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Manually deserialize struct
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<a href="." >Manually deserialize struct</a>
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<h1 id="manually-implementing-deserialize-for-a-struct"><a name="manually-implementing-deserialize-for-a-struct" class="plugin-anchor" href="#manually-implementing-deserialize-for-a-struct"><i class="fa fa-link" aria-hidden="true"></i></a>Manually implementing Deserialize for a struct</h1>
<p>Only when <a href="derive.html">derive</a> is not getting the job done.</p>
<p>The <code>Deserialize</code> impl below corresponds to the following struct:</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Duration</span></span> {
secs: <span class="hljs-keyword">u64</span>,
nanos: <span class="hljs-keyword">u32</span>,
}
</code></pre>
<p>Deserializing a struct is somewhat more complicated than <a href="deserialize-map.html">deserializing a
map</a> in order to avoid allocating a String to hold the field
names. Instead there is a <code>Field</code> enum which is deserialized from a <code>&str</code>.</p>
<p>The implementation supports two possible ways that a struct may be represented
by a data format: as a seq like in Postcard, and as a map like in JSON.</p>
<pre><a class="playground-link" target="_blank" href="https://play.rust-lang.org/?edition=2018&gist=2e212d29e38110fc3d8f22ff920712be"></a><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>, Deserialize, Deserializer, Visitor, SeqAccess, MapAccess};
<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> Duration {
<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>>,
{
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Field</span></span> { Secs, Nanos }
<span class="hljs-comment">// This part could also be generated independently by:</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// #[derive(Deserialize)]</span>
<span class="hljs-comment">// #[serde(field_identifier, rename_all = "lowercase")]</span>
<span class="hljs-comment">// enum Field { Secs, Nanos }</span>
<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> Field {
<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><Field, D::Error>
<span class="hljs-keyword">where</span>
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">FieldVisitor</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> FieldVisitor {
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = Field;
<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">"`secs` or `nanos`"</span>)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_str</span></span><E>(<span class="hljs-keyword">self</span>, value: &<span class="hljs-keyword">str</span>) -> <span class="hljs-built_in">Result</span><Field, E>
<span class="hljs-keyword">where</span>
E: de::Error,
{
<span class="hljs-keyword">match</span> value {
<span class="hljs-string">"secs"</span> => <span class="hljs-literal">Ok</span>(Field::Secs),
<span class="hljs-string">"nanos"</span> => <span class="hljs-literal">Ok</span>(Field::Nanos),
_ => <span class="hljs-literal">Err</span>(de::Error::unknown_field(value, FIELDS)),
}
}
}
deserializer.deserialize_identifier(FieldVisitor)
}
}
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">DurationVisitor</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> DurationVisitor {
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = Duration;
<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">"struct Duration"</span>)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_seq</span></span><V>(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> seq: V) -> <span class="hljs-built_in">Result</span><Duration, V::Error>
<span class="hljs-keyword">where</span>
V: SeqAccess<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">let</span> secs = seq.next_element()?
.ok_or_else(|| de::Error::invalid_length(<span class="hljs-number">0</span>, &<span class="hljs-keyword">self</span>))?;
<span class="hljs-keyword">let</span> nanos = seq.next_element()?
.ok_or_else(|| de::Error::invalid_length(<span class="hljs-number">1</span>, &<span class="hljs-keyword">self</span>))?;
<span class="hljs-literal">Ok</span>(Duration::new(secs, nanos))
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_map</span></span><V>(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> map: V) -> <span class="hljs-built_in">Result</span><Duration, V::Error>
<span class="hljs-keyword">where</span>
V: MapAccess<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> secs = <span class="hljs-literal">None</span>;
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> nanos = <span class="hljs-literal">None</span>;
<span class="hljs-keyword">while</span> <span class="hljs-keyword">let</span> <span class="hljs-literal">Some</span>(key) = map.next_key()? {
<span class="hljs-keyword">match</span> key {
Field::Secs => {
<span class="hljs-keyword">if</span> secs.is_some() {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Err</span>(de::Error::duplicate_field(<span class="hljs-string">"secs"</span>));
}
secs = <span class="hljs-literal">Some</span>(map.next_value()?);
}
Field::Nanos => {
<span class="hljs-keyword">if</span> nanos.is_some() {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Err</span>(de::Error::duplicate_field(<span class="hljs-string">"nanos"</span>));
}
nanos = <span class="hljs-literal">Some</span>(map.next_value()?);
}
}
}
<span class="hljs-keyword">let</span> secs = secs.ok_or_else(|| de::Error::missing_field(<span class="hljs-string">"secs"</span>))?;
<span class="hljs-keyword">let</span> nanos = nanos.ok_or_else(|| de::Error::missing_field(<span class="hljs-string">"nanos"</span>))?;
<span class="hljs-literal">Ok</span>(Duration::new(secs, nanos))
}
}
<span class="hljs-keyword">const</span> FIELDS: &[&<span class="hljs-keyword">str</span>] = &[<span class="hljs-string">"secs"</span>, <span class="hljs-string">"nanos"</span>];
deserializer.deserialize_struct(<span class="hljs-string">"Duration"</span>, FIELDS, DurationVisitor)
}
}
</code></pre>
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