[#error_handling]

Errors can occur during `initialize`, or during method dispatch, if the method's
registry contains the `runtime_checks` policy. If the registry contains an
`error_handler` policy, its `error_handler::error` member function is called
with an error object, before terminating the program with a call to `abort`.
`default_registry` contains such a policy: `default_error_handler`. It wraps the
error object in a variant, and calls a handler via a `std::function`,
initialized to a function that prints a short description of the error to
`stderr`. The function can be changed, for example, to throw an exception:

[source,c++]
----
include::example$default_error_handler.cpp[tag=example]
----

Output:

[source,console]
----
spin
not implemented
spin
----

We can also replace the `error_handler` policy with our own.
For example:


[source,c++]
----
include::example$throw_error_handler.cpp[tag=example]
----

[source,console]
----
spin
not implemented
spin
----

Stock policy `throw_error_handler` does this for all the error types:

```c++
namespace boost::openmethod::policies {

struct throw_error_handler : error_handler {
    template<class Error>
    [[noreturn]] static auto error(const Error& error) -> void {
        throw error;
    }
};

} // namespace boost::openmethod::policies
```