve Wishnousky stwish@microsoft.com