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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>&amp;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&amp;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>&lt;<span class="hljs-symbol">&apos;de</span>&gt; Deserialize&lt;<span class="hljs-symbol">&apos;de</span>&gt; <span class="hljs-keyword">for</span> Duration {
    <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span>&lt;D&gt;(deserializer: D) -&gt; <span class="hljs-built_in">Result</span>&lt;<span class="hljs-keyword">Self</span>, D::Error&gt;
    <span class="hljs-keyword">where</span>
        D: Deserializer&lt;<span class="hljs-symbol">&apos;de</span>&gt;,
    {
        <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 = &quot;lowercase&quot;)]</span>
        <span class="hljs-comment">//    enum Field { Secs, Nanos }</span>
        <span class="hljs-keyword">impl</span>&lt;<span class="hljs-symbol">&apos;de</span>&gt; Deserialize&lt;<span class="hljs-symbol">&apos;de</span>&gt; <span class="hljs-keyword">for</span> Field {
            <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span>&lt;D&gt;(deserializer: D) -&gt; <span class="hljs-built_in">Result</span>&lt;Field, D::Error&gt;
            <span class="hljs-keyword">where</span>
                D: Deserializer&lt;<span class="hljs-symbol">&apos;de</span>&gt;,
            {
                <span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">FieldVisitor</span></span>;

                <span class="hljs-keyword">impl</span>&lt;<span class="hljs-symbol">&apos;de</span>&gt; Visitor&lt;<span class="hljs-symbol">&apos;de</span>&gt; <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>(&amp;<span class="hljs-keyword">self</span>, formatter: &amp;<span class="hljs-keyword">mut</span> fmt::Formatter) -&gt; fmt::<span class="hljs-built_in">Result</span> {
                        formatter.write_str(<span class="hljs-string">&quot;`secs` or `nanos`&quot;</span>)
                    }

                    <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_str</span></span>&lt;E&gt;(<span class="hljs-keyword">self</span>, value: &amp;<span class="hljs-keyword">str</span>) -&gt; <span class="hljs-built_in">Result</span>&lt;Field, E&gt;
                    <span class="hljs-keyword">where</span>
                        E: de::Error,
                    {
                        <span class="hljs-keyword">match</span> value {
                            <span class="hljs-string">&quot;secs&quot;</span> =&gt; <span class="hljs-literal">Ok</span>(Field::Secs),
                            <span class="hljs-string">&quot;nanos&quot;</span> =&gt; <span class="hljs-literal">Ok</span>(Field::Nanos),
                            _ =&gt; <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>&lt;<span class="hljs-symbol">&apos;de</span>&gt; Visitor&lt;<span class="hljs-symbol">&apos;de</span>&gt; <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>(&amp;<span class="hljs-keyword">self</span>, formatter: &amp;<span class="hljs-keyword">mut</span> fmt::Formatter) -&gt; fmt::<span class="hljs-built_in">Result</span> {
                formatter.write_str(<span class="hljs-string">&quot;struct Duration&quot;</span>)
            }

            <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_seq</span></span>&lt;V&gt;(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> seq: V) -&gt; <span class="hljs-built_in">Result</span>&lt;Duration, V::Error&gt;
            <span class="hljs-keyword">where</span>
                V: SeqAccess&lt;<span class="hljs-symbol">&apos;de</span>&gt;,
            {
                <span class="hljs-keyword">let</span> secs = seq.next_element()?
                    .ok_or_else(|| de::Error::invalid_length(<span class="hljs-number">0</span>, &amp;<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>, &amp;<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>&lt;V&gt;(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> map: V) -&gt; <span class="hljs-built_in">Result</span>&lt;Duration, V::Error&gt;
            <span class="hljs-keyword">where</span>
                V: MapAccess&lt;<span class="hljs-symbol">&apos;de</span>&gt;,
            {
                <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 =&gt; {
                            <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">&quot;secs&quot;</span>));
                            }
                            secs = <span class="hljs-literal">Some</span>(map.next_value()?);
                        }
                        Field::Nanos =&gt; {
                            <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">&quot;nanos&quot;</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">&quot;secs&quot;</span>))?;
                <span class="hljs-keyword">let</span> nanos = nanos.ok_or_else(|| de::Error::missing_field(<span class="hljs-string">&quot;nanos&quot;</span>))?;
                <span class="hljs-literal">Ok</span>(Duration::new(secs, nanos))
            }
        }

        <span class="hljs-keyword">const</span> FIELDS: &amp;[&amp;<span class="hljs-keyword">str</span>] = &amp;[<span class="hljs-string">&quot;secs&quot;</span>, <span class="hljs-string">&quot;nanos&quot;</span>];
        deserializer.deserialize_struct(<span class="hljs-string">&quot;Duration&quot;</span>, FIELDS, DurationVisitor)
    }
}
</code></pre>

                                
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