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<h1 class="title">Trait Objects</h1>
<p><small>There is a new edition of the book and this is an old link.</small></p>
<blockquote>
<p>Trait objects combine the data made up of the pointer to a concrete object with the behavior of the methods defined in the trait.
A trait defines behavior that we need in a given situation.
We can then use a trait as a trait object in places where we would use a concrete type or a generic type.</p>
</blockquote>
<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><pre class="rust rust-example-rendered ignore">
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">InputBox</span> {
<span class="kw">pub</span> <span class="ident">label</span>: <span class="ident">String</span>,
}
<span class="kw">impl</span> <span class="ident">Draw</span> <span class="kw">for</span> <span class="ident">InputBox</span> {
<span class="kw">fn</span> <span class="ident">draw</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="comment">// Code to actually draw an input box</span>
}
}
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Button</span> {
<span class="kw">pub</span> <span class="ident">label</span>: <span class="ident">String</span>,
}
<span class="kw">impl</span> <span class="ident">Draw</span> <span class="kw">for</span> <span class="ident">Button</span> {
<span class="kw">fn</span> <span class="ident">draw</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="comment">// Code to actually draw a button</span>
}
}
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Screen</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Draw</span><span class="op">></span> {
<span class="kw">pub</span> <span class="ident">components</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Screen</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Draw</span> {
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">run</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="kw">for</span> <span class="ident">component</span> <span class="kw">in</span> <span class="self">self</span>.<span class="ident">components</span>.<span class="ident">iter</span>() {
<span class="ident">component</span>.<span class="ident">draw</span>();
}
}
}
<span class="kw">fn</span> <span class="ident">main</span>() {
<span class="kw">let</span> <span class="ident">screen</span> <span class="op">=</span> <span class="ident">Screen</span> {
<span class="ident">components</span>: <span class="macro">vec</span><span class="macro">!</span>[
<span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">InputBox</span> {
<span class="ident">label</span>: <span class="ident">String</span>::<span class="ident">from</span>(<span class="string">"OK"</span>),
}),
<span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">Button</span> {
<span class="ident">label</span>: <span class="ident">String</span>::<span class="ident">from</span>(<span class="string">"OK"</span>),
}),
],
};
<span class="ident">screen</span>.<span class="ident">run</span>();
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Apub%20struct%20InputBox%20%7B%0A%20%20%20%20pub%20label%3A%20String%2C%0A%7D%0A%0Aimpl%20Draw%20for%20InputBox%20%7B%0A%20%20%20%20fn%20draw(%26self)%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20Code%20to%20actually%20draw%20an%20input%20box%0A%20%20%20%20%7D%0A%7D%0A%0Apub%20struct%20Button%20%7B%0A%20%20%20%20pub%20label%3A%20String%2C%0A%7D%0A%0Aimpl%20Draw%20for%20Button%20%7B%0A%20%20%20%20fn%20draw(%26self)%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20Code%20to%20actually%20draw%20a%20button%0A%20%20%20%20%7D%0A%7D%0A%0Apub%20struct%20Screen%3CT%3A%20Draw%3E%20%7B%0A%20%20%20%20pub%20components%3A%20Vec%3CT%3E%2C%0A%7D%0A%0Aimpl%3CT%3E%20Screen%3CT%3E%0A%20%20%20%20where%20T%3A%20Draw%20%7B%0A%20%20%20%20pub%20fn%20run(%26self)%20%7B%0A%20%20%20%20%20%20%20%20for%20component%20in%20self.components.iter()%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20component.draw()%3B%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D%0A%7D%0A%0Afn%20main()%20%7B%0A%20%20%20%20let%20screen%20%3D%20Screen%20%7B%0A%20%20%20%20%20%20%20%20components%3A%20vec!%5B%0A%20%20%20%20%20%20%20%20%20%20%20%20Box%3A%3Anew(InputBox%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20label%3A%20String%3A%3Afrom(%22OK%22)%2C%0A%20%20%20%20%20%20%20%20%20%20%20%20%7D)%2C%0A%20%20%20%20%20%20%20%20%20%20%20%20Box%3A%3Anew(Button%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20label%3A%20String%3A%3Afrom(%22OK%22)%2C%0A%20%20%20%20%20%20%20%20%20%20%20%20%7D)%2C%0A%20%20%20%20%20%20%20%20%5D%2C%0A%20%20%20%20%7D%3B%0A%0A%20%20%20%20screen.run()%3B%0A%7D">Run</a></pre>
<hr />
<p>Here are the relevant sections in the new and old books:</p>
<ul>
<li><strong><a href="second-edition/ch17-02-trait-objects.html">In the second edition: Ch 17.02 — Trait Objects</a></strong></li>
<li><small><a href="first-edition/trait-objects.html">In the first edition: Ch 3.22 — Trait Objects</a></small></li>
</ul>
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