GSL/README.md

# GSL: Guidelines Support Library
[![Build Status](https://dev.azure.com/cppstat/GSL/_apis/build/status/microsoft.GSL?branchName=main)](https://dev.azure.com/cppstat/GSL/_build/latest?definitionId=1&branchName=main)

The Guidelines Support Library (GSL) contains functions and types that are suggested for use by the
[C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines) maintained by the [Standard C++ Foundation](https://isocpp.org).
This repo contains Microsoft's implementation of GSL.

The entire implementation is provided inline in the headers under the [gsl](./include/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](./include/gsl/span)),
it is simplest to just include [gsl/gsl](./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](./CONTRIBUTING.md) for more information about contributing.

# Project Code of Conduct
This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/). For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or contact [opencode@microsoft.com](mailto:opencode@microsoft.com) with any additional questions or comments.

# Usage of Third Party Libraries
This project makes use of the [Google Test](https://github.com/google/googletest) testing library. Please see the [ThirdPartyNotices.txt](./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**][cg-views]                                                 |            |
[owner](docs/headers.md#user-content-H-pointers-owner)                   | ☑   | An alias for a raw pointer
[not_null](docs/headers.md#user-content-H-pointers-not_null)             | ☑   | Restricts a pointer/smart pointer to hold non-null values
[span](docs/headers.md#user-content-H-span-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](docs/headers.md#user-content-H-zstring)                  | ☑   | A pointer to a C-string (zero-terminated array) with a templated char type
[zstring](docs/headers.md#user-content-H-zstring)                        | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char`
[czstring](docs/headers.md#user-content-H-zstring)                       | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char`
[wzstring](docs/headers.md#user-content-H-zstring)                       | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `wchar_t`
[cwzstring](docs/headers.md#user-content-H-zstring)                      | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const wchar_t`
[u16zstring](docs/headers.md#user-content-H-zstring)                     | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char16_t`
[cu16zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char16_t`
[u32zstring](docs/headers.md#user-content-H-zstring)                     | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char32_t`
[cu32zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char32_t`
[**2. Owners**][cg-owners]                                               |            |
[unique_ptr](docs/headers.md#user-content-H-pointers-unique_ptr)         | ☑   | An alias to `std::unique_ptr`
[shared_ptr](docs/headers.md#user-content-H-pointers-shared_ptr)         | ☑   | An alias to `std::shared_ptr`
stack_array                                                              | ☐   | A stack-allocated array
dyn_array                                                                | ☐   | A heap-allocated array
[**3. Assertions**][cg-assertions]                                       |            |
[Expects](docs/headers.md#user-content-H-assert-expects)                 | ☑   | A precondition assertion; on failure it terminates
[Ensures](docs/headers.md#user-content-H-assert-ensures)                 | ☑   | A postcondition assertion; on failure it terminates
[**4. Utilities**][cg-utilities]                                         |            |
move_owner                                                               | ☐   | A helper function that moves one `owner` to the other
[byte](docs/headers.md#user-content-H-byte-byte)                         | ☑   | Either an alias to `std::byte` or a byte type
[final_action](docs/headers.md#user-content-H-util-final_action)         | ☑   | A RAII style class that invokes a functor on its destruction
[finally](docs/headers.md#user-content-H-util-finally)                   | ☑   | A helper function instantiating [final_action](docs/headers.md#user-content-H-util-final_action)
[GSL_SUPPRESS](docs/headers.md#user-content-H-assert-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](docs/headers.md#user-content-H-util-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](docs/headers.md#user-content-H-narrow-narrow)                   | ☑   | A checked version of `narrow_cast`; it can throw [narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error)
[narrow_cast](docs/headers.md#user-content-H-util-narrow_cast)           | ☑   | A narrowing cast for values and a synonym for `static_cast`
[narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error) | ☑   | A custom exception type thrown by [narrow](docs/headers.md#user-content-H-narrow-narrow)
[**5. Concepts**][cg-concepts]                                           | ☐   |

## The following features do not exist in or have been removed from the C++ Core Guidelines:
Feature                            | Supported? | Description
-----------------------------------|:----------:|-------------
[strict_not_null](docs/headers.md#user-content-H-pointers-strict_not_null) | ☑   | A stricter version of [not_null](docs/headers.md#user-content-H-pointers-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](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gsl-guidelines-support-library).

[cg-views]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslview-views
[cg-owners]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslowner-ownership-pointers
[cg-assertions]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslassert-assertions
[cg-utilities]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslutil-utilities
[cg-concepts]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gslconcept-concepts

# 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 | ![CI_iOS](https://github.com/microsoft/GSL/workflows/CI_iOS/badge.svg)
Android | ![CI_Android](https://github.com/microsoft/GSL/workflows/CI_Android/badge.svg)

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](http://cmake.org), 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](https://github.com/Microsoft/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](https://github.com/Microsoft/vcpkg) 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](./include/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.

```cmake
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](https://cmake.org/cmake/help/latest/module/FetchContent.html).
This allows you to easily incorporate GSL into your project.

```cmake
# 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 "v4.0.0"
    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](./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)](https://aka.ms/gsl) of applied data management and system research see <https://aka.ms/gsl>.