GSL

Guidelines Support Library

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Quick Overview

The Microsoft Guidelines Support Library (GSL) is a collection of C++ core guidelines support types and functions. It aims to provide standardized, efficient, and safe implementations of common programming patterns, helping developers write more robust and less error-prone code in modern C++.

Pros

Improves code safety and reduces common programming errors
Lightweight and header-only library, easy to integrate
Implements many of the C++ Core Guidelines recommendations
Actively maintained by Microsoft and the community

Cons

May require a learning curve for developers unfamiliar with C++ Core Guidelines
Some features might have a small performance overhead
Not all C++ compilers fully support all GSL features
Can introduce additional dependencies in projects

Code Examples

Using gsl::span for safer array access:

#include <gsl/span>
#include <vector>

void process_data(gsl::span<int> data) {
    for (int& item : data) {
        item *= 2;
    }
}

int main() {
    std::vector<int> vec = {1, 2, 3, 4, 5};
    process_data(vec);  // Safely pass vector as a span
    return 0;
}

Using gsl::not_null to ensure non-null pointers:

#include <gsl/pointers>

void print_value(gsl::not_null<int*> ptr) {
    std::cout << *ptr << std::endl;
}

int main() {
    int value = 42;
    print_value(&value);  // OK
    // print_value(nullptr);  // Compile-time error
    return 0;
}

Using gsl::finally for guaranteed cleanup:

#include <gsl/util>

void resource_intensive_function() {
    auto cleanup = gsl::finally([]{ /* cleanup code */ });
    // Function body...
    // Cleanup will be called automatically when exiting the function
}

Getting Started

To use GSL in your project, follow these steps:

Clone the repository or download the headers:

git clone https://github.com/microsoft/GSL.git

Add the include directory to your project's include path.
Include the desired GSL headers in your C++ files:
```
#include <gsl/gsl>
```
Compile your project with a C++14 or later compiler.
Start using GSL types and functions in your code to improve safety and follow C++ Core Guidelines.

Competitor Comparisons

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Pros of type_safe

Focuses on strong typing and type safety, providing more specialized tools for type-safe programming
Offers a wider range of vocabulary types, including strong_typedef for creating distinct types
Provides more comprehensive constexpr support, enabling better compile-time checks

Cons of type_safe

Smaller community and less widespread adoption compared to GSL
May have a steeper learning curve due to its more specialized nature
Less comprehensive documentation and examples available

Code Comparison

GSL:

gsl::span<int> arr = ...;
gsl::not_null<int*> ptr = ...;

type_safe:

type_safe::array_ref<int> arr = ...;
type_safe::object_ref<int> ptr = ...;

Summary

While GSL provides a broader set of guidelines and utilities for C++ programming, type_safe focuses more specifically on enhancing type safety. GSL has wider adoption and more extensive documentation, but type_safe offers more specialized tools for creating type-safe code. The choice between the two depends on the specific needs of the project and the developer's preferences for type safety enforcement.

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Pros of Abseil

More comprehensive library with a wider range of utilities and data structures
Actively maintained and regularly updated by Google
Designed for better compatibility across different C++ versions

Cons of Abseil

Larger codebase and potentially higher learning curve
May introduce dependencies that are not needed for smaller projects
Some features might be Google-specific and less applicable to general use cases

Code Comparison

GSL example:

gsl::span<int> mySpan(myVector);
gsl::not_null<int*> ptr = &myInt;

Abseil example:

absl::Span<int> mySpan(myVector);
absl::optional<int> maybeInt = absl::nullopt;

Both libraries provide similar functionality for spans, but Abseil offers a broader range of utilities. GSL focuses more on core guidelines support, while Abseil includes additional features like string formatting, time utilities, and synchronization primitives.

llvm-project

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The LLVM Project is a collection of modular and reusable compiler and toolchain technologies.

Pros of LLVM Project

Comprehensive compiler infrastructure with multiple frontends and backends
Extensive toolchain including debugger, linker, and optimization passes
Large, active community with frequent updates and contributions

Cons of LLVM Project

Steeper learning curve due to its size and complexity
Heavier resource requirements for building and using
May be overkill for projects not requiring full compiler infrastructure

Code Comparison

GSL (Guidelines Support Library):

#include <gsl/gsl>

void example(gsl::span<int> arr) {
    for (int& elem : arr) {
        elem *= 2;
    }
}

LLVM Project:

#include "llvm/IR/IRBuilder.h"

LLVMContext Context;
IRBuilder<> Builder(Context);
auto *FunctionType = FunctionType::get(Builder.getVoidTy(), false);
Function *MyFunction = Function::Create(FunctionType, Function::ExternalLinkage, "example", Module);

Summary

While GSL focuses on providing lightweight abstractions for C++ programming guidelines, LLVM Project offers a complete compiler infrastructure. GSL is more suitable for projects aiming to improve C++ code safety and readability, while LLVM is ideal for tasks involving code generation, optimization, and analysis across multiple programming languages.

folly

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An open-source C++ library developed and used at Facebook.

Pros of Folly

Broader scope with a more extensive collection of utilities and components
Optimized for high-performance systems and large-scale applications
Active development with frequent updates and contributions from Facebook engineers

Cons of Folly

Larger codebase and more complex API, potentially steeper learning curve
May include unnecessary features for smaller projects
Heavier dependency footprint compared to GSL's lightweight design

Code Comparison

GSL (Guidelines Support Library):

#include <gsl/gsl>

void example(gsl::span<int> data) {
    for (int& item : data) {
        // Process item
    }
}

Folly:

#include <folly/FBVector.h>

folly::fbvector<int> data;
data.reserve(1000);
for (int i = 0; i < 1000; ++i) {
    data.push_back(i);
}

GSL focuses on providing lightweight abstractions for safer C++ programming, while Folly offers a broader range of utilities optimized for performance in large-scale applications. GSL's span example demonstrates its emphasis on bounds-safe views, whereas Folly's FBVector showcases its high-performance container implementations.

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README

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 the latest and previous major versions of VS with MSVC & LLVM, GCC, Clang, and XCode with Apple-Clang. Within these two major versions, we try to target the latest minor updates / revisions (although this may be affected by delays between a toolchain's release and when it becomes widely available for use). Below is a table showing the versions currently being tested.

Compiler	Toolset Versions Currently Tested
XCode	13.2.1 & 12.5.1
GCC	11¹ & 10²
Clang	12² & 11²
Visual Studio with MSVC	VS2022³ & VS2019⁴
Visual Studio with LLVM	VS2022³ & VS2019⁴

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.

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
```
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
```
Build the test suite (in this case, in the Debug configuration, Release is another good choice).
```
 cmake --build . --config Debug
```
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 "v4.0.0"
    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 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.

Precise version may be found in the latest CI results. ↩
Precise version may be found in the latest CI results. Should be the version specified here. ↩ ↩² ↩³
Precise version may be found in the latest CI results. Should be the version specified here. ↩ ↩²
Precise version may be found in the latest CI results. Should be the version specified here. ↩ ↩²

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Quick Overview

Pros

Cons

Code Examples

Getting Started

Competitor Comparisons

Pros of type_safe

Cons of type_safe

Code Comparison

Summary

Pros of Abseil

Cons of Abseil

Code Comparison

Pros of LLVM Project

Cons of LLVM Project

Code Comparison

Summary

Pros of Folly

Cons of Folly

Code Comparison

Convert designs to code with AI

README

GSL: Guidelines Support Library

Project Code of Conduct

Usage of Third Party Libraries

Supported features

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

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

Quick Start

Supported Compilers / Toolsets

Building the tests

Building GSL - Using vcpkg

Using the libraries

Usage in CMake

FetchContent

Debugging visualization support

Footnotes

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