STL Features and Fixes in VS 2017 15.8

Billy O'Neal

15.7 was our first feature complete C++17 library (except floating-point <charconv>), and in 15.8 we have addressed large numbers of outstanding bugs. As usual, we’ve maintained a detailed list of the STL fixes that are available. Visual Studio 2017 15.8 is available at

New Features
  • Implemented floating-point from_chars() in C++17 <charconv>. It’s approximately 40% faster than the CRT’s strtod()/strtof().
  • Implemented C++20 feature-test macros in the compiler and STL (except for __has_cpp_attribute which will be implemented in the future). The feature-test macros are always provided regardless of the Standard mode option (/std:c++14 vs. /std:c++17 etc.) – that is, individual feature-test macros are defined (or not) according to the Standard mode option and fine-grained options, but there’s no option for enabling or disabling feature-test macros in their entirety.
  • Implemented std::is_aggregate.
  • Added officially supported hooks to allow user iterator type interaction with iterator debugging, already in use by the Guidelines Support Library. See gsl::span for an example implementation of the following customization points, or the type _String_view_iterator in the internal header <xstring>.
    The customization points are, given a const user_iterator_type cit; and a user_iterator_type it:

    _Verify_range(cit, cit)
    Requests that the iterator type check that the parameters form a valid [First, Last) range iterator pair. The standard library provides a version for pointers under _ITERATOR_DEBUG_LEVEL != 0. We strongly recommend this customization point be implemented as a “hidden friend function”, as demonstrated in the gsl::span example.
    cit._Verify_offset(difference_type off)
    For random-access iterators, verifies that cit + off is a valid iterator. Typically used to provide better diagnostics of the form “the iterator range passed to this algorithm was invalid” rather than “the iterator was incremented off the end of the range”.
    static constexpr bool user_iterator_type::_Unwrap_when_unverified
    Declares to the standard library whether an iterator type wants to be “unchecked” or “unwrapped” even if it can’t be verified in advance by _Verify_range or _Verify_offset. For example, the standard containers set this to true if and only if _ITERATOR_DEBUG_LEVEL == 0. This allows an iterator type to request range checks even in release builds (as desired by gsl::span). If this member is not present, it defaults to false.
    Returns an “unchecked” or “unwrapped” iterator denoting the same element as cit. This will be called by the standard library after a _Verify_range or _Verify_offset call, or if _Unwrap_when_unverified is true.
    it._Seek_to(cit) / it._Seek_to(return value from _Unwrapped())
    Moves the position of the “wrapped” or “checked” iterator it to the position of the “unwrapped” / “unchecked” iterator supplied. This is generally not intended to perform range checks. This allows algorithms that return iterators like std::fill_n to form their return values without needing each iterator operation in the body of the algorithm to be subject to debugging checks.
  • The header structure of the STL was changed to allow use of a subset of the library in conditions where the user can’t link with msvcp140.dll, such as driver development. (Previously the hard dependency on msvcp and pragma detect mismatch were injected by our base configuration header included by everything.) The following headers are now considered “core” and don’t inject our runtime dependencies (though we do still assume some form of CRT headers are present):
    • <cstddef>
    • <cstdlib>
    • <initializer_list>
    • <ratio>
    • <type_traits>

    We aren’t actually driver developers ourselves and are interested in feedback in this area if there are things we can do to make these more usable in constrained environments.

Correctness Fixes
  • Implemented Library Issues 2510, 2729, and 2958 for pair and tuple, constraining their assignment operators. This implementation contained a minor regression affecting assignments to std::pair from a type that implicitly converts to std::pair. We’ve fixed that regression for VS 2019 16.0 and are planning to backport the fix to VS 2017 15.9. In the meantime, if you encounter that compiler error, explicitly converting to std::pair is a workaround.
  • Because user iterators can now officially participate in the iterator debugging system, the _SCL_SECURE_NO_WARNINGS family of warnings has been removed. The previously-pseudo-documented struct _Is_checked_helper remains present in order to avoid breaking user specializations, but it is no longer used by the library directly.
  • Changed num_get overloads to store default values in the event of a failed parse, as the standard requires.
  • Fixed incorrect values numeric_limits<integral>::is_modulo and numeric_limits<floating-point>::tinyness_before.
  • Rewrote several streambuf operations to resolve cases where the streambuf would become confused; for example, cases where seekg(0) would not reset the streambuf position even when the current offset was nonzero.
  • Fixed cases where basic_filebuf would become confused when set to some multibyte locales; the lead byte table was populated using the “fake POSIX support” locale function _ismbslead, but then used the standard locale function _isleadbyte later in processing, corrupting the state of the basic_filebuf.
  • Fixed conditions where one could form a corrupted basic_stringbuf (or stringstream) by forcing it to attempt to represent a string of more than INT_MAX characters. The INT_MAX size restriction unfortunately must remain due to ABI compatibility, but at least we now consistently enforce that restriction.
  • Overhauled std::ws to Do What The Standard Says.
  • The off-by-default warning C4365 “conversion from ‘type_1’ to ‘type_2’, signed/unsigned mismatch” is now supported by the STL.
  • Fixed a case C4365 found in put_money where a narrowed wchar_t was emitted instead of the proper raw wchar_t.
  • Fixed creating an istreambuf_iterator::proxy returning the value stored in the proxy rather than returning the value one would expect from a copied istreambuf_iterator.
  • Fixed a case where linear_congruential_engine would use unsigned int to do intermediate calculations even when unsigned int could not properly store the intermediate values.
  • Fixed linear_congruential_engine::seed() to Do What The Standard Says.
  • Changed parallel algorithms’ partitioning behavior to not special case the foreground thread, improving performance in situations where the system threadpool detects an I/O bound workload and decides to oversubscribe the machine.
  • Fixed a race condition in std::async that could cause crashes or deadlocks.
  • Fixed C++17 std::filesystem‘s support .obj containing a __declspec(dllexport), causing unusual conditions like .exes with export tables.
  • Fixed recursive_directory_iterator::disable_recursion_pending() not being reset after an increment operation.
  • Fixed a long-standing bug that caused aligned_storage to treat requests for alignment greater than alignof(max_align_t) as requests for alignment equal to alignof(max_align_t). We can’t enable this fix unconditionally without breaking ABI since doing so changes layout of types with members whose type is a highly-aligned specialization of aligned_storage. As an interim solution until a future ABI-breaking release, compilation of programs that use highly-aligned aligned_storage produces a compile error explaining this issue and asking the user to define either _ENABLE_EXTENDED_ALIGNED_STORAGE to request the conforming-but-potentially-ABI-breaking behavior, or _DISABLE_EXTENDED_ALIGNED_STORAGE to request the previous incorrect-but-ABI-safe behavior.
Performance and Throughput Improvements
  • Fixed iterator wrappers like move_iterator and reverse_iterator defeating iterator debugging optimizations. These wrappers remained “checked” even after verified by algorithms. For example, fill(someVectorOfInt.rbegin(), someVectorOfInt.rend()) will properly check the range and convert to reverse_iterator<int *> instead of staying a reverse_iterator<vector::iterator> as in previous releases.
  • Changed several calls of basic_string::append(1, ch) to basic_string::push_back for micro-perf improvements in iostreams machinery.
  • Implemented a fast path for binary I/O in iostreams, quadrupling bulk I/O performance.
Other Changes
  • Removed _NOEXCEPT and other internal non-Standard macros.
  • Increased readability by consistently using nullptr in the STL’s headers.

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