Microsoft Visual Studio Team just opened sourced the C++ Standard Library (aka STL)

What is the C++ STL ?

The STL (Standard Template Library) was developed separately, and then submitted to the C++ standard committee for consideration, giving them the option of adopting it into the language. But it was not developed as part of the C++ standard, and for this reason, it is designed in a style that is very different from the rest of the C++ standard library. 

The STL at it's code provides the most fundamental functionality you expect of a standard library: The ability to store sequences of data, and the ability to process these sequences.

Every other language has a Collections/Containers part of its standard library, containing implementations of dynamic arrays (known as arraylists in Java, List in C#, and vectors in C++), linked lists, dictionaries and other common data structures.

They also typically provide some mechanims for traversing these structures. (Enumerators or iterators, for example)

The STL provides the same functionality in C++, but does it in a unusually elegant way, and with some interesting abstractions.

Press Release

Today (Sept 16, 2019) at CppCon 2019, we (the MSVC team) announced that we’re releasing our implementation of the C++ Standard Library (also known as the STL) as open source.

https://github.com/microsoft/STL is our new repository, containing all of our product source code, a new CMake build system, and a README with more information.

Sources: 

https://stackoverflow.com/questions/827236/what-is-the-stl
https://devblogs.microsoft.com/cppblog/open-sourcing-msvcs-stl/