# `BOOST_OUTCOME_TRY(var, expr)`

Evaluate an expression which results in a type matching the following customisation points, assigning `T` to a decl called `var` if successful, immediately returning `try_operation_return_as(X)` from the calling function if unsuccessful:

- `BOOST_OUTCOME_V2_NAMESPACE::``try_operation_has_value(X)`
- `BOOST_OUTCOME_V2_NAMESPACE::``try_operation_return_as(X)`
- `BOOST_OUTCOME_V2_NAMESPACE::``try_operation_extract_value(X)`

Default overloads for these customisation points are provided. See the recipe for supporting foreign input to `BOOST_OUTCOME_TRY`.

Hints are given to the compiler that the expression will be successful. If you expect failure, you should use `BOOST_OUTCOME_TRY_FAILURE_LIKELY(var, expr)` instead.

An internal temporary to hold the value of the expression is created, which generally invokes a copy/move. If you wish to never copy/move, you can tell this macro to create the internal temporary as a reference instead.

Overridable: Not overridable.

Definition: See `BOOST_OUTCOME_TRYV(expr)` for most of the mechanics.

If successful, `var` is initialised or assigned to the expression result’s `.assume_value()` if available, else to its `.value()`. This binds a reference possibly to the `T` stored inside the bound result of the expression, but possibly also to a temporary emitted from the value observer function.

Header: `<boost/outcome/try.hpp>`

Legacy: Before Outcome v2.2, `var` was always declared as an automatic rvalue ref. You can use the backwards compatibility macro `OUTCOME21_TRY()` if wish to retain the old behaviour.

Last revised: February 12, 2021 at 20:25:43 UTC