beman.optional26: C++26 Extensions for std::optional

This repository implements std::optional extensions targeting C++26. The beman.optional26 library aims to evaluate the stability, the usability, and the performance of these proposed changes before they are officially adopted by WG21 into the C++ Working Draft. Additionally, it allows developers to use these new features before they are implemented in major standard library compilers.

Implements:

• Give std::optional Range Support (P3168R2)
• std::optional<T&> (P2988R5)

Table of Contents

• beman.optional26: C++26 Extensions for std::optional
  • Table of Contents
  • License
  • Examples
    • range_loop
    • optional_ref
  • How to Build
    • Compiler Support
    • Dependencies
    • Instructions
      • Preset CMake Flows
      • Custom CMake Flows
        • Build and Run Tests
        • Build Production, but Skip Tests
  • Papers

License

Source is licensed with the Apache 2.0 license with LLVM exceptions

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

Documentation and associated papers are licensed with the Creative Commons Attribution 4.0 International license.

// SPDX-License-Identifier: CC-BY-4.0

The intent is that the source and documentation are available for use by people implementing their own optional types as well as people using the optional presented here as-is.

The README itself is licensed with CC0 1.0 Universal. Copy the contents and incorporate in your own work as you see fit.

// SPDX-License-Identifier: CC0-1.0

Examples

Full runable examples can be found in examples/ - please check ./examples/README.md.

range_loop

The next code snippet shows optional range support added in Give std::optional Range Support(P3168R2):

#include <beman/optional26/optional.hpp>
...

// Example from P3168R2: basic range loop over C++26 optional.

beman::optional26::optional<int> empty_opt{};
for ([[maybe_unused]] const auto& i : empty_opt) {
    // Should not see this in console.
    std::cout << "\"for each loop\" over C++26 optional: empty_opt\n";
}

beman::optional26::optional<int> opt{26};
for (const auto& i : opt) {
    // Should see this in console.
    std::cout << "\"for each loop\" over C++26 optional: opt = " << i << "\n";
}

Full code can be found in ./examples/range_loop.cpp. Build and run instructions in ./examples/README.md. Or try it on Compiler Explorer.

optional_ref

The next code snippet shows optional reference support added in std::optional<T&> (P2988R5):

#include <beman/optional26/optional.hpp>
...

{
    // Empty optional example.
    std::optional<int>             std_empty_opt;
    beman::optional26::optional<int> beman_empty_opt;
    assert(!std_empty_opt.has_value() &&
            !beman_empty_opt.has_value()); // or assert(!std_empty_opt && !beman_empty_opt);
    std::cout << "std_vs_beman: .has_value() matches?: "
              << (std_empty_opt.has_value() == beman_empty_opt.has_value() ? "yes" : "no") << "\n";
}

{
    // Optional with value example.
    std::optional<int>             std_opt   = 26;
    beman::optional26::optional<int> beman_opt = 26;
    assert(std_opt.has_value() && beman_opt.has_value()); // or assert(std_opt && beman_opt);
    assert(std_opt.value() == beman_opt.value());         // or assert(*std_opt == *beman_opt);
    std::cout << "std_vs_beman: .value() matches?: " << (std_opt.value() == beman_opt.value() ? "yes" : "no")
              << "\n";
}

Full code can be found in ./examples/optional_ref.cpp. Build and run instructions in ./examples/README.md.

How to Build

Compiler Support

This is a modern C++ project which can be compiled with the latest C++ standards (C++20 or later).

Default build: C++23. Please check etc/${compiler}-flags.cmake.

Dependencies

This project is mainly tested on Ubuntu 22.04 and Ubuntu 24.04, but it should be as portable as CMake is. This project has no C or C++ dependencies.

Build-time dependencies:

• cmake
• ninja, make, or another CMake-supported build system
  • CMake defaults to "Unix Makefiles" on POSIX systems

Example of installation on Ubuntu 24.04:

# Install tools:
apt-get install -y cmake make ninja-build

# Example of toolchains:
apt-get install                           \
  g++-14 gcc-14 gcc-13 g++-14             \
  clang-18 clang++-18 clang-17 clang++-17

Instructions

Full set of supported toolchains can be found in .github/workflows/ci.yml.

Preset CMake Flows

This project strives to be as normal and simple a CMake project as possible. This build workflow in particular will work, producing a static beman_optional26 library, ready to package:

# List available preset configurations:
$ cmake --workflow --list-presets
Available workflow presets:

  "system"
  "gcc-14"
  "gcc-13"
  "clang-18"
  "clang-17"

# Run examples:
$ cmake --workflow --preset gcc-14
cmake --workflow --preset gcc-14
Executing workflow step 1 of 3: configure preset "gcc-14"
...
-- Build files have been written to: /path/to/repo/.build/gcc-14

Executing workflow step 2 of 3: build preset "gcc-14"

ninja: no work to do.

Executing workflow step 3 of 3: test preset "gcc-14"

Test project /path/to/repo/.build/gcc-14
        Start   1: OptionalTest.TestGTest
  1/... Test   #1: OptionalTest.TestGTest ...........................   Passed    0.00 sec
...
        Start   x: RangeSupportTest.RangeConcepts
.../... Test   #x: RangeSupportTest.RangeConcepts ...................   Passed    0.00 sec
        Start x+1: RangeSupportTest.IteratorConcepts
.../... Test #x+1: RangeSupportTest.IteratorConcepts ................   Passed    0.00 sec
...

100% tests passed, 0 tests failed out of ...

Total Test time (real) =   0.09 sec

This should build and run the tests with GCC 14 with the address and undefined behavior sanitizers enabled.

Custom CMake Flows

Build and Run Tests

CI current build and test flows:

# Configure build: default build production code + tests (OPTIONAL26_ENABLE_TESTING=ON by default).
$ cmake -G "Ninja Multi-Config" \
      -DCMAKE_CONFIGURATION_TYPES="RelWithDebInfo;Asan" \
      -DCMAKE_TOOLCHAIN_FILE=etc/clang-19-toolchain.cmake \
      -B .build -S .
-- The CXX compiler identification is Clang 19.0.0
...
-- Build files have been written to: /path/to/optional26/.build

# Build.
$ cmake --build .build --config Asan --target all -- -k 0
...
[30/30] Linking CXX executable ... # Note: 30 targets here (including tests).

# Run tests.
$ ctest --build-config Asan --output-on-failure --test-dir .build
Internal ctest changing into directory: /path/to/optional26/.build
Test project /path/to/optional26/.build
...
100% tests passed, 0 tests failed out of 82

Total Test time (real) =   0.67 sec

Build Production, but Skip Tests

By default, we build and run tests. You can provide -DOPTIONAL26_ENABLE_TESTING=OFF and completely disable building tests:

# Configure build: build production code, skip tests (OPTIONAL26_ENABLE_TESTING=OFF).
$ cmake -G "Ninja Multi-Config" \
      -DCMAKE_CONFIGURATION_TYPES="RelWithDebInfo;Asan" \
      -DCMAKE_TOOLCHAIN_FILE=etc/clang-19-toolchain.cmake \
      -DOPTIONAL26_ENABLE_TESTING=OFF \
      -B .build -S .
-- The CXX compiler identification is Clang 19.0.0
...
-- Build files have been written to: /path/to/optional26/.build

# Build.
$ cmake --build .build --config Asan --target all -- -k 0
...
[15/15] Linking CXX executable ... # Note: 15 targets here (tests were not built).

# Check that tests are not built/installed.
$ ctest --build-config Asan --output-on-failure --test-dir .build
Internal ctest changing into directory: /path/to/beman.optional26/.build
Test project /path/to/beman.optional26/.build
No tests were found!!!

Pre-Commit for Linting

Various linting tools are configured and installed via the pre-commit framework. This requires a working python environment, but also allows the tools, such as clang-format and cmake-lint, to be versioned on a per project basis rather than being installed globally. Version changes in lint checks often means differences in success or failure between the versions in CI and the versions used by a developer. By using the same configurations, this problem is avoided.

In order to set up a python environment, using a python virtual environment can simplify maintaining different configurations between projects. There is no particular dependency on a particular python3 version.

Creating and configuring a venv

python3 -m venv .venv
. .venv/bin/activate && python3 -m pip install --upgrade pip setuptools wheel
. .venv/bin/activate && python3 -m pip install pip-tools
. .venv/bin/activate && python3 -m piptools sync requirements.txt
. .venv/bin/activate && python3 -m piptools sync requirements-dev.txt
. .venv/bin/activate && exec bash

This will create the venv, install the python and python development tools, and run bash with the PATH and other environment variables set to use the venv preferentially.

Running the linting tools

pre-commit run -a

This will download and configure the lint tools specified in .pre-commit-config.yaml.

There is also a Makefile that will automate this process and keep everything up to date.

make lint

Papers

Latest revision(s) of the papers can be built / found at:

• give-std-optional-range-support for Give *std::optional* Range Support (P3168)
  • issue: #1831
  • LEWG:
    • Reviewed in Tokyo 2024.
    • Forwarded by LEWG April electronic poll to LWG.
  • LWG:
    • Reviewed and approved in Saint Louis 2024.
• ./papers/P2988/README.md for std::optional<T&> (P2988).
  • issue: #1661
  • LEWG:
    • Reviewed in Tokyo 2024.