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<p class='location'>Trait FnBox</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#associated-types">Associated Types</a><div class="sidebar-links"><a href="#associatedtype.Output">Output</a></div><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.call_box">call_box</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>boxed</a></p><script>window.sidebarCurrent = {name: 'FnBox', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></script></div>
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<h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>boxed</a>::<wbr><a class="trait" href=''>FnBox</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version '></span><span id='render-detail'>
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</span><a class='srclink' href='../../src/alloc/boxed.rs.html#774-778' title='goto source code'>[src]</a></span></h1>
<pre class='rust trait'>pub trait FnBox<A> {
type <a href='#associatedtype.Output' class="type">Output</a>;
fn <a href='#tymethod.call_box' class='fnname'>call_box</a>(self: <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><Self>, args: A) -> Self::<a class="type" href="../../std/boxed/trait.FnBox.html#associatedtype.Output" title="type std::boxed::FnBox::Output">Output</a>;
}</pre><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>fnbox </code><a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div><div class='docblock'><p><code>FnBox</code> is a version of the <code>FnOnce</code> intended for use with boxed
closure objects. The idea is that where one would normally store a
<code>Box<FnOnce()></code> in a data structure, you should use
<code>Box<FnBox()></code>. The two traits behave essentially the same, except
that a <code>FnBox</code> closure can only be called if it is boxed. (Note
that <code>FnBox</code> may be deprecated in the future if <code>Box<FnOnce()></code>
closures become directly usable.)</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Here is a snippet of code which creates a hashmap full of boxed
once closures and then removes them one by one, calling each
closure as it is removed. Note that the type of the closures
stored in the map is <code>Box<FnBox() -> i32></code> and not <code>Box<FnOnce() -> i32></code>.</p>
<pre class="rust rust-example-rendered">
<span class="attribute">#![<span class="ident">feature</span>(<span class="ident">fnbox</span>)]</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">boxed</span>::<span class="ident">FnBox</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">collections</span>::<span class="ident">HashMap</span>;
<span class="kw">fn</span> <span class="ident">make_map</span>() <span class="op">-></span> <span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">Box</span><span class="op"><</span><span class="ident">FnBox</span>() <span class="op">-></span> <span class="ident">i32</span><span class="op">>></span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">map</span>: <span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">Box</span><span class="op"><</span><span class="ident">FnBox</span>() <span class="op">-></span> <span class="ident">i32</span><span class="op">>></span> <span class="op">=</span> <span class="ident">HashMap</span>::<span class="ident">new</span>();
<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">1</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="op">||</span> <span class="number">22</span>));
<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">2</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="op">||</span> <span class="number">44</span>));
<span class="ident">map</span>
}
<span class="kw">fn</span> <span class="ident">main</span>() {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">map</span> <span class="op">=</span> <span class="ident">make_map</span>();
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="kw-2">&</span>[<span class="number">1</span>, <span class="number">2</span>] {
<span class="kw">let</span> <span class="ident">f</span> <span class="op">=</span> <span class="ident">map</span>.<span class="ident">remove</span>(<span class="kw-2">&</span><span class="ident">i</span>).<span class="ident">unwrap</span>();
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">f</span>(), <span class="ident">i</span> <span class="op">*</span> <span class="number">22</span>);
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Bfeature(fnbox)%5D%0A%0Ause%20std%3A%3Aboxed%3A%3AFnBox%3B%0Ause%20std%3A%3Acollections%3A%3AHashMap%3B%0A%0Afn%20make_map()%20-%3E%20HashMap%3Ci32%2C%20Box%3CFnBox()%20-%3E%20i32%3E%3E%20%7B%0A%20%20%20%20let%20mut%20map%3A%20HashMap%3Ci32%2C%20Box%3CFnBox()%20-%3E%20i32%3E%3E%20%3D%20HashMap%3A%3Anew()%3B%0A%20%20%20%20map.insert(1%2C%20Box%3A%3Anew(%7C%7C%2022))%3B%0A%20%20%20%20map.insert(2%2C%20Box%3A%3Anew(%7C%7C%2044))%3B%0A%20%20%20%20map%0A%7D%0A%0Afn%20main()%20%7B%0A%20%20%20%20let%20mut%20map%20%3D%20make_map()%3B%0A%20%20%20%20for%20i%20in%20%26%5B1%2C%202%5D%20%7B%0A%20%20%20%20%20%20%20%20let%20f%20%3D%20map.remove(%26i).unwrap()%3B%0A%20%20%20%20%20%20%20%20assert_eq!(f()%2C%20i%20*%2022)%3B%0A%20%20%20%20%7D%0A%7D%0A&version=nightly">Run</a></pre>
</div>
<h2 id='associated-types' class='small-section-header'>
Associated Types<a href='#associated-types' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='associatedtype.Output' class='method'><span id='Output.t' class='invisible'><code>type <a href='#associatedtype.Output' class="type">Output</a></code></span></h3><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>fnbox </code><a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div></div>
<h2 id='required-methods' class='small-section-header'>
Required Methods<a href='#required-methods' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='tymethod.call_box' class='method'><span id='call_box.v' class='invisible'><code>fn <a href='#tymethod.call_box' class='fnname'>call_box</a>(self: <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><Self>, args: A) -> Self::<a class="type" href="../../std/boxed/trait.FnBox.html#associatedtype.Output" title="type std::boxed::FnBox::Output">Output</a></code></span></h3><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>fnbox </code><a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div></div>
<h2 id='implementors' class='small-section-header'>
Implementors<a href='#implementors' class='anchor'></a>
</h2>
<ul class='item-list' id='implementors-list'>
<li><div class='out-of-band'><a class='srclink' href='../../src/alloc/boxed.rs.html#782-790' title='goto source code'>[src]</a></div><code>impl<A, F> FnBox<A> for F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>, </span><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = <F as <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>>::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;</span></code></li>
</ul><script type="text/javascript" async
src="../../implementors/alloc/boxed/trait.FnBox.js">
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