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Deserialize for custom map type
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<h1 id="implement-deserialize-for-a-custom-map-type"><a name="implement-deserialize-for-a-custom-map-type" class="plugin-anchor" href="#implement-deserialize-for-a-custom-map-type"><i class="fa fa-link" aria-hidden="true"></i></a>Implement Deserialize for a custom map type</h1>
<pre><a class="playground-link" target="_blank" href="https://play.rust-lang.org/?edition=2018&gist=72f10ca685c08f8afeb618efdabfed6a"></a><code class="lang-rust"><span class="hljs-keyword">use</span> std::fmt;
<span class="hljs-keyword">use</span> std::marker::PhantomData;
<span class="hljs-keyword">use</span> serde::de::{Deserialize, Deserializer, Visitor, MapAccess};
<span class="hljs-comment">// A Visitor is a type that holds methods that a Deserializer can drive</span>
<span class="hljs-comment">// depending on what is contained in the input data.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// In the case of a map we need generic type parameters K and V to be</span>
<span class="hljs-comment">// able to set the output type correctly, but don't require any state.</span>
<span class="hljs-comment">// This is an example of a "zero sized type" in Rust. The PhantomData</span>
<span class="hljs-comment">// keeps the compiler from complaining about unused generic type</span>
<span class="hljs-comment">// parameters.</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">MyMapVisitor</span></span><K, V> {
marker: PhantomData<<span class="hljs-function"><span class="hljs-keyword">fn</span></span>() -> MyMap<K, V>>
}
<span class="hljs-keyword">impl</span><K, V> MyMapVisitor<K, V> {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">new</span></span>() -> <span class="hljs-keyword">Self</span> {
MyMapVisitor {
marker: PhantomData
}
}
}
<span class="hljs-comment">// This is the trait that Deserializers are going to be driving. There</span>
<span class="hljs-comment">// is one method for each type of data that our type knows how to</span>
<span class="hljs-comment">// deserialize from. There are many other methods that are not</span>
<span class="hljs-comment">// implemented here, for example deserializing from integers or strings.</span>
<span class="hljs-comment">// By default those methods will return an error, which makes sense</span>
<span class="hljs-comment">// because we cannot deserialize a MyMap from an integer or string.</span>
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, K, V> Visitor<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> MyMapVisitor<K, V>
<span class="hljs-keyword">where</span>
K: Deserialize<<span class="hljs-symbol">'de</span>>,
V: Deserialize<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">// The type that our Visitor is going to produce.</span>
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = MyMap<K, V>;
<span class="hljs-comment">// Format a message stating what data this Visitor expects to receive.</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">"a very special map"</span>)
}
<span class="hljs-comment">// Deserialize MyMap from an abstract "map" provided by the</span>
<span class="hljs-comment">// Deserializer. The MapAccess input is a callback provided by</span>
<span class="hljs-comment">// the Deserializer to let us see each entry in the map.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_map</span></span><M>(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> access: M) -> <span class="hljs-built_in">Result</span><Self::Value, M::Error>
<span class="hljs-keyword">where</span>
M: MapAccess<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> map = MyMap::with_capacity(access.size_hint().unwrap_or(<span class="hljs-number">0</span>));
<span class="hljs-comment">// While there are entries remaining in the input, add them</span>
<span class="hljs-comment">// into our map.</span>
<span class="hljs-keyword">while</span> <span class="hljs-keyword">let</span> <span class="hljs-literal">Some</span>((key, value)) = access.next_entry()? {
map.insert(key, value);
}
<span class="hljs-literal">Ok</span>(map)
}
}
<span class="hljs-comment">// This is the trait that informs Serde how to deserialize MyMap.</span>
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, K, V> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> MyMap<K, V>
<span class="hljs-keyword">where</span>
K: Deserialize<<span class="hljs-symbol">'de</span>>,
V: Deserialize<<span class="hljs-symbol">'de</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>>,
{
<span class="hljs-comment">// Instantiate our Visitor and ask the Deserializer to drive</span>
<span class="hljs-comment">// it over the input data, resulting in an instance of MyMap.</span>
deserializer.deserialize_map(MyMapVisitor::new())
}
}
</code></pre>
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