A lightweight C++11-compatible error-handling mechanism


A Modern C++ Result Type

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Result is a modern, simple, and light-weight error-handling alternative to exceptions with a rich feature-set.


✔️ Offers a coherent, light-weight alternative to exceptions
✔️ Compatible with C++11 (with more features in C++14 and C++17)
✔️ Single-header, header-only solution -- easily drops into any project
✔️ Zero overhead abstractions -- don't pay for what you don't use.
✔️ No dependencies
✔️ Support for value-type, reference-type, and void-type values in result
✔️ Monadic composition functions like map, flat_map, and map_error for easy functional use
✔️ Optional support to disable all exceptions and rename the cpp namespace
✔️ Comprehensively unit tested for both static behavior and runtime validation
✔️ Incurs minimal cost when optimized, especially for trivial types

Check out the tutorial to see what other features Result offers.

If you're interested in how cpp::result deviates from std::expected proposals, please see this page.


enum class narrow_error{ none, loss_of_data };

template <typename To, typename From>
auto try_narrow(const From& from) noexcept -> cpp::result
   auto to = 

    if (
     (to) != from) {

     return to;

     struct {
     template <
     typename T>
     const T& x) -> std::string {
} to_string;

     main() -> int {
      map(to_string) == 

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Quick References

Why result?

Error cases in C++ are often difficult to discern from the API. Any function not marked noexcept can be assumed to throw an exception, but the exact type of exception, and if it even derives from std::exception, is ambiguous. Nothing in the language forces which exceptions may propagate from an API, which can make dealing with such APIs complicated.

Often it is more desirable to achieve noexcept functions where possible, since this allows for better optimizations in containers (e.g. optimal moves/swaps) and less cognitive load on consumers.

Having a result type on your API not only semantically encodes that a function is able to fail, it also indicates to the caller how the function may fail, and what discrete, testable conditions may cause it to fail -- which is what this library intends to solve.

As a simple example, compare these two identical functions:

// (1)
auto to_uint32(const std::string& x) -> std::uint32_t;

// (2)
enum class parse_error { overflow=1, underflow=2, bad_input=3};
auto to_uint32(const std::string& x) noexcept -> result

In (1), it is ambiguous what (if anything) this function may throw on failure, or how this error case may be accounted for.

In (2), on the other hand, it is explicit that to_uint32 cannot throw -- so there is no need for a catch handler. It's also clear that it may fail for whatever reasons are in parse_error, which discretely enumerates any possible case for failure.

Compiler Compatibility

Result is compatible with any compiler capable of compiling valid C++11. Specifically, this has been tested and is known to work with:

  • GCC 5, 6, 7, 8, 9, 10
  • Clang 3.5, 3.6, 3.7, 3.8, 3.9, 4, 5, 6, 7, 8, 9, 10, 11
  • Apple Clang (Xcode) 10.3, 11.2, 11.3, 12.3
  • Visual Studio 2017[1], 2019

Latest patch level releases are assumed in the versions listed above.

Note: Visual Studios 2015 is not currently supported due to an internal compiler error experienced in the default constructor of result. Support for this will be added at a later time.

[1] Visual Studios 2017 is officially supported, though toolchain 14.16 has some issues properly compiling map_error due to insufficient support for SFINAE.


Result is licensed under the MIT License:

Copyright © 2017-2021 Matthew Rodusek

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.



  • P0323R9: std::expected proposal was used as an inspiration for the general template structure.
  • bit::stl: the original version that seeded this repository, based off an earlier proposal version.
  • Improve CMake usage

    Improve CMake usage


    • [x] I have read the CONTRIBUTING guidelines
    • [x] The coding style is consistent with the rest of the library
    • [x] My branch's history is clean and coherent. This could be done through at least one of the following practices:
      • Rebasing my branch off of the branch being merged to
      • Squashing commits to create a more cohesive history
    • [x] If relevant, I have included unit-tests (for new code/bugfixes)


    I realize that my commits don't live up to your standards and that's fine for me, there is only so much grunt work I'm willing to do and I haven't adapted the CI files yet.

    Some things to note about the changes:

    • The documentation falsely claimed that modern CMake starts with 3.4, however 3.4 hasn't added anything significant that would make it modern. Modern CMake starts with 3.8, as that is the version which added meta compile features to specify the C++ standard requirement as a lower bound. The (CMAKE_)?\<LANG>_STANDARD variables/properties define an upper bound, which may decay to earlier standards.
    • The root project CML and test project CML files now only deal with their respective requirements. The root project CML must be nothing more than (almost) pure build and usage requirements of the project. Things like hardcoded conan details, compiler flags and alike are not requirements of the project, but the developer developing the library, and as such there is a separate superbuild CML file (in the all directory) that wires things together in a way that things work.
    • I haven't actually found a way to cleanly represent subprojects' requirements from a root conanfile.py and I don't write conan recipes myself, so I have no clue how to have a root conanfile.py without the test subprojects' details poisoning it. For this reason, I just dropped a conanfile.txt for catch in the test directory and called it a day. This point is also similar to the above, as the conanfile.py describes your library, not your tests and the test_package tests whether your library generates the conan package as it should and can be used as it should, nothing about your unit tests, etc.

    The following commands show how to run things, but you can also look at https://github.com/erikzenker/hsm/blob/master/.github/workflows/windows-ci.yml (this uses conan for its Boost dependency) and https://github.com/friendlyanon/cxx-static-shared-example/blob/master/.github/workflows/ci.yml (this uses a superbuild CML to build things in CI) for real world examples.

    How to build and install the library with this PR:

    cmake -S . -B build
    # nothing to build, header only
    cmake --install build

    How to build the tests with this PR:

    cmake -S test -B build
    cmake --build build
    (cd build; ctest)

    How to do the above using the superbuild with this PR:

    conan install test -if build/deps
    cmake -S all -B build/all -D "CMAKE_MODULE_PATH=${PWD}/build/deps"
    cmake --build build/all
    cmake --install build/all
    (cd build/all/sub/test; ctest)

    How to do the above using the superbuild with this PR on Windows (or multi-config generators in general):

    conan install test -if build\deps
    cmake -S all -B build\all -D "CMAKE_MODULE_PATH=%cd:\=/%/build/deps"
    cmake --build build\all --config Release
    cmake --install build\all --config Release
    %comspec% /C "cd build\all\sub\test & ctest -C Release"

    Also, I found a bug with Catch2's conan package, because it does not provide the proper module path to enable include(Catch). You should report this to them.

    For rationale behind everything in the refactored CML files you can read around in my PR to https://github.com/Dobiasd/FunctionalPlus and my commit messages in https://github.com/friendlyanon/cxx-static-shared-example

    So you may ask, how to move forward with this PR? Like I said, the list of requirements for PRs is gigantic and there is only so much mundane programming I'm willing to do. This PR shows a project structure to strive for that doesn't look like the build tooling was bolted on as an afterthough with no care put into it.
    You may take the code in this PR as is, split commits up, make them adhere to your personal guidelines.

    opened by friendlyanon 2
  • Error compilling result.hpp on apple-clang.

    Error compilling result.hpp on apple-clang.

    Linux Compier: GCC 11 Mac Compiler: latest apple-clang

    The resulting code works on linux / gcc, but on mac I get:

    /result.hpp:3524:82: error: ISO C++ requires the name after '::~' to be found in the same scope as the name before '::~' [-Werror,-Wdtor-name]
    inline RESULT_INLINE_VISIBILITY RESULT_NS_IMPL::detail::result_union<T, E, false>::~result_union() noexcept(
    opened by tcanabrava 4
  • v1.0.0(Mar 12, 2021)

    This is the official release of the Result library!

    This provides a result-API similar to other modern languages like Rust and Swift

    The full API reference for this version can be found here: v1.0.0 API


    • result<T,E> for representing fallible types
    • result<T&,E> for fallible reference types
    • result<void,E> for fallible non-value types (e.g. void returns)
    • failure<T> for disambiguation for error types
    • failure<T&> for referential access of T types
    • Support for comparison operators of result and failure types
    • Support for std::hash for result and failure types
    • Support for monadic functions for result (map, flat_map, map_error, flat_map_error, value_or, and_then)
    • Full constexpr support for result and failure (more constexpr support in C++14 and above)
    • Support for CTAD for failure types when working in C++17


    This release is compatible with any C++11 compiler or later.

    In particular, it's known to be compatible with the following compilers:

    • GCC: 5, 6, 7, 8, 9, 10
    • Clang: 3.5, 3.6, 3.7, 3.8, 3.9, 4, 5, 6, 7, 8, 9, 10, 11
    • Apple Clang (Xcode) 10.3, 11.2, 11.3, 12.3
    • Visual Studio 2017[1], 2019

    [1] Visual Studios 2017 is officially supported, though toolchain 14.16 has some issues properly compiling map_error due to insufficient support for SFINAE.

    Source code(tar.gz)
    Source code(zip)
    result.hpp(207.21 KB)
  • v0.0.1+test7(Dec 10, 2020)

  • v0.0.1+test4(Nov 30, 2020)

Matthew Rodusek
C++ Software developer by day, C++ template metaprogrammer by night.
Matthew Rodusek
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