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<h1>
<i class="fa fa-circle-o-notch fa-spin"></i>
<a href="." >Serde data model</a>
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<h1 id="serde-data-model"><a name="serde-data-model" class="plugin-anchor" href="#serde-data-model"><i class="fa fa-link" aria-hidden="true"></i></a>Serde data model</h1>
<p>The Serde data model is the API by which data structures and data formats
interact. You can think of it as Serde's type system.</p>
<p>In code, the serialization half of the Serde data model is defined by the
<a href="https://docs.rs/serde/1/serde/trait.Serializer.html" target="_blank"><code>Serializer</code></a> trait and the deserialization half is defined by the
<a href="https://docs.rs/serde/1/serde/trait.Deserializer.html" target="_blank"><code>Deserializer</code></a> trait. These are a way of mapping every Rust data structure
into one of 29 possible types. Each method of the <code>Serializer</code> trait corresponds
to one of the types of the data model.</p>
<p>When serializing a data structure to some format, the <a href="https://docs.rs/serde/1/serde/trait.Serialize.html" target="_blank"><code>Serialize</code></a>
implementation for the data structure is responsible for mapping the data
structure into the Serde data model by invoking exactly one of the <code>Serializer</code>
methods, while the <code>Serializer</code> implementation for the data format is
responsible for mapping the Serde data model into the intended output
representation.</p>
<p>When deserializing a data structure from some format, the <a href="https://docs.rs/serde/1/serde/trait.Deserialize.html" target="_blank"><code>Deserialize</code></a>
implementation for the data structure is responsible for mapping the data
structure into the Serde data model by passing to the <code>Deserializer</code> a
<a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> implementation that can receive the various types of the data model,
while the <code>Deserializer</code> implementation for the data format is responsible for
mapping the input data into the Serde data model by invoking exactly one of the
<code>Visitor</code> methods.</p>
<h2 id="types"><a name="types" class="plugin-anchor" href="#types"><i class="fa fa-link" aria-hidden="true"></i></a>Types</h2>
<p>The Serde data model is a simplified form of Rust's type system. It consists of
the following 29 types:</p>
<ul>
<li><strong>14 primitive types</strong><ul>
<li>bool</li>
<li>i8, i16, i32, i64, i128</li>
<li>u8, u16, u32, u64, u128</li>
<li>f32, f64</li>
<li>char</li>
</ul>
</li>
<li><strong>string</strong><ul>
<li>UTF-8 bytes with a length and no null terminator. May contain 0-bytes.</li>
<li>When serializing, all strings are handled equally. When deserializing, there
are three flavors of strings: transient, owned, and borrowed. This
distinction is explained in <a href="lifetimes.html">Understanding deserializer lifetimes</a> and is a
key way that Serde enabled efficient zero-copy deserialization.</li>
</ul>
</li>
<li><strong>byte array</strong> - [u8]<ul>
<li>Similar to strings, during deserialization byte arrays can be transient,
owned, or borrowed.</li>
</ul>
</li>
<li><strong>option</strong><ul>
<li>Either none or some value.</li>
</ul>
</li>
<li><strong>unit</strong><ul>
<li>The type of <code>()</code> in Rust. It represents an anonymous value containing no
data.</li>
</ul>
</li>
<li><strong>unit_struct</strong><ul>
<li>For example <code>struct Unit</code> or <code>PhantomData<T></code>. It represents a named value
containing no data.</li>
</ul>
</li>
<li><strong>unit_variant</strong><ul>
<li>For example the <code>E::A</code> and <code>E::B</code> in <code>enum E { A, B }</code>.</li>
</ul>
</li>
<li><strong>newtype_struct</strong><ul>
<li>For example <code>struct Millimeters(u8)</code>.</li>
</ul>
</li>
<li><strong>newtype_variant</strong><ul>
<li>For example the <code>E::N</code> in <code>enum E { N(u8) }</code>.</li>
</ul>
</li>
<li><strong>seq</strong><ul>
<li>A variably sized heterogeneous sequence of values, for example <code>Vec<T></code> or
<code>HashSet<T></code>. When serializing, the length may or may not be known before
iterating through all the data. When deserializing, the length is determined
by looking at the serialized data. Note that a homogeneous Rust collection
like <code>vec![Value::Bool(true), Value::Char('c')]</code> may serialize as a
heterogeneous Serde seq, in this case containing a Serde bool followed by a
Serde char.</li>
</ul>
</li>
<li><strong>tuple</strong><ul>
<li>A statically sized heterogeneous sequence of values for which the length
will be known at deserialization time without looking at the serialized
data, for example <code>(u8,)</code> or <code>(String, u64, Vec<T>)</code> or <code>[u64; 10]</code>.</li>
</ul>
</li>
<li><strong>tuple_struct</strong><ul>
<li>A named tuple, for example <code>struct Rgb(u8, u8, u8)</code>.</li>
</ul>
</li>
<li><strong>tuple_variant</strong><ul>
<li>For example the <code>E::T</code> in <code>enum E { T(u8, u8) }</code>.</li>
</ul>
</li>
<li><strong>map</strong><ul>
<li>A variably sized heterogeneous key-value pairing, for example <code>BTreeMap<K,
V></code>. When serializing, the length may or may not be known before iterating
through all the entries. When deserializing, the length is determined by
looking at the serialized data.</li>
</ul>
</li>
<li><strong>struct</strong><ul>
<li>A statically sized heterogeneous key-value pairing in which the keys are
compile-time constant strings and will be known at deserialization time
without looking at the serialized data, for example <code>struct S { r: u8, g:
u8, b: u8 }</code>.</li>
</ul>
</li>
<li><strong>struct_variant</strong><ul>
<li>For example the <code>E::S</code> in <code>enum E { S { r: u8, g: u8, b: u8 } }</code>.</li>
</ul>
</li>
</ul>
<h2 id="mapping-into-the-data-model"><a name="mapping-into-the-data-model" class="plugin-anchor" href="#mapping-into-the-data-model"><i class="fa fa-link" aria-hidden="true"></i></a>Mapping into the data model</h2>
<p>In the case of most Rust types, their mapping into the Serde data model is
straightforward. For example the Rust <code>bool</code> type corresponds to Serde's bool
type. The Rust tuple struct <code>Rgb(u8, u8, u8)</code> corresponds to Serde's tuple
struct type.</p>
<p>But there is no fundamental reason that these mappings need to be
straightforward. The <a href="https://docs.rs/serde/1/serde/trait.Serialize.html" target="_blank"><code>Serialize</code></a> and <a href="https://docs.rs/serde/1/serde/trait.Deserialize.html" target="_blank"><code>Deserialize</code></a> traits can perform <em>any</em>
mapping between Rust type and Serde data model that is appropriate for the use
case.</p>
<p>As an example, consider Rust's <a href="https://doc.rust-lang.org/std/ffi/struct.OsString.html" target="_blank"><code>std::ffi::OsString</code></a> type. This type represents
a platform-native string. On Unix systems they are arbitrary non-zero bytes and
on Windows systems they are arbitrary non-zero 16-bit values. It may seem
natural to map <code>OsString</code> into the Serde data model as one of the following
types:</p>
<ul>
<li>As a Serde <strong>string</strong>. Unfortunately serialization would be brittle because an
<code>OsString</code> is not guaranteed to be representable in UTF-8 and deserialization
would be brittle because Serde strings are allowed to contain 0-bytes.</li>
<li>As a Serde <strong>byte array</strong>. This fixes both problems with using string, but now
if we serialize an <code>OsString</code> on Unix and deserialize it on Windows we end up
with <a href="https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/" target="_blank">the wrong string</a>.</li>
</ul>
<p>Instead the <code>Serialize</code> and <code>Deserialize</code> impls for <code>OsString</code> map into the
Serde data model by treating <code>OsString</code> as a Serde <strong>enum</strong>. Effectively it acts
as though <code>OsString</code> were defined as the following type, even though this does
not match its definition on any individual platform.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">OsString</span></span> {
Unix(<span class="hljs-built_in">Vec</span><<span class="hljs-keyword">u8</span>>),
Windows(<span class="hljs-built_in">Vec</span><u16>),
<span class="hljs-comment">// and other platforms</span>
}
</code></pre>
<p>The flexibility around mapping into the Serde data model is profound and
powerful. When implementing <code>Serialize</code> and <code>Deserialize</code>, be aware of the
broader context of your type that may make the most instinctive mapping not the
best choice.</p>
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