Editing Microsoft Visual C++ (section)

== Compatibility ==

=== ABI ===
The Visual C++ compiler [[Application binary interface|ABI]] has historically changed between major compiler releases.<ref>Microsoft MSDN: [http://msdn.microsoft.com/en-us/library/bb531344.aspx Breaking Changes in Visual C++]</ref> This is especially the case for STL containers, where container sizes have varied a lot between compiler releases.<ref>Microsoft MSDN: [http://msdn.microsoft.com/en-us/library/vstudio/hh438470.aspx Containers (Modern C++)]</ref> Microsoft therefore recommends against using C++ interfaces at module boundaries when one wants to enable client code compiled using a different compiler version. Instead of C++, Microsoft recommends using [[C (programming language)|C]]<ref>Microsoft MSDN: [http://msdn.microsoft.com/en-us/library/vstudio/hh438475.aspx Portability At ABI Boundaries (Modern C++)]</ref> or [[Component Object Model|COM]]<ref>Microsoft forum: [http://social.msdn.microsoft.com/Forums/en/vcgeneral/thread/86eda6a7-4d90-4e19-a9d4-6cbe22b661f4 Binary compatibility across Visual C++ versions] {{Webarchive|url=https://archive.today/20130216110824/http://social.msdn.microsoft.com/Forums/en/vcgeneral/thread/86eda6a7-4d90-4e19-a9d4-6cbe22b661f4 |date=2013-02-16 }}</ref> interfaces, which are designed to have a stable ABI between compiler releases.

All 14.x MSVC releases have a stable ABI,<ref>docs.microsoft.com: [https://docs.microsoft.com/en-us/cpp/porting/binary-compat-2015-2017? C++ binary compatibility between Visual Studio 2015, 2017, and 2019]</ref> and binaries built with these versions can be mixed in a forwards-compatible manner, noting the following restrictions:
* The toolset version used must be equal to or higher than the highest toolset version used to build any linked binaries.
* The MSVC Redistributable version must be equal to or higher than the toolset version used by any application component.
* Static libraries or object files compiled with /GL (Whole program optimisation) are not binary compatible between versions and must use the exact same toolset.

=== C runtime libraries ===
Visual C++ ships with different versions of C runtime libraries.<ref name="MSVCRuntime">[http://msdn.microsoft.com/en-us/library/abx4dbyh.aspx C Run-Time Libraries]</ref> This means users can compile their code with any of the available libraries. However, this can cause some problems when using different components ([[Dynamic-link library|DLLs]], [[EXE]]s) in the same program. A typical example is a program using different [[Library (computing)|libraries]]. The user should use the same [[C standard library|C Run-Time]] for all the program's components unless the implications are understood. Microsoft recommends using the [[Thread (computer science)|multithreaded]], dynamic link library (/MD or /MDd compiler option) to avoid possible problems.<ref name="MSVCRuntime" />

==== POSIX ====
Although Microsoft's CRT implements a large subset of [[POSIX]] interfaces, the Visual C++ compiler will emit a warning on ''every'' use of such functions by default. The rationale is that C and C++ standards require an underscore prefix before implementation-defined interfaces, so the use of these functions are non-standard.<ref>{{cite web |title=Compatibility |url=https://docs.microsoft.com/en-us/cpp/c-runtime-library/compatibility |website=Microsoft: CRT library features |date=26 October 2022 |language=en-us}}</ref> However, systems that are actually POSIX-compliant would not accept these underscored names, and it is more portable to just turn off the warning instead.

=== C ===
Although the product originated as an IDE for the [[C (programming language)|C]] programming language, for many years the compiler's support for that language conformed only to the original edition of the [[ANSI C|C standard]], dating from 1989, but not the [[C99]] revision of the standard. There had been no plans to support C99 even in 2011, more than a decade after its publication.<ref>{{cite web|url=https://connect.microsoft.com/VisualStudio/feedback/details/653336/c99-support|title=C99 support|work=Microsoft Connect|archive-url=https://web.archive.org/web/20160304185822/http://connect.microsoft.com/VisualStudio/feedback/details/653336/c99-support|archive-date=2016-03-04|url-status=dead|access-date=2015-09-21}}</ref>

Visual C++ 2013 finally added support for various C99 features in its C mode (including designated initializers, compound literals, and the <code>_Bool</code> type),<ref>{{cite web|title=What's New for Visual C++ in Visual Studio 2013|url=https://msdn.microsoft.com/en-us/library/hh409293(v=vs.120).aspx|work=Microsoft Developer Network| date=28 April 2015 |publisher=Microsoft}}</ref> though it was still not complete.<ref>{{cite web|url=http://blogs.msdn.com/b/vcblog/archive/2013/07/19/c99-library-support-in-visual-studio-2013.aspx|title=C99 library support in Visual Studio 2013|author=Pat Brenner|date=July 19, 2013|publisher=Microsoft|work=Visual C++ Team Blog}}</ref> Visual C++ 2015 further improved the C99 support, with full support of the C99 Standard Library, except for features that require C99 language features not yet supported by the compiler.<ref>{{cite web|url=https://msdn.microsoft.com/en-us/library/hh409293(v=vs.140).aspx|title=What's New for Visual C++ in Visual Studio 2015|work=Microsoft Developer Network|date=3 January 2018 |publisher=Microsoft}}</ref>

Most of the changes from the [[C11 (C standard revision)|C11]] revision of the standard were still not supported by Visual C++ 2017.<ref>{{cite web|url=https://docs.microsoft.com/en-us/cpp/build/walkthrough-compile-a-c-program-on-the-command-line|title=Walkthrough: Compile a C program on the command line|work=Visual C++ Documentation|date=10 May 2022 |publisher=Microsoft}}</ref> For example, generic selections via the <code>_Generic</code> keyword are not supported by the compiler and result in a syntax error.<ref>{{cite web|url=https://developercommunity.visualstudio.com/content/problem/250665/msvc-2017-does-not-support-generic-type-generic-ma.html|title=MSVC 2017 does not support _Generic (type generic macros) in C}}</ref>

The preprocessor was overhauled in 2018, with C11 in sight:<ref>{{cite web |last=Luvsanbat |first=Bat-Ulzii |date=July 6, 2018 |title=MSVC Preprocessor Progress towards Conformance |url=https://devblogs.microsoft.com/cppblog/msvc-preprocessor-progress-towards-conformance/ |publisher=Microsoft |work=Visual Studio Team Blog |url-status=live |archive-url=https://web.archive.org/web/20240303170213/https://devblogs.microsoft.com/cppblog/msvc-preprocessor-progress-towards-conformance/ |archive-date=3 Mar 2024}}</ref>

{{blockquote|Full C11 conformance is on our roadmap, and updating the preprocessor is just the first step in that process. The C11 <code>_Generic</code> feature is not actually part of the preprocessor, so it has not yet been implemented. When implemented I expect the feature to work independently of if the traditional or updated preprocessor logic is used.}}

<code>_Generic</code> support has been committed to MSVC as of February 2020.<ref>{{cite web|url=https://www.reddit.com/r/cpp/comments/hjn4uv/c20_features_and_fixes_in_vs_2019_161_through_166/fwsil0q/?context=3 |title=C++20 Features and Fixes |date=July 3, 2020 |website=[[reddit]]}}</ref>

In September 2020, Microsoft announced C11 and C17 standards support in MSVC would arrive in version 16.8.<ref>{{cite web|url=https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-arriving-in-msvc/|title=C11 and C17 Standard Support Arriving in MSVC|date=September 14, 2020}}</ref> This did not include optional features but Microsoft indicated that they were planning to add support for atomics and threads at a later date. In version 17.5, partial (since [[Lock (computer science)|atomic locks]] are missing) and experimental (meaning hidden behind the compiler flag <code>/experimental:c11atomics</code>) support for atomics was added<ref>{{cite web|url=https://devblogs.microsoft.com/cppblog/c11-atomics-in-visual-studio-2022-version-17-5-preview-2/|title=C11 Atomics in Visual Studio 2022 version 17.5 Preview 2|date=2022-12-13}}</ref> and in version 17.8, support for threads was added, this time not behind a compiler flag.<ref>{{cite web|url=https://devblogs.microsoft.com/cppblog/c11-threads-in-visual-studio-2022-version-17-8-preview-2/|title=C11 Threads in Visual Studio 2022 version 17.8 Preview 2|date=2023-09-26}}</ref><ref>{{Cite web |last=Murphy |first=Adrian |date=2023-11-14 |title=Visual Studio 17.8 now available! |url=https://devblogs.microsoft.com/visualstudio/visual-studio-17-8-now-available/ |access-date=2024-01-12 |website=Visual Studio Blog |language=en-US}}</ref>

=== C++ ===
With default settings MSVC does not do two-phase name lookup which prevents it from flagging a wide range of invalid code.  Most checks are deferred to template instantiation.  More recent versions remedy this behavior, but it needs to be enabled by the command-line option <code>/permissive-</code>.<ref>{{cite web|url=https://devblogs.microsoft.com/cppblog/two-phase-name-lookup-support-comes-to-msvc/|title=Two-phase name lookup support comes to MSVC|date=2017-09-11}}</ref>