You now know everything you need to get started with using Outcome immediately.

The initial temptation for most beginners will be to use a bespoke strongly typed enumeration on a case by case basis i.e. a “once off” custom E type. This is usually due to experience in other languages with sum types e.g. Rust, Haskell, Swift etc.

However this is C++! Not Rust, not Swift, not Haskell! I must caution you to always avoid using custom E types in public APIs. The reason is that every time library A using custom E1 type must interface with library B using custom E2 type, you must map between those E1 and E2 types.

This is information lossy, i.e. fidelity of failure gets lost after multiple translations. It involves writing, and then maintaining, a lot of annoying boilerplate. It leaks internal implementation detail, and fails to separate concerns. And one cannot use BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr) if there is no convertibility between E types.

The C++ 11 standard library, and Boost, specifically ships <system_error> for the purpose of wrapping up individual custom E types into a generic framework, where disparate custom E types can discover and interact with one another. That ships with every C++ compiler.

For all these reasons, this is why result and outcome default the EC type to error code. You should leave that default alone where possible.

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tl;dr;

Please plug your library into std::error_code, or equivalent, and do not expose custom E types in ANY public API. result and outcome default EC to an error code for good reason.