GSL: Guidelines Support Library

The Guidelines Support Library (GSL) contains functions and types that are suggested for use by the C++ Core Guidelines maintained by the Standard C++ Foundation. This repo contains Microsoft's implementation of GSL.

The entire implementation is provided inline in the headers under the gsl directory. The implementation generally assumes a platform that implements C++14 support.

While some types have been broken out into their own headers (e.g. gsl/span), it is simplest to just include gsl/gsl and gain access to the entire library.

NOTE: We encourage contributions that improve or refine any of the types in this library as well as ports to other platforms. Please see CONTRIBUTING.md for more information about contributing.

Project 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 opencode@microsoft.com with any additional questions or comments.

Usage of Third Party Libraries

This project makes use of the Google Test testing library. Please see the ThirdPartyNotices.txt file for details regarding the licensing of Google Test.

Supported features

Microsoft GSL implements the following from the C++ Core Guidelines:

Feature	Supported?	Description
1. Views
owner	☑	An alias for a raw pointer
not_null	☑	Restricts a pointer/smart pointer to hold non-null values
span	☑	A view over a contiguous sequence of memory. Based on the standardized version of std::span, however gsl::span enforces bounds checking.
span_p	☐	Spans a range starting from a pointer to the first place for which the predicate is true
basic_zstring	☑	A pointer to a C-string (zero-terminated array) with a templated char type
zstring	☑	An alias to basic_zstring with dynamic extent and a char type of char
czstring	☑	An alias to basic_zstring with dynamic extent and a char type of const char
wzstring	☑	An alias to basic_zstring with dynamic extent and a char type of wchar_t
cwzstring	☑	An alias to basic_zstring with dynamic extent and a char type of const wchar_t
u16zstring	☑	An alias to basic_zstring with dynamic extent and a char type of char16_t
cu16zstring	☑	An alias to basic_zstring with dynamic extent and a char type of const char16_t
u32zstring	☑	An alias to basic_zstring with dynamic extent and a char type of char32_t
cu32zstring	☑	An alias to basic_zstring with dynamic extent and a char type of const char32_t
2. Owners
unique_ptr	☑	An alias to std::unique_ptr
shared_ptr	☑	An alias to std::shared_ptr
stack_array	☐	A stack-allocated array
dyn_array	☐	A heap-allocated array
3. Assertions
Expects	☑	A precondition assertion; on failure it terminates
Ensures	☑	A postcondition assertion; on failure it terminates
4. Utilities
move_owner	☐	A helper function that moves one owner to the other
byte	☑	Either an alias to std::byte or a byte type
final_action	☑	A RAII style class that invokes a functor on its destruction
finally	☑	A helper function instantiating final_action
GSL_SUPPRESS	☑	A macro that takes an argument and turns it into [[gsl::suppress(x)]] or [[gsl::suppress("x")]]
implicit	☐	A "marker" to put on single-argument constructors to explicitly make them non-explicit
index	☑	A type to use for all container and array indexing (currently an alias for std::ptrdiff_t)
joining_thread	☐	A RAII style version of std::thread that joins
narrow	☑	A checked version of narrow_cast; it can throw narrowing_error
narrow_cast	☑	A narrowing cast for values and a synonym for static_cast
narrowing_error	☑	A custom exception type thrown by narrow
5. Concepts	☐

The following features do not exist in or have been removed from the C++ Core Guidelines:

Feature	Supported?	Description
strict_not_null	☑	A stricter version of not_null with explicit constructors
multi_span	☐	Deprecated. Multi-dimensional span.
strided_span	☐	Deprecated. Support for this type has been discontinued.
basic_string_span	☐	Deprecated. Like span but for strings with a templated char type
string_span	☐	Deprecated. An alias to basic_string_span with a char type of char
cstring_span	☐	Deprecated. An alias to basic_string_span with a char type of const char
wstring_span	☐	Deprecated. An alias to basic_string_span with a char type of wchar_t
cwstring_span	☐	Deprecated. An alias to basic_string_span with a char type of const wchar_t
u16string_span	☐	Deprecated. An alias to basic_string_span with a char type of char16_t
cu16string_span	☐	Deprecated. An alias to basic_string_span with a char type of const char16_t
u32string_span	☐	Deprecated. An alias to basic_string_span with a char type of char32_t
cu32string_span	☐	Deprecated. An alias to basic_string_span with a char type of const char32_t

This is based on CppCoreGuidelines semi-specification.

Quick Start

Supported Compilers / Toolsets

The GSL officially supports recent major versions of Visual Studio with both MSVC and LLVM, GCC, Clang, and XCode with Apple-Clang. For each of these major versions, the GSL officially supports C++14, C++17, C++20, and C++23 (when supported by the compiler). Below is a table showing the versions currently being tested (also see [.github/workflows/compilers.yml](the workflow).)

Compiler	Toolset Versions Currently Tested
GCC	10, 11, 12
XCode	14.3.1, 15.4
Clang	13, 14, 15
Visual Studio with MSVC	VS2019, VS2022
Visual Studio with LLVM	VS2019, VS2022

---

If you successfully port GSL to another platform, we would love to hear from you!

• Submit an issue specifying the platform and target.
• Consider contributing your changes by filing a pull request with any necessary changes.
• If at all possible, add a CI/CD step and add the button to the table below!

Target	CI/CD Status
iOS
Android

Note: These CI/CD steps are run with each pull request, however failures in them are non-blocking.

Building the tests

To build the tests, you will require the following:

• CMake, version 3.14 or later to be installed and in your PATH.

These steps assume the source code of this repository has been cloned into a directory named c:\GSL.

1. Create a directory to contain the build outputs for a particular architecture (we name it c:\GSL\build-x86 in this example).

    cd GSL
    md build-x86
    cd build-x86
   

2. Configure CMake to use the compiler of your choice (you can see a list by running cmake --help).

    cmake -G "Visual Studio 15 2017" c:\GSL
   

3. Build the test suite (in this case, in the Debug configuration, Release is another good choice).

    cmake --build . --config Debug
   

4. Run the test suite.

    ctest -C Debug
   

All tests should pass - indicating your platform is fully supported and you are ready to use the GSL types!

Building GSL - Using vcpkg

You can download and install GSL using the vcpkg dependency manager:

git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
vcpkg install ms-gsl

The GSL port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please create an issue or pull request on the vcpkg repository.

Using the libraries

As the types are entirely implemented inline in headers, there are no linking requirements.

You can copy the gsl directory into your source tree so it is available to your compiler, then include the appropriate headers in your program.

Alternatively set your compiler's include path flag to point to the GSL development folder (c:\GSL\include in the example above) or installation folder (after running the install). Eg.

MSVC++

/I c:\GSL\include

GCC/clang

-I$HOME/dev/GSL/include

Include the library using:

#include <gsl/gsl>

Usage in CMake

The library provides a Config file for CMake, once installed it can be found via find_package.

Which, when successful, will add library target called Microsoft.GSL::GSL which you can use via the usual target_link_libraries mechanism.

find_package(Microsoft.GSL CONFIG REQUIRED)

target_link_libraries(foobar PRIVATE Microsoft.GSL::GSL)

FetchContent

If you are using CMake version 3.11+ you can use the official FetchContent module. This allows you to easily incorporate GSL into your project.

# NOTE: This example uses CMake version 3.14 (FetchContent_MakeAvailable).
# Since it streamlines the FetchContent process
cmake_minimum_required(VERSION 3.14)

include(FetchContent)

FetchContent_Declare(GSL
    GIT_REPOSITORY "https://github.com/microsoft/GSL"
    GIT_TAG "v4.1.0"
    GIT_SHALLOW ON
)

FetchContent_MakeAvailable(GSL)

target_link_libraries(foobar PRIVATE Microsoft.GSL::GSL)

Debugging visualization support

For Visual Studio users, the file GSL.natvis in the root directory of the repository can be added to your project if you would like more helpful visualization of GSL types in the Visual Studio debugger than would be offered by default.

See Also

For information on Microsoft Gray Systems Lab (GSL) of applied data management and system research see https://aka.ms/gsl.