These are the predefined policies built into Outcome:

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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. The compiler literally compiles no code for an invalid observation – the CPU “runs off” into the unknown.

As bad as this may sound, it generates the most optimal code, and such hard UB is very tool-friendly for detection e.g. undefined behaviour sanitiser, valgrind memcheck, etc.

If you are considering choosing this policy, definitely read static void _ub(Impl &&) first.

Note that unchecked<T, E = varies> aliases a basic_result with the all_narrow no-value policy.

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terminate

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

Note that configuring EC = void or EP = void causes default_policy to choose terminate as the no-value policy.

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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.

Note that if is_error_code_available<T> is true for EC, and (if basic_outcome) is_exception_ptr_available<T> is true for EP, default_policy chooses error_code_throw_as_system_error<T, EC, EP> as the no-value policy.

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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.

Note that if is_exception_ptr_available<T> is true for EC, or (if basic_outcome) is_exception_ptr_available<T> is true for EP, default_policy chooses exception_ptr_rethrow<T, EC, EP> as the no-value policy.

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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.

Note that checked<T, E = varies> aliases a basic_result with the throw_bad_result_access<EC> no-value policy.