These are the predefined policies built into Outcome:

all_narrow

If there is an observation of a value/error/exception which is not present, the program is put into a hard undefined behaviour situation. However this is a tool-friendly UB using intrinsics such as __builtin_unreachable() that allows tools to make use of it, e.g., better bug detection or optimizations.

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terminate

Observation of a missing value/error/exception causes a call to std::terminate().

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error_code_throw_as_system_error<T, EC, EP>

This policy assumes that EC has the interface of std::error_code, and EP has the interface of std::exception_ptr. Upon missing value observation:

• if an exception is stored through pointer of type EP it is rethrown;
• otherwise, if an error of type EC is stored, it is converted to error_code and then thrown as system_error.

Upon missing error observation throws:

• bad_result_access("no error") from instances of basic_result<>.
• bad_outcome_access("no error") from instances of basic_outcome<>.

Upon missing exception observation throws bad_outcome_access("no exception").

Overloads are provided for boost::system::error_code and boost::exception_ptr.

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exception_ptr_rethrow<T, EC, EP>

This policy assumes that either EC or EP (unless void) has the interface of std::exception_ptr. Upon missing value observation:

• in instances of basic_result<>, rethrows exception pointed to by EC;
• in instances of basic_outcome<>, if exception EP is present rethrows it, otherwise rethrows EC.

Upon missing error observation:

• in instances of basic_result<>, throws bad_result_access("no error") ;
• in instances of basic_outcome<>, throws bad_outcome_access("no error").

Upon missing exception observation throws bad_outcome_access("no exception").

Overloads are provided for boost::exception_ptr.

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throw_bad_result_access<EC>

Upon missing value observation throws bad_result_access_with<EC>(ec), where ec is the value of the stored error. If error is not stored, the behaviour is undefined.

Upon missing error observation throws bad_result_access("no error").

This policy can be used with basic_outcome<> instances, where it always throws bad_outcome_access for all no-value/error/exception observations.