Function replace_with::replace_with_or_default
source · Expand description
Temporarily takes ownership of a value at a mutable location, and replace it with a new value
based on the old one. Replaces with Default::default()
on panic.
We move out of the reference temporarily, to apply a closure f
, returning a new value, which
is then placed at the original value’s location.
An important note
On panic (or to be more precise, unwinding) of the closure f
, T::default()
will be called to
provide a replacement value. T::default()
should not panic – doing so will constitute a double
panic and will most likely abort the process.
Equivalent to replace_with(dest, T::default, f)
.
Differs from *dest = mem::replace(dest, Default::default())
in that Default::default()
will
only be called on panic.
Example
enum States {
A(String),
B(String),
}
impl Default for States {
fn default() -> Self {
States::A(String::new())
}
}
impl States {
fn poll(&mut self) {
replace_with_or_default(self, |self_| match self_ {
States::A(a) => States::B(a),
States::B(a) => States::A(a),
});
}
}