doc: develop: Improve C++ language support documentation

This commit reworks the C++ language support documentation such that:

1. it contains more up-to-date information regarding the available
   features.

2. it makes a clear distinction between the Zephyr C++ subsystem and
   the C++ standard library and describe the functionalities they
   provide.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

doc/develop/languages/cpp/index.rst

@@ -0,0 +1,101 @@
+.. _language_cpp:
+
+C++ Language Support
+####################
+
+C++ is a general-purpose object-oriented programming language that is based on
+the C language.
+
+Enabling C++ Support
+********************
+
+Zephyr supports applications written in both C and C++. However, to use C++ in
+an application you must configure Zephyr to include C++ support by selecting
+the :kconfig:option:`CONFIG_CPLUSPLUS` in the application configuration file.
+
+To enable C++ support, the compiler toolchain must also include a C++ compiler
+and the included compiler must be supported by the Zephyr build system. The
+:ref:`toolchain_zephyr_sdk`, which includes the GNU C++ Compiler (part of GCC),
+is supported by Zephyr, and the features and their availability documented
+here assume the use of the Zephyr SDK.
+
+When compiling a source file, the build system selects the C++ compiler based
+on the suffix (extension) of the files. Files identified with either a **cpp**
+or a **cxx** suffix are compiled using the C++ compiler. For example,
+:file:`myCplusplusApp.cpp` is compiled using C++.
+
+.. note::
+    Do not use C++ for kernel, driver, or system initialization code.
+
+Language Features
+*****************
+
+Zephyr currently provides only a subset of C++ functionality. The following
+features are *not* supported:
+
+* Static global object destruction
+* OS-specific C++ standard library classes (e.g. ``std::thread``,
+  ``std::mutex``)
+
+While not an exhaustive list, support for the following functionality is
+included:
+
+* Inheritance
+* Virtual functions
+* Virtual tables
+* Static global object constructors
+* Dynamic object management with the **new** and **delete** operators
+* Exceptions
+* :abbr:`RTTI (runtime type information)`
+* Standard Template Library (STL)
+
+Static global object constructors are initialized after the drivers are
+initialized but before the application :c:func:`main()` function. Therefore,
+use of C++ is restricted to application code.
+
+In order to make use of the C++ exceptions, the
+:kconfig:option:`CONFIG_EXCEPTIONS` must be selected in the application
+configuration file.
+
+Zephyr C++ Subsystem
+********************
+
+Zephyr C++ subsystem (:file:`subsys/cpp`) provides a minimal subset of the C++
+standard library and application binary interface (ABI) functions to enable
+basic C++ language support. This includes:
+
+* ``new`` and ``delete`` operators
+* virtual function stub and vtables
+* static global initializers for global constructors
+
+The scope of the C++ subsystem is strictly limited to providing the basic C++
+language support, and it does not implement any `Standard Template Library
+(STL)`_ classes and functions. For this reason, it is only suitable for use in
+the applications that implement their own (non-standard) class library and do
+rely on the Standard Template Library (STL) components.
+
+Any application that makes use of the Standard Template Library (STL)
+components, such as ``std::string`` and ``std::vector``, must enable the C++
+standard library support.
+
+C++ Standard Library
+********************
+
+The `C++ Standard Library`_ is a collection of classes and functions that are
+part of the ISO C++ standard (``std`` namespace).
+
+Zephyr does not include any C++ standard library implementation in source code
+form. Instead, it allows configuring the build system to link against the
+pre-built C++ standard library included in the C++ compiler toolchain.
+
+For instance, when building with the :ref:`toolchain_zephyr_sdk`, the build
+system can be configured to link against the GNU C++ Standard Library
+(``libstdc++.a``) included in the Zephyr SDK, which is a fully featured C++
+standard library that provides all features required by the ISO C++ standard
+including the Standard Template Library (STL).
+
+To enable C++ standard library, select the
+:kconfig:option:`CONFIG_LIB_CPLUSPLUS` in the application configuration file.
+
+.. _`C++ Standard Library`: https://en.wikipedia.org/wiki/C%2B%2B_Standard_Library
+.. _`Standard Template Library (STL)`: https://en.wikipedia.org/wiki/Standard_Template_Library