Outcome 2.2 library

At the end of December 2021, Standalone Outcome went guaranteed future ABI stable. From v2.2.3 onwards, you get ABI compatibilty guarantees across Outcome releases.

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, Executor = void>/atomic_eager<T, Executor = void> , lazy<T, Executor = void>/atomic_lazy<T, Executor = void> and generator<T, Executor = void> 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
  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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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]