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Outcome 2.2 library

Niall Douglas

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at

Table of Contents

Build and install
Error codes
Plugging a library into std::error_code
Plugging a library into boost::system::error_code
Narrow contracts
ASIO/Networking TS : Boost < 1.70
ASIO/Networking TS: Boost >= 1.70
The main advantages
Approximate map between error code designs
Major differences
status_result and status_outcome
Worked example: Custom domain
Tying it all together
Using Outcome from C code
API reference
Frequently asked questions
Upgrade guide v2.1 => v2.2
v2.2 major changes



At the end of December 2020, Outcome v2.2 replaced v2.1 in develop branch. This is a breaking change and all Outcome v2.1 code will need to be upgraded using the v2.1 => v2.2 upgrade guide. See also the list of v2.2 major changes.

Outcome is a set of tools for reporting and handling function failures in contexts where directly using C++ exception handling is unsuitable. Such contexts include:

Outcome addresses failure handling through returning a special type from functions, which is able to store either a successfully computed value (or void), or the information about failure. Outcome also comes with a set of idioms for dealing with such types.

Particular care has been taken to ensure that Outcome has the lowest possible impact on build times, thus making it suitable for use in the global headers of really large codebases. Storage layout is guaranteed and is C-compatible for result<T, E>1, thus making Outcome based code long term ABI-stable.

Fully deterministic all-noexcept C++ Coroutine support in Outcome is particularly strong, and we supply Outcome-optimising eager<T>/atomic_eager<T> and lazy<T>/atomic_lazy<T> awaitables which work for any user type.

Sample usage

The main workhorse in the Outcome library is result<T>: it represents either a successfully computed value of type T, or a std::error_code/boost::system::error_code2 representing the reason for failure. You use it in the function’s return type:

outcome::result<string> data_from_file(string_view path) noexcept;
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It is possible to inspect the state manually:

if (outcome::result<string> rslt = data_from_file("config.cfg"))
  use_string(rslt.value());                   // returns string
  throw LibError{rslt.error(), "config.cfg"}; // returns error_code
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Or, if this function is called in another function that also returns result<T>, you can use a dedicated control statement:

outcome::result<int> process(const string& content) noexcept;

outcome::result<int> int_from_file(string_view path) noexcept
  BOOST_OUTCOME_TRY(auto str, data_from_file(path));
  // if control gets here data_from_file() has succeeded
  return process(str);  // decltype(str) == string
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BOOST_OUTCOME_TRY is a control statement. If the returned result<T> object contains an error information, the enclosing function is immediately returned with result<U> containing the same failure information; otherwise an automatic object of type T is available in scope.


This library joined the Boost C++ libraries in the 1.70 release (Spring 2019). It can be grafted into much older Boost releases if desired.

  1. If you choose a C-compatible T and E type. [return]
  2. result<T> defaults to std::error_code for Standalone Outcome, and to boost::system::error_code for Boost.Outcome. You can mandate a choice using std_result<T> or boost_result<T>. [return]

Last revised: February 12, 2021 at 20:25:43 UTC

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