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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">2.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../idioms/deref.html"><strong aria-hidden="true">2.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../idioms/dtor-finally.html" class="active"><strong aria-hidden="true">2.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../idioms/mem-replace.html"><strong aria-hidden="true">2.7.</strong> mem::{take(), replace()}</a></li><li class="chapter-item expanded "><a href="../idioms/on-stack-dyn-dispatch.html"><strong aria-hidden="true">2.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/intro.html"><strong aria-hidden="true">2.9.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/ffi/errors.html"><strong aria-hidden="true">2.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/accepting-strings.html"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/passing-strings.html"><strong aria-hidden="true">2.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">2.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">2.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html"><strong aria-hidden="true">2.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">2.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">2.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/strategy.html"><strong aria-hidden="true">3.1.5.</strong> Strategy</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/visitor.html"><strong aria-hidden="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li><li class="chapter-item expanded "><a href="../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li></ol></li><li class="chapter-item expanded "><a href="../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<h1 id="finalisation-in-destructors"><a class="header" href="#finalisation-in-destructors">Finalisation in destructors</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>Rust does not provide the equivalent to <code>finally</code> blocks - code that will be
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executed no matter how a function is exited. Instead an object's destructor can
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be used to run code that must be run before exit.</p>
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<h2 id="example"><a class="header" href="#example">Example</a></h2>
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<pre><code class="language-rust ignore">fn bar() -> Result<(), ()> {
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// These don't need to be defined inside the function.
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struct Foo;
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// Implement a destructor for Foo.
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impl Drop for Foo {
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fn drop(&mut self) {
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println!("exit");
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}
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}
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// The dtor of _exit will run however the function `bar` is exited.
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let _exit = Foo;
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// Implicit return with `?` operator.
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baz()?;
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// Normal return.
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Ok(())
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}
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</code></pre>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>If a function has multiple return points, then executing code on exit becomes
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difficult and repetitive (and thus bug-prone). This is especially the case where
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return is implicit due to a macro. A common case is the <code>?</code> operator which
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returns if the result is an <code>Err</code>, but continues if it is <code>Ok</code>. <code>?</code> is used as
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an exception handling mechanism, but unlike Java (which has <code>finally</code>), there is
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no way to schedule code to run in both the normal and exceptional cases.
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Panicking will also exit a function early.</p>
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<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
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<p>Code in destructors will (nearly) always be run - copes with panics, early
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returns, etc.</p>
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<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
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<p>It is not guaranteed that destructors will run. For example, if there is an
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infinite loop in a function or if running a function crashes before exit.
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Destructors are also not run in the case of a panic in an already panicking
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thread. Therefore destructors cannot be relied on as finalisers where it is
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absolutely essential that finalisation happens.</p>
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<p>This pattern introduces some hard to notice, implicit code. Reading a function
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gives no clear indication of destructors to be run on exit. This can make
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debugging tricky.</p>
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<p>Requiring an object and <code>Drop</code> impl just for finalisation is heavy on boilerplate.</p>
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<h2 id="discussion"><a class="header" href="#discussion">Discussion</a></h2>
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<p>There is some subtlety about how exactly to store the object used as a
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finaliser. It must be kept alive until the end of the function and must then be
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destroyed. The object must always be a value or uniquely owned pointer (e.g.,
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<code>Box<Foo></code>). If a shared pointer (such as <code>Rc</code>) is used, then the finaliser can
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be kept alive beyond the lifetime of the function. For similar reasons, the
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finaliser should not be moved or returned.</p>
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<p>The finaliser must be assigned into a variable, otherwise it will be destroyed
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immediately, rather than when it goes out of scope. The variable name must start
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with <code>_</code> if the variable is only used as a finaliser, otherwise the compiler
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will warn that the finaliser is never used. However, do not call the variable
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<code>_</code> with no suffix - in that case it will be destroyed immediately.</p>
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<p>In Rust, destructors are run when an object goes out of scope. This happens
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whether we reach the end of block, there is an early return, or the program
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panics. When panicking, Rust unwinds the stack running destructors for each
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object in each stack frame. So, destructors get called even if the panic happens
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in a function being called.</p>
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<p>If a destructor panics while unwinding, there is no good action to take, so Rust
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aborts the thread immediately, without running further destructors. This means
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that destructors are not absolutely guaranteed to run. It also means that you
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must take extra care in your destructors not to panic, since it could leave
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resources in an unexpected state.</p>
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<h2 id="see-also"><a class="header" href="#see-also">See also</a></h2>
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<p><a href="../patterns/behavioural/RAII.html">RAII guards</a>.</p>
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