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61 lines
2.2 KiB
Rust
61 lines
2.2 KiB
Rust
// Sometimes, we have structs which hold on to data temporarily. A use-case of
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// this could be a routing component which accepts a connection and returns it to
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// another recipient. To avoid copying the data, we just accept a reference with
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// lifetime and return this reference later.
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//
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// In the example below, we create a `Router` instance in a limited scope. It
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// accepts a connection reference created in the enclosing scope and returns it.
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// In theory, this should be possible given that the connection reference outlives
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// the scope from which it is returned. However, the borrow checker does not
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// seem to understand it. What can we do about that?
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struct Router<'a> {
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connection: Option<&'a u64>,
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// ^^ lifetime of the connection reference
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// which may outlive the `Router` itself
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}
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impl<'a> Router<'a> {
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fn new() -> Self {
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Self { connection: None }
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}
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fn accept_connection(&mut self, connection: &'a u64) {
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self.connection = Some(connection);
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}
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fn return_connection(&mut self) -> Option<&'a u64> {
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// added lifetime annotation ^^
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//
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// Without annotation, the compiler infers the output reference
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// to have the lifetime of the only input reference
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// -> the lifetime of `&mut self`.
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self.connection.take()
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}
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}
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fn main() {
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let connection = &123;
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let returned_connection = {
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// Create router within scope
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let mut router = Router::new();
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// Accept connection which lives longer than the router
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router.accept_connection(connection);
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// Return connection which **should** live longer than the router
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router.return_connection()
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// ^^^^^^^^^^^^^^^^^^^^
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//
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// Without the explicit lifetime annotation in `return_connection`,
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// the reference from `return_connection` has the lifetime of `router`.
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// We are returning the reference from the scope, requiring it to outlive it,
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// so the compiler complains about `router` not living long enough.
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};
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if let Some(connection) = returned_connection {
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println!("The connection is {connection}");
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}
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}
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