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class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class=menu__link tabindex=0 href=/docs/next/guide/xlang/troubleshooting><span title=Troubleshooting class=linkLabel_WmDU>Troubleshooting</span></a></ul></ul></ul></nav></div></div></aside><main class=docMainContainer_TBSr><div class="container padding-top--md padding-bottom--lg"><div class=row><div class="col docItemCol_VOVn"><div class="theme-doc-version-banner alert alert--warning margin-bottom--md" role=alert><div>This is unreleased documentation for <!-- -->Apache Fory™<!-- --> <b>dev</b> version.</div><div class=margin-top--md>For up-to-date documentation, see the <b><a href=/docs/guide/xlang/field_nullability>latest version</a></b> (<!-- -->1.3.0<!-- -->).</div></div><div class=docItemContainer_Djhp><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label=Breadcrumbs><ul class=breadcrumbs><li class=breadcrumbs__item><a aria-label="Home page" class=breadcrumbs__link href=/><svg viewBox="0 0 24 24" class=breadcrumbHomeIcon_YNFT><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill=currentColor /></svg></a><li class=breadcrumbs__item><span class=breadcrumbs__link>Guide</span><li class=breadcrumbs__item><a class=breadcrumbs__link href=/docs/next/guide/xlang/><span>Cross Language</span></a><li class="breadcrumbs__item breadcrumbs__item--active"><span class=breadcrumbs__link>Field Nullability</span></ul></nav><span class="theme-doc-version-badge badge badge--secondary">Version: dev</span><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type=button class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>Field Nullability</h1></header><p>This page explains how Fory handles field nullability in cross-language (xlang) serialization mode.</p> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=default-behavior>Default Behavior<a href=#default-behavior class=hash-link aria-label="Direct link to Default Behavior" title="Direct link to Default Behavior" translate=no></a></h2> |
| <p>In xlang mode, <strong>fields are non-nullable by default</strong>. This means:</p> |
| <ul> |
| <li class="">Values must always be present (non-null)</li> |
| <li class="">No null flag byte is written for the field</li> |
| <li class="">Serialization is more compact</li> |
| </ul> |
| <p>The following types are nullable by default:</p> |
| <ul> |
| <li class=""><code>Optional<T></code> (Java, C++)</li> |
| <li class="">Java boxed types (<code>Integer</code>, <code>Long</code>, <code>Double</code>, etc.)</li> |
| <li class="">Go pointer types (<code>*int32</code>, <code>*string</code>, etc.)</li> |
| <li class="">Rust <code>Option<T></code></li> |
| <li class="">Python <code>Optional[T]</code></li> |
| <li class="">Scala <code>Option[T]</code></li> |
| </ul> |
| <table><thead><tr><th>Field Type<th>Default Nullable<th>Null Flag Written<tbody><tr><td>Primitives (<code>int</code>, <code>bool</code>, <code>float</code>, etc.)<td>No<td>No<tr><td><code>String</code><td>No<td>No<tr><td><code>List<T></code>, <code>Map<K,V></code>, <code>Set<T></code><td>No<td>No<tr><td>Custom structs<td>No<td>No<tr><td>Enums<td>No<td>No<tr><td>Java boxed types (<code>Integer</code>, <code>Long</code>, etc.)<td>Yes<td>Yes<tr><td>Go pointer types (<code>*int32</code>, <code>*string</code>)<td>Yes<td>Yes<tr><td><code>Optional<T></code> / <code>Option<T></code><td>Yes<td>Yes</table> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=wire-format>Wire Format<a href=#wire-format class=hash-link aria-label="Direct link to Wire Format" title="Direct link to Wire Format" translate=no></a></h2> |
| <p>The nullable flag controls whether a <strong>null flag byte</strong> is written before the field value:</p> |
| <div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">Non-nullable field: [value data]</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">Nullable field: [null_flag] [value data if not null]</span><br/></div></code></pre></div></div> |
| <p>Where <code>null_flag</code> is:</p> |
| <ul> |
| <li class=""><code>-1</code> (NULL_FLAG): Value is null</li> |
| <li class=""><code>-2</code> (NOT_NULL_VALUE_FLAG): Value is present</li> |
| </ul> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=nullable-vs-reference-tracking>Nullable vs Reference Tracking<a href=#nullable-vs-reference-tracking class=hash-link aria-label="Direct link to Nullable vs Reference Tracking" title="Direct link to Nullable vs Reference Tracking" translate=no></a></h2> |
| <p>These are related but distinct concepts:</p> |
| <table><thead><tr><th>Concept<th>Purpose<th>Flag Values<tbody><tr><td><strong>Nullable</strong><td>Allow null values for a field<td><code>-1</code> (null), <code>-2</code> (not null)<tr><td><strong>Reference Tracking</strong><td>Deduplicate shared object references<td><code>-1</code> (null), <code>-2</code> (not null), <code>≥0</code> (ref ID)</table> |
| <p>Key differences:</p> |
| <ul> |
| <li class=""><strong>Nullable only</strong>: Writes <code>-1</code> or <code>-2</code> flag, no reference deduplication</li> |
| <li class=""><strong>Reference tracking</strong>: Extends nullable semantics with reference IDs (<code>≥0</code>) for previously seen objects</li> |
| <li class="">Both use the same flag byte position—ref tracking is a superset of nullable</li> |
| </ul> |
| <p>When <code>refTracking=true</code>, the null flag byte doubles as a ref flag:</p> |
| <div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">ref_flag = -1 → null value</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">ref_flag = -2 → new object (first occurrence)</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">ref_flag >= 0 → reference to object at index ref_flag</span><br/></div></code></pre></div></div> |
| <p>For detailed reference tracking behavior, see <a class="" href=/docs/next/guide/xlang/reference_tracking>Reference Tracking</a>.</p> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=language-specific-examples>Language-Specific Examples<a href=#language-specific-examples class=hash-link aria-label="Direct link to Language-Specific Examples" title="Direct link to Language-Specific Examples" translate=no></a></h2> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=java>Java<a href=#java class=hash-link aria-label="Direct link to Java" title="Direct link to Java" translate=no></a></h3> |
| <div class="language-java codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-java codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>public</span><span class="token plain"> </span><span class="token keyword" style=color:#00009f>class</span><span class="token plain"> </span><span class="token class-name">Person</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Non-nullable by default in xlang mode</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">String</span><span class="token plain"> name</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Must not be null</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token keyword" style=color:#00009f>int</span><span class="token plain"> age</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Primitive, always non-nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">List</span><span class="token generics punctuation" style=color:#393A34><</span><span class="token generics class-name">String</span><span class="token generics punctuation" style=color:#393A34>></span><span class="token plain"> tags</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Must not be null</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Explicitly nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token annotation punctuation" style=color:#393A34>@Nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">String</span><span class="token plain"> nickname</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be null</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Optional wrapper - nullable by default</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">Optional</span><span class="token generics punctuation" style=color:#393A34><</span><span class="token generics class-name">String</span><span class="token generics punctuation" style=color:#393A34>></span><span class="token plain"> bio</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be empty/null</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token class-name">Fory</span><span class="token plain"> fory </span><span class="token operator" style=color:#393A34>=</span><span class="token plain"> </span><span class="token class-name">Fory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>builder</span><span class="token punctuation" style=color:#393A34>(</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>withXlang</span><span class="token punctuation" style=color:#393A34>(</span><span class="token boolean" style=color:#36acaa>true</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>build</span><span class="token punctuation" style=color:#393A34>(</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">fory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>register</span><span class="token punctuation" style=color:#393A34>(</span><span class="token class-name">Person</span><span class="token punctuation" style=color:#393A34>.</span><span class="token keyword" style=color:#00009f>class</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> </span><span class="token string" style=color:#e3116c>"example.Person"</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>;</span><br/></div></code></pre></div></div> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=python>Python<a href=#python class=hash-link aria-label="Direct link to Python" title="Direct link to Python" translate=no></a></h3> |
| <div class="language-python codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-python codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>from</span><span class="token plain"> dataclasses </span><span class="token keyword" style=color:#00009f>import</span><span class="token plain"> dataclass</span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token keyword" style=color:#00009f>from</span><span class="token plain"> typing </span><span class="token keyword" style=color:#00009f>import</span><span class="token plain"> Optional</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> List</span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token keyword" style=color:#00009f>import</span><span class="token plain"> pyfory</span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token decorator annotation punctuation" style=color:#393A34>@dataclass</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token keyword" style=color:#00009f>class</span><span class="token plain"> </span><span class="token class-name">Person</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Non-nullable by default</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> name</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token builtin">str</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Must have a value</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> age</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> pyfory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token plain">Int32 </span><span class="token comment" style=color:#999988;font-style:italic># Primitive</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> tags</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> List</span><span class="token punctuation" style=color:#393A34>[</span><span class="token builtin">str</span><span class="token punctuation" style=color:#393A34>]</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Must not be None</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Optional makes it nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> nickname</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> Optional</span><span class="token punctuation" style=color:#393A34>[</span><span class="token builtin">str</span><span class="token punctuation" style=color:#393A34>]</span><span class="token plain"> </span><span class="token operator" style=color:#393A34>=</span><span class="token plain"> </span><span class="token boolean" style=color:#36acaa>None</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Can be None</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> bio</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> Optional</span><span class="token punctuation" style=color:#393A34>[</span><span class="token builtin">str</span><span class="token punctuation" style=color:#393A34>]</span><span class="token plain"> </span><span class="token operator" style=color:#393A34>=</span><span class="token plain"> </span><span class="token boolean" style=color:#36acaa>None</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic># Can be None</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">fory </span><span class="token operator" style=color:#393A34>=</span><span class="token plain"> pyfory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token plain">Fory</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">xlang</span><span class="token operator" style=color:#393A34>=</span><span class="token boolean" style=color:#36acaa>True</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">fory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token plain">register_type</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">Person</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> name</span><span class="token operator" style=color:#393A34>=</span><span class="token string" style=color:#e3116c>"example.Person"</span><span class="token punctuation" style=color:#393A34>)</span><br/></div></code></pre></div></div> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=rust>Rust<a href=#rust class=hash-link aria-label="Direct link to Rust" title="Direct link to Rust" translate=no></a></h3> |
| <div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>use</span><span class="token plain"> </span><span class="token namespace" style=opacity:0.7>fory</span><span class="token namespace punctuation" style=opacity:0.7;color:#393A34>::</span><span class="token punctuation" style=color:#393A34>{</span><span class="token class-name">Fory</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> </span><span class="token class-name">ForyStruct</span><span class="token punctuation" style=color:#393A34>}</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token attribute attr-name" style=color:#00a4db>#[derive(ForyStruct)]</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token keyword" style=color:#00009f>struct</span><span class="token plain"> </span><span class="token type-definition class-name">Person</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Non-nullable by default</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> name</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token class-name">String</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> age</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token keyword" style=color:#00009f>i32</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> tags</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token class-name">Vec</span><span class="token operator" style=color:#393A34><</span><span class="token class-name">String</span><span class="token operator" style=color:#393A34>></span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Option<T> is nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> nickname</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token class-name">Option</span><span class="token operator" style=color:#393A34><</span><span class="token class-name">String</span><span class="token operator" style=color:#393A34>></span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be None</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> bio</span><span class="token punctuation" style=color:#393A34>:</span><span class="token plain"> </span><span class="token class-name">Option</span><span class="token operator" style=color:#393A34><</span><span class="token class-name">String</span><span class="token operator" style=color:#393A34>></span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be None</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><br/></div></code></pre></div></div> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=go>Go<a href=#go class=hash-link aria-label="Direct link to Go" title="Direct link to Go" translate=no></a></h3> |
| <div class="language-go codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-go codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>type</span><span class="token plain"> Person </span><span class="token keyword" style=color:#00009f>struct</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Non-nullable by default</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> Name </span><span class="token builtin">string</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> Age </span><span class="token builtin">int32</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> Tags </span><span class="token punctuation" style=color:#393A34>[</span><span class="token punctuation" style=color:#393A34>]</span><span class="token builtin">string</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Pointer types for nullable fields</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> Nickname </span><span class="token operator" style=color:#393A34>*</span><span class="token builtin">string</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be nil</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> Bio </span><span class="token operator" style=color:#393A34>*</span><span class="token builtin">string</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Can be nil</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">fory </span><span class="token operator" style=color:#393A34>:=</span><span class="token plain"> forygo</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>NewFory</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">forygo</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>WithXlang</span><span class="token punctuation" style=color:#393A34>(</span><span class="token boolean" style=color:#36acaa>true</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain">fory</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>RegisterStructByName</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">Person</span><span class="token punctuation" style=color:#393A34>{</span><span class="token punctuation" style=color:#393A34>}</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> </span><span class="token string" style=color:#e3116c>"example.Person"</span><span class="token punctuation" style=color:#393A34>)</span><br/></div></code></pre></div></div> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=c>C++<a href=#c class=hash-link aria-label="Direct link to C++" title="Direct link to C++" translate=no></a></h3> |
| <div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>struct</span><span class="token plain"> </span><span class="token class-name">Person</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Non-nullable by default</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string name</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token keyword" style=color:#00009f>int32_t</span><span class="token plain"> age</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">vector</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> tags</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// std::optional for nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">optional</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> nickname</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">optional</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> bio</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token function" style=color:#d73a49>FORY_STRUCT</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">Person</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> name</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> age</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> tags</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> nickname</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> bio</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>;</span><br/></div></code></pre></div></div> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=customizing-nullability>Customizing Nullability<a href=#customizing-nullability class=hash-link aria-label="Direct link to Customizing Nullability" title="Direct link to Customizing Nullability" translate=no></a></h2> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=java-nullable-annotation>Java: @Nullable Annotation<a href=#java-nullable-annotation class=hash-link aria-label="Direct link to Java: @Nullable Annotation" title="Direct link to Java: @Nullable Annotation" translate=no></a></h3> |
| <div class="language-java codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-java codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>public</span><span class="token plain"> </span><span class="token keyword" style=color:#00009f>class</span><span class="token plain"> </span><span class="token class-name">Config</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token annotation punctuation" style=color:#393A34>@Nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">String</span><span class="token plain"> optionalSetting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Explicitly nullable</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token class-name">String</span><span class="token plain"> requiredSetting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"> </span><span class="token comment" style=color:#999988;font-style:italic>// Explicitly non-nullable (default)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><br/></div></code></pre></div></div> |
| <h3 class="anchor anchorTargetStickyNavbar_Vzrq" id=c-fory_struct-field-config>C++: FORY_STRUCT Field Config<a href=#c-fory_struct-field-config class=hash-link aria-label="Direct link to C++: FORY_STRUCT Field Config" title="Direct link to C++: FORY_STRUCT Field Config" translate=no></a></h3> |
| <div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>struct</span><span class="token plain"> </span><span class="token class-name">Config</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">optional</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> optional_setting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string required_setting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token function" style=color:#d73a49>FORY_STRUCT</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">Config</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">optional_setting</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> fory</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token function" style=color:#d73a49>F</span><span class="token punctuation" style=color:#393A34>(</span><span class="token number" style=color:#36acaa>1</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">required_setting</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> fory</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token function" style=color:#d73a49>F</span><span class="token punctuation" style=color:#393A34>(</span><span class="token number" style=color:#36acaa>2</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>;</span><br/></div></code></pre></div></div> |
| <p>For nullable pointer carriers, opt in with <code>.nullable()</code>:</p> |
| <div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token keyword" style=color:#00009f>struct</span><span class="token plain"> </span><span class="token class-name">ConfigRef</span><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>{</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">shared_ptr</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> optional_setting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">shared_ptr</span><span class="token operator" style=color:#393A34><</span><span class="token plain">std</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token plain">string</span><span class="token operator" style=color:#393A34>></span><span class="token plain"> required_setting</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>}</span><span class="token punctuation" style=color:#393A34>;</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain" style=display:inline-block></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token function" style=color:#d73a49>FORY_STRUCT</span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">ConfigRef</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">optional_setting</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> fory</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token function" style=color:#d73a49>F</span><span class="token punctuation" style=color:#393A34>(</span><span class="token number" style=color:#36acaa>1</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>.</span><span class="token function" style=color:#d73a49>nullable</span><span class="token punctuation" style=color:#393A34>(</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"> </span><span class="token punctuation" style=color:#393A34>(</span><span class="token plain">required_setting</span><span class="token punctuation" style=color:#393A34>,</span><span class="token plain"> fory</span><span class="token double-colon punctuation" style=color:#393A34>::</span><span class="token function" style=color:#d73a49>F</span><span class="token punctuation" style=color:#393A34>(</span><span class="token number" style=color:#36acaa>2</span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>)</span><span class="token plain"></span><br/></div><div class=token-line style=color:#393A34><span class="token plain"></span><span class="token punctuation" style=color:#393A34>)</span><span class="token punctuation" style=color:#393A34>;</span><br/></div></code></pre></div></div> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=null-value-handling>Null Value Handling<a href=#null-value-handling class=hash-link aria-label="Direct link to Null Value Handling" title="Direct link to Null Value Handling" translate=no></a></h2> |
| <p>When a non-nullable field receives a null value:</p> |
| <table><thead><tr><th>Language<th>Behavior<tbody><tr><td>Java<td>Throws <code>NullPointerException</code> or serialization error<tr><td>Python<td>Raises <code>TypeError</code> or serialization error<tr><td>Rust<td>Compile-time error (non-Option types can't be None)<tr><td>Go<td>Zero value is used (empty string, 0, etc.)<tr><td>C++<td>Default-constructed value or undefined behavior</table> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=schema-compatibility>Schema Compatibility<a href=#schema-compatibility class=hash-link aria-label="Direct link to Schema Compatibility" title="Direct link to Schema Compatibility" translate=no></a></h2> |
| <p>The nullable flag is part of the struct schema fingerprint. When compatible mode is disabled, changing a field's nullability is a <strong>breaking change</strong> that will cause schema version mismatch errors.</p> |
| <div class="language-text codeBlockContainer_Ckt0 theme-code-block" style=--prism-color:#393A34;--prism-background-color:#f6f8fa><div class=codeBlockContent_QJqH><pre tabindex=0 class="prism-code language-text codeBlock_bY9V thin-scrollbar" style=color:#393A34;background-color:#f6f8fa><code class=codeBlockLines_e6Vv><div class=token-line style=color:#393A34><span class="token plain">Schema A: { name: String (non-nullable) }</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">Schema B: { name: String (nullable) }</span><br/></div><div class=token-line style=color:#393A34><span class="token plain">// These have different fingerprints when compatible mode is disabled</span><br/></div></code></pre></div></div> |
| <p>In compatible mode, top-level scalar fields can still be matched when their scalar type is otherwise compatible and the nullability or optional wrapper differs. Present values are read through compatible scalar conversion and must satisfy the normal lossless conversion checks. Remote null values follow the compatible-read null/default behavior for the local field.</p> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=best-practices>Best Practices<a href=#best-practices class=hash-link aria-label="Direct link to Best Practices" title="Direct link to Best Practices" translate=no></a></h2> |
| <ol> |
| <li class=""><strong>Use non-nullable by default</strong>: Only make fields nullable when null is a valid semantic value</li> |
| <li class=""><strong>Use Optional/Option wrappers</strong>: Instead of raw types with nullable annotation</li> |
| <li class=""><strong>Be consistent across languages</strong>: Use the same nullability for corresponding fields</li> |
| <li class=""><strong>Document nullable fields</strong>: Make it clear which fields can be null in your API</li> |
| </ol> |
| <h2 class="anchor anchorTargetStickyNavbar_Vzrq" id=see-also>See Also<a href=#see-also class=hash-link aria-label="Direct link to See Also" title="Direct link to See Also" translate=no></a></h2> |
| <ul> |
| <li class=""><a class="" href=/docs/next/guide/xlang/reference_tracking>Reference Tracking</a> - Shared and circular reference handling</li> |
| <li class=""><a class="" href=/docs/next/guide/xlang/serialization>Serialization</a> - Basic cross-language serialization</li> |
| <li class=""><a class="" href=/docs/next/specification/xlang_type_mapping>Type Mapping</a> - Cross-language type mapping reference</li> |
| <li class=""><a class="" href=/docs/next/specification/xlang_serialization_spec>Xlang Specification</a> - Binary protocol details</li> |
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