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MSVC's implementation of the C++ Standard Library.

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Microsoft's C++ Standard Library

This is the official repository for Microsoft's implementation of the C++ Standard Library (also known as the STL), which ships as part of the MSVC toolset and the Visual Studio IDE.

Build Status

What This Repo Is Useful For

If you're a programmer who just wants to use the STL, you don't need this repo. Simply install the Visual Studio IDE and select the "Desktop development with C++" workload.

If you want to participate in the STL's development, welcome! You can report issues, comment on pull requests, and learn about what we're working on. You can also submit pull requests to fix bugs or add features (see below).

Finally, you can take our code and use it in other apps and libraries (according to the terms of our license, like everything else).

GitHub Migration Status

We're in the process of moving all of our work on the STL to GitHub. Current status:

  • Code: Done. Our source code is available under the Apache License v2.0 with LLVM Exception. (See LICENSE.txt and NOTICE.txt for more information.)

  • Build System: In progress. We're working on a CMake build system, which is currently capable of building one flavor of the STL (native desktop). We need to extend this to build all of the flavors required for the MSVC toolset (e.g.

    , OneCore, Spectre). Until that's done, we're keeping our legacy build system around in the
    subdirectory. (We're keeping those files in this repo, even though they're unusable outside of Microsoft, because they need to be updated whenever source files are added/renamed/deleted. We'll delete the legacy machinery as soon as possible.)
  • Tests: In progress. We rely on three test suites: std, tr1, and libcxx. We've partially ported std and tr1, and fully ported libcxx to run under lit using the various configurations/compilers we test internally.

  • Continuous Integration: In progress. We've set up Azure Pipelines to validate changes to the repository. Currently, it builds the STL (native desktop for x86, x64, ARM, and ARM64). Also, it strictly verifies that all of our files have been formatted with clang-format and follow our other whitespace conventions.

  • Contribution Guidelines: Coming soon. Working on the STL's code involves following many rules. We have codebase conventions, Standard requirements, Microsoft-specific requirements, binary compatibility (ABI) requirements, and more. We're eager to begin accepting features and fixes from the community, but in addition to setting up a CI system, we need to write down all of the rules that are currently stored in our brains. (The ABI rules may be useful to other C++ libraries.)

  • Issues: In progress. We're going to use GitHub issues to track all of the things that we need to work on. This includes C++20 features, LWG issues, conformance bugs, performance improvements, and other todos. There are approximately 200 active bugs in the STL's Microsoft-internal database; we need to manually replicate all of them to GitHub issues. Currently, the cxx20 tag and LWG tag are done; every remaining work item is tracked by a GitHub issue. The bug tag and enhancement tag are being populated.

  • Plans: In progress. We're writing up our Roadmap.


We're implementing the latest C++ Working Draft, currently N4868, which will eventually become the next C++ International Standard, C++20. The terms Working Draft (WD) and Working Paper (WP) are interchangeable; we often informally refer to these drafts as "the Standard" while being aware of the difference. (There are other relevant Standards; for example, supporting

involves understanding how the C++14 and C++17 Standards differ from the Working Paper, and we often need to refer to the C Standard Library and ECMAScript regular expression specifications.)

Our primary goals are conformance, performance, usability, and compatibility.

  • Conformance: The Working Paper is a moving target; as features and LWG issue resolutions are added, we need to implement them. That can involve a lot of work, because the STL is required to behave in very specific ways and to handle users doing very unusual things.

  • Performance: The STL needs to be extremely fast at runtime; speed is one of C++'s core strengths, and most C++ programs use the STL extensively. As a result, we spend more time on optimization than most general-purpose libraries. (However, we're wary of changes that improve some scenarios at the expense of others, or changes that make code significantly more complicated and fragile. That is, there's a "complexity budget" that must be spent carefully.)

  • Usability: This includes parts of the programming experience like compiler throughput, diagnostic messages, and debugging checks. For example, we've extensively marked the STL with

    attributes because this helps programmers avoid bugs.
  • Compatibility: This includes binary compatibility and source compatibility. We're keeping VS 2019 binary-compatible with VS 2017 and VS 2015, which restricts what we can change in VS 2019 updates. (We've found that significant changes are possible even though other changes are impossible, which we'll be documenting in our Contribution Guidelines soon.) While there are a few exceptions to this rule (e.g. if a feature is added to the Working Paper, we implement it, and then the feature is significantly changed before the International Standard is finalized, we reserve the right to break binary compatibility because

    offers an experimental preview of such features), binary compatibility generally overrides all other considerations, even conformance. Source compatibility refers to being able to successfully recompile user code without changes. We consider source compatibility to be important, but not all-important; breaking source compatibility can be an acceptable cost, if done for the right reasons in the right way (e.g. in a controlled manner with escape hatches).


There are things that we aren't interested in doing with this project, for various reasons (most importantly, we need to focus development effort on our goals). Some examples:

  • Non-goal: Porting to other platforms.

  • Non-goal: Adding non-Standard extensions.

  • Non-goal: Implementing Technical Specifications. (We're prioritizing features in the Working Paper. Occasionally, we might implement some or all of a TS, often when we're working on the specification itself.)

If you're proposing a feature to WG21 (the C++ Standardization Committee), you're welcome (and encouraged!) to use our code as a base for a proof-of-concept implementation. These non-goals simply mean that we're unable to consider pull requests for a proposed feature until it has been voted into a Working Paper. After that happens, we'll be delighted to review a production-ready pull request.

Reporting Issues

You can report STL bugs here, where they'll be directly reviewed by maintainers. You can also report STL bugs through Developer Community, or the VS IDE (Help > Send Feedback > Report a Problem...).

Please help us efficiently process bug reports by following these rules:

  • Only STL bugs should be reported here. If it's a bug in the compiler, CRT, or IDE, please report it through Developer Community or Report A Problem. If it's a bug in the Windows SDK, please report it through the Feedback Hub app. If you aren't sure, try to reduce your test case and see if you can eliminate the STL's involvement while still reproducing the bug.

  • You should be reasonably confident that you're looking at an actual implementation bug, instead of undefined behavior or surprising-yet-Standard behavior. Comparing against other implementations can help (but remember that implementations can differ while conforming to the Standard); try Godbolt's Compiler Explorer and Wandbox. If you still aren't sure, ask the nearest C++ expert.

  • You should prepare a self-contained command-line test case, ideally as small as possible. We need a source file, a command line, what happened (e.g. a compiler error, runtime misbehavior), and what you expected to happen. By "self-contained", we mean that your source file has to avoid including code that we don't have. Ideally, only CRT and STL headers should be included. If you have to include other MSVC libraries, or the Windows SDK, to trigger an STL bug, that's okay. But if you need parts of your own source code to trigger the STL bug, you need to extract that for us. (On Developer Community, we'll accept zipped IDE projects if you have no other way to reproduce a bug, but this is very time-consuming for us to reduce.)

  • A good title is helpful. We prefer "

    : Short description of your issue". You don't usually need to
    or C++. For example, "
    should be true for
    enum class FluffyKittens

It's okay if you report an apparent STL bug that turns out to be a compiler bug, or surprising-yet-Standard behavior. Just try to follow these rules, so we can spend more time fixing bugs and implementing features.

How To Build With The Visual Studio IDE

The STL uses boost-math headers to provide P0226R1 Mathematical Special Functions. We recommend using vcpkg to acquire this dependency.

  1. Install Visual Studio 2019 16.9 Preview 1 or later.
    • We recommend selecting "C++ CMake tools for Windows" in the VS Installer. This will ensure that you're using supported versions of CMake and Ninja.
    • Otherwise, install CMake 3.18 or later, and Ninja 1.8.2 or later.
  2. Open Visual Studio, and choose the "Clone or check out code" option. Enter the URL of this repository,
  3. Open a terminal in the IDE with
    Ctrl + `
    (by default) or press on "View" in the top bar, and then "Terminal".
  4. In the terminal, invoke
    git submodule update --init --progress llvm-project vcpkg
  5. In the terminal, invoke
  6. In the terminal, invoke
    .\vcpkg\vcpkg.exe install boost-math:x86-windows boost-math:x64-windows
  7. Choose the architecture you wish to build in the IDE, and build as you would any other project. All necessary CMake settings are set by

How To Build With A Native Tools Command Prompt

  1. Install Visual Studio 2019 16.9 Preview 1 or later.
    • We recommend selecting "C++ CMake tools for Windows" in the VS Installer. This will ensure that you're using supported versions of CMake and Ninja.
    • Otherwise, install CMake 3.18 or later, and Ninja 1.8.2 or later.
  2. Open a command prompt.
  3. Change directories to a location where you'd like a clone of this STL repository.
  4. git clone
  5. cd STL
  6. git submodule update --init --progress llvm-project vcpkg
  7. .\vcpkg\bootstrap-vcpkg.bat
  8. .\vcpkg\vcpkg.exe install boost-math:x86-windows boost-math:x64-windows

To build the x86 target:

  1. Open an "x86 Native Tools Command Prompt for VS 2019".
  2. Change directories to the previously cloned
  3. cmake -G Ninja -S . -B out\build\x86
  4. ninja -C out\build\x86

To build the x64 target:

  1. Open an "x64 Native Tools Command Prompt for VS 2019".
  2. Change directories to the previously cloned
  3. cmake -G Ninja -S . -B out\build\x64
  4. ninja -C out\build\x64

How To Consume

Consumption of the built library is largely based on the build system you're using. There are at least 2 directories you need to hook up. Assuming you built the x64 target with the Visual Studio IDE, with the STL repository cloned to

, build outputs will end up at
. Ensure that the
directory is searched for headers, and that
is searched for link libraries, before any defaults supplied by MSVC. The names of the import and static libraries are the same as those that ship with MSVC. As a result, the compiler
, or
switches will work without modification of your build scripts or command-line muscle memory.

Should you choose to use the DLL flavors, the DLLs to deploy are built to

. Note that the DLLs generated by the CMake build system here have a suffix, defaulting to
, which distinguishes them from the binaries that ship with MSVC. That avoids any conflict with the DLLs installed by the redistributables into System32, and ensures that other components wanting to be a "guest in your process", like print drivers and shell extensions, see the export surface of the STL they were built with. Otherwise, the "
" you deployed in the same directory as your .exe would "win" over the versions in System32.

Complete Example Using x64 DLL Flavor

The compiler looks for include directories according to the

environment variable, and the linker looks for import library directories according to the
environment variable, and the Windows loader will (eventually) look for DLL dependencies according to directories in the
environment variable. From an "x64 Native Tools Command Prompt for VS 2019":
C:\Users\username\Desktop>set INCLUDE=C:\Dev\STL\out\build\x64\out\inc;%INCLUDE%

C:\Users\username\Desktop>set LIB=C:\Dev\STL\out\build\x64\out\lib\amd64;%LIB%

C:\Users\username\Desktop>set PATH=C:\Dev\STL\out\build\x64\out\bin\amd64;%PATH%

C:\Users\username\Desktop>type example.cpp #include

int main() { std::cout << "Hello STL OSS world!\n"; }

C:\Users\username\Desktop>cl /nologo /EHsc /W4 /WX /MDd /std:c++latest .\example.cpp example.cpp

C:\Users\username\Desktop>.\example.exe Hello STL OSS world!

C:\Users\username\Desktop>dumpbin /IMPORTS .\example.exe | findstr msvcp msvcp140d_oss.dll

How To Run The Tests With A Native Tools Command Prompt

  1. Follow either How To Build With A Native Tools Command Prompt or How To Build With The Visual Studio IDE.
  2. Acquire Python 3.9 or newer and have it on the
    (or run it directly using its absolute or relative path).
  3. Have LLVM's
    directory on the
    is available).
    • We recommend selecting "C++ Clang tools for Windows" in the VS Installer. This will automatically add LLVM to the
      of the x86 and x64 Native Tools Command Prompts, and will ensure that you're using a supported version.
    • Otherwise, use LLVM's installer and choose to add LLVM to your
      during installation.
  4. Follow the instructions below.

Running All The Tests

After configuring and building the project, running

from the build output directory will run all the tests. CTest will only display the standard error output of tests that failed. In order to get more details from CTest's
invocations, run the tests with
ctest -V

Running A Subset Of The Tests

can be invoked on a subdirectory of a testsuite and will execute all the tests under that subdirectory. This can mean executing the entirety of a single testsuite, running all tests under a category in libcxx, or running a single test in


These examples assume that your current directory is

  • This command will run all of the testsuites with verbose output.
    • ctest -V
  • This command will also run all of the testsuites.
    • python tests\utils\stl-lit\ ..\..\..\llvm-project\libcxx\test ..\..\..\tests\std ..\..\..\tests\tr1
  • This command will run all of the std testsuite.
    • python tests\utils\stl-lit\ ..\..\..\tests\std
  • If you want to run a subset of a testsuite, you need to point it to the right place in the sources. The following will run the single test found under VSO0000000anycallingconventions.
    • python tests\utils\stl-lit\ ..\..\..\tests\std\tests\VSO_0000000_any_calling_conventions
  • You can invoke
    with any arbitrary subdirectory of a testsuite. In libcxx this allows you to have finer control over what category of tests you would like to run. The following will run all the libcxx map tests.
    • python tests\utils\stl-lit\ ..\..\..\llvm-project\libcxx\test\std\containers\associative\map

Interpreting The Results Of Tests


When running the tests via CTest, all of the testsuites are considered to be a single test. If any single test in a testsuite fails, CTest will simply report that the

test failed.

Example: ``` 0% tests passed, 1 tests failed out of 1

Total Test time (real) = 2441.55 sec

The following tests FAILED: 1 - stl (Failed) ```

The primary utility of CTest in this case is to conveniently invoke
with the correct set of arguments.

CTest will output everything that was sent to stderr for each of the failed testsuites, which can be used to identify which individual test within the testsuite failed. It can sometimes be helpful to run CTest with the

option in order to see the stdout of the tests.


When running the tests directly via the generated
script the result of each test will be printed. The format of each result is
{Result Code}: {Testsuite Name} :: {Test Name}:{Configuration Number}

Example: ``` -- Testing: 28 tests, 12 workers -- PASS: tr1 :: tests/cwchar1:01 (1 of 28) PASS: tr1 :: tests/cwchar1:11 (2 of 28) PASS: tr1 :: tests/cwchar1:02 (3 of 28) PASS: tr1 :: tests/cwchar1:03 (4 of 28) PASS: tr1 :: tests/cwchar1:00 (5 of 28) PASS: tr1 :: tests/cwchar1:04 (6 of 28) PASS: tr1 :: tests/cwchar1:05 (7 of 28) PASS: tr1 :: tests/cwchar1:09 (8 of 28) PASS: tr1 :: tests/cwchar1:06 (9 of 28) UNSUPPORTED: tr1 :: tests/cwchar1:20 (10 of 28) UNSUPPORTED: tr1 :: tests/cwchar1:21 (11 of 28) UNSUPPORTED: tr1 :: tests/cwchar1:22 (12 of 28) UNSUPPORTED: tr1 :: tests/cwchar1:23 (13 of 28) UNSUPPORTED: tr1 :: tests/cwchar1:24 (14 of 28) PASS: tr1 :: tests/cwchar1:07 (15 of 28) PASS: tr1 :: tests/cwchar1:08 (16 of 28) PASS: tr1 :: tests/cwchar1:10 (17 of 28) PASS: tr1 :: tests/cwchar1:16 (18 of 28) PASS: tr1 :: tests/cwchar1:17 (19 of 28) PASS: tr1 :: tests/cwchar1:14 (20 of 28) PASS: tr1 :: tests/cwchar1:12 (21 of 28) PASS: tr1 :: tests/cwchar1:13 (22 of 28) PASS: tr1 :: tests/cwchar1:19 (23 of 28) PASS: tr1 :: tests/cwchar1:18 (24 of 28) PASS: tr1 :: tests/cwchar1:15 (25 of 28) PASS: tr1 :: tests/cwchar1:25 (26 of 28) PASS: tr1 :: tests/cwchar1:26 (27 of 28) PASS: tr1 :: tests/cwchar1:27 (28 of 28)

Testing Time: 3.96s Expected Passes : 23 Unsupported Tests : 5 ```

In the above example we see that 23 tests succeeded and 5 were unsupported.

Result Code Values

Our tests use the standard lit result codes, and an undocumented result code:

. For our tests, only the
, and
result codes are relevant.


result codes are self-explanatory. We want our tests to
and not


result codes are less obvious.
is actually a failure result and indicates that we expected a test to fail but it passed.
is a successful result and indicates that we expected the test to fail and it did. Typically an
result means that the
file for the testsuite needs to be modified. If the
result is a test legitimately passing, the usual course of action would be to remove a
entry from the
. However, some tests from
mark themselves as
(meaning they expect to fail) for features they have added tests for but have yet to implement in
. If the STL implements those features first the tests will begin passing unexpectedly for us and return
results. In order to resolve this it is necessary to add a
entry to the
of the testsuite in question.


result code means that the requirements for a test are not met and so it will not be run. Currently all tests which use the
options are unsupported.


result code indicates that a given test was explicitly skipped by adding a
entry to the
. A test may be skipped for a number of reasons, which include, but are not limited to: * being an incorrect test * taking a very long time to run * failing or passing for the incorrect reason

Block Diagram

The STL is built atop other compiler support libraries that ship with Windows and Visual Studio, like the UCRT, VCRuntime, and VCStartup. The following diagram describes the dependencies between those components and their ship vehicles.

MSVC Libraries Block Diagram


This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit

When you submit a pull request, a CLA bot will automatically determine whether you need to provide a CLA and decorate the PR appropriately (e.g., status check, comment). Simply follow the instructions provided by the bot. You will only need to do this once across all repos using our CLA.

Code Of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact [email protected] with any additional questions or comments.


Copyright (c) Microsoft Corporation.

SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

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