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<a href="." >Implementing Deserialize</a>
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<h1 id="implementing-deserialize"><a name="implementing-deserialize" class="plugin-anchor" href="#implementing-deserialize"><i class="fa fa-link" aria-hidden="true"></i></a>Implementing Deserialize</h1>
<p>The <a href="https://docs.rs/serde/1/serde/de/trait.Deserialize.html" target="_blank"><code>Deserialize</code></a> trait looks like this:</p>
<pre><code class="lang-rust"><span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">trait</span> <span class="hljs-title">Deserialize</span></span><<span class="hljs-symbol">'de</span>>: <span class="hljs-built_in">Sized</span> {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span><D>(deserializer: D) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">Self</span>, D::Error>
<span class="hljs-keyword">where</span>
D: Deserializer<<span class="hljs-symbol">'de</span>>;
}
</code></pre>
<p>This method's job is to map the type into the <a href="data-model.html">Serde data model</a> by providing
the <a href="https://docs.rs/serde/1/serde/trait.Deserializer.html" target="_blank"><code>Deserializer</code></a> with a <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> that can be driven by the <code>Deserializer</code>
to construct an instance of your type.</p>
<p>In most cases Serde's <a href="derive.html">derive</a> is able to generate an appropriate implementation
of <code>Deserialize</code> for structs and enums defined in your crate. Should you need to
customize the deserialization behavior for a type in a way that derive does not
support, you can implement <code>Deserialize</code> yourself. Implementing <code>Deserialize</code>
for a type tends to be more complicated than implementing <code>Serialize</code>.</p>
<p>The <code>Deserializer</code> trait supports two entry point styles which enables different
kinds of deserialization.</p>
<ol>
<li><p>The <code>deserialize_any</code> method. Self-describing data formats like JSON are able
to look at the serialized data and tell what it represents. For example the
JSON deserializer may see an opening curly brace (<code>{</code>) and know that it is
seeing a map. If the data format supports <code>Deserializer::deserialize_any</code>, it
will drive the Visitor using whatever type it sees in the input. JSON uses
this approach when deserializing <code>serde_json::Value</code> which is an enum that
can represent any JSON document. Without knowing what is in a JSON document,
we can deserialize it to <code>serde_json::Value</code> by going through
<code>Deserializer::deserialize_any</code>.</p>
</li>
<li><p>The various other <code>deserialize_*</code> methods. Non-self-describing formats like
Postcard need to be told what is in the input in order to deserialize it. The
<code>deserialize_*</code> methods are hints to the deserializer for how to interpret
the next piece of input. Non-self-describing formats are not able to
deserialize something like <code>serde_json::Value</code> which relies on
<code>Deserializer::deserialize_any</code>.</p>
</li>
</ol>
<p>When implementing <code>Deserialize</code>, you should avoid relying on
<code>Deserializer::deserialize_any</code> unless you need to be told by the Deserializer
what type is in the input. Know that relying on <code>Deserializer::deserialize_any</code>
means your data type will be able to deserialize from self-describing formats
only, ruling out Postcard and many others.</p>
<h2 id="the-visitor-trait"><a name="the-visitor-trait" class="plugin-anchor" href="#the-visitor-trait"><i class="fa fa-link" aria-hidden="true"></i></a>The Visitor trait</h2>
<p>A <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> is instantiated by a <code>Deserialize</code> impl and passed to a
<code>Deserializer</code>. The <code>Deserializer</code> then calls a method on the <code>Visitor</code> in order
to construct the desired type.</p>
<p>Here is a <code>Visitor</code> that is able to deserialize a primitive <code>i32</code> from a variety
of types.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std::fmt;
<span class="hljs-keyword">use</span> serde::de::{<span class="hljs-keyword">self</span>, Visitor};
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">I32Visitor</span></span>;
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Visitor<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> I32Visitor {
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = <span class="hljs-keyword">i32</span>;
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">expecting</span></span>(&<span class="hljs-keyword">self</span>, formatter: &<span class="hljs-keyword">mut</span> fmt::Formatter) -> fmt::<span class="hljs-built_in">Result</span> {
formatter.write_str(<span class="hljs-string">"an integer between -2^31 and 2^31"</span>)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_i8</span></span><E>(<span class="hljs-keyword">self</span>, value: <span class="hljs-keyword">i8</span>) -> <span class="hljs-built_in">Result</span><Self::Value, E>
<span class="hljs-keyword">where</span>
E: de::Error,
{
<span class="hljs-literal">Ok</span>(i32::from(value))
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_i32</span></span><E>(<span class="hljs-keyword">self</span>, value: <span class="hljs-keyword">i32</span>) -> <span class="hljs-built_in">Result</span><Self::Value, E>
<span class="hljs-keyword">where</span>
E: de::Error,
{
<span class="hljs-literal">Ok</span>(value)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_i64</span></span><E>(<span class="hljs-keyword">self</span>, value: <span class="hljs-keyword">i64</span>) -> <span class="hljs-built_in">Result</span><Self::Value, E>
<span class="hljs-keyword">where</span>
E: de::Error,
{
<span class="hljs-keyword">use</span> std::<span class="hljs-keyword">i32</span>;
<span class="hljs-keyword">if</span> value >= i64::from(i32::MIN) && value <= i64::from(i32::MAX) {
<span class="hljs-literal">Ok</span>(value <span class="hljs-keyword">as</span> <span class="hljs-keyword">i32</span>)
} <span class="hljs-keyword">else</span> {
<span class="hljs-literal">Err</span>(E::custom(<span class="hljs-built_in">format!</span>(<span class="hljs-string">"i32 out of range: {}"</span>, value)))
}
}
<span class="hljs-comment">// Similar for other methods:</span>
<span class="hljs-comment">// - visit_i16</span>
<span class="hljs-comment">// - visit_u8</span>
<span class="hljs-comment">// - visit_u16</span>
<span class="hljs-comment">// - visit_u32</span>
<span class="hljs-comment">// - visit_u64</span>
}
</code></pre>
<p>The <code>Visitor</code> trait has lots more methods that are not implemented for
<code>I32Visitor</code>. Leaving them unimplemented means a <a href="https://docs.rs/serde/1/serde/de/trait.Error.html#method.invalid_type" target="_blank">type error</a> is returned if
they get called. For example <code>I32Visitor</code> does not implement
<code>Visitor::visit_map</code>, so trying to deserialize an i32 when the input contains a
map is a type error.</p>
<h2 id="driving-a-visitor"><a name="driving-a-visitor" class="plugin-anchor" href="#driving-a-visitor"><i class="fa fa-link" aria-hidden="true"></i></a>Driving a Visitor</h2>
<p>Deserialize a value by passing a <code>Visitor</code> to the given <code>Deserializer</code>. The
<code>Deserializer</code> will call one of the <code>Visitor</code> methods depending on the input
data, which is known as "driving" the <code>Visitor</code>.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> <span class="hljs-keyword">i32</span> {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span><D>(deserializer: D) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">i32</span>, D::Error>
<span class="hljs-keyword">where</span>
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
deserializer.deserialize_i32(I32Visitor)
}
}
</code></pre>
<p>Note that a <code>Deserializer</code> will not necessarily follow the type hint, so the
call to <code>deserialize_i32</code> does not necessarily mean the <code>Deserializer</code> will call
<code>I32Visitor::visit_i32</code>. For example JSON treats all signed integer types alike.
The JSON <code>Deserializer</code> will call <code>visit_i64</code> for any signed integer and
<code>visit_u64</code> for any unsigned integer, even if hinted a different type.</p>
<h2 id="other-examples"><a name="other-examples" class="plugin-anchor" href="#other-examples"><i class="fa fa-link" aria-hidden="true"></i></a>Other examples</h2>
<ul>
<li><a href="deserialize-map.html">Deserializing a map</a></li>
<li><a href="deserialize-struct.html">Deserializing a struct</a></li>
</ul>
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