+++
title = "Hook result"
description = ""
weight = 30
+++

We now tell Outcome that for every instance of our localised `result<T>`, that
on failure construction only, we want custom code to be run which increments the current
slot in TLS storage and writes the current stack backtrace into it.

{{% snippet "error_code_extended.cpp" "error_code_extended3" %}}

The only non-obvious part above is the call to {{< api "void set_spare_storage(basic_result|basic_outcome *, uint16_t) noexcept" >}}.

Both `result` and `outcome` keep their internal state metadata in a `uint32_t`,
half of which is not used by Outcome. As it can be very useful to keep a small
unique number attached to any particular `result` or `outcome` instance, we
permit user code to set those sixteen bits to anything they feel like.
The corresponding function to retrieve those sixteen bits is {{< api "uint16_t spare_storage(const basic_result|basic_outcome *) noexcept" >}}.

The state of the sixteen bits of spare storage are ignored during comparison operations.

The sixteen bits of spare storage propagate during the following operations:

1. Copy and move construction between `result`'s.
2. Copy and move construction between `outcome`'s.
3. Copy and move construction from a `result` to an `outcome`.
4. Converting copy and move constructions for all the above.
5. Assignment for all of the above.

They are NOT propagated in these operations:

1. Any conversion or translation which goes through a `failure_type` or `success_type`.
2. Any conversion or translation which goes through a `ValueOrError` concept match.
3. Any unpacking or repacking of value/error/exception e.g. a manual repack of an
`outcome` into a `result`.
