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| <section id="data-types"> |
| <h1>Data Types<a class="headerlink" href="#data-types" title="Permalink to this heading">#</a></h1> |
| <div class="admonition seealso"> |
| <p class="admonition-title">See also</p> |
| <p><a class="reference internal" href="api/datatype.html"><span class="doc">Datatype API reference</span></a>.</p> |
| </div> |
| <p>Data types govern how physical data is interpreted. Their <a class="reference internal" href="../format/Columnar.html#format-columnar"><span class="std std-ref">specification</span></a> allows binary interoperability between different Arrow |
| implementations, including from different programming languages and runtimes |
| (for example it is possible to access the same data, without copying, from |
| both Python and Java using the <code class="xref py py-mod docutils literal notranslate"><span class="pre">pyarrow.jvm</span></code> bridge module).</p> |
| <p>Information about a data type in C++ can be represented in three ways:</p> |
| <ol class="arabic simple"> |
| <li><p>Using a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a> instance (e.g. as a function argument)</p></li> |
| <li><p>Using a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a> concrete subclass (e.g. as a template |
| parameter)</p></li> |
| <li><p>Using a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow4Type4typeE" title="arrow::Type::type"><code class="xref cpp cpp-type docutils literal notranslate"><span class="pre">arrow::Type::type</span></code></a> enum value (e.g. as the condition of |
| a switch statement)</p></li> |
| </ol> |
| <p>The first form (using a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a> instance) is the most idiomatic |
| and flexible. Runtime-parametric types can only be fully represented with |
| a DataType instance. For example, a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow13TimestampTypeE" title="arrow::TimestampType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::TimestampType</span></code></a> needs to be |
| constructed at runtime with a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8TimeUnit4typeE" title="arrow::TimeUnit::type"><code class="xref cpp cpp-type docutils literal notranslate"><span class="pre">arrow::TimeUnit::type</span></code></a> parameter; a |
| <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow14Decimal128TypeE" title="arrow::Decimal128Type"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Decimal128Type</span></code></a> with <em>scale</em> and <em>precision</em> parameters; |
| a <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8ListTypeE" title="arrow::ListType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::ListType</span></code></a> with a full child type (itself a |
| <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a> instance).</p> |
| <p>The two other forms can be used where performance is critical, in order to |
| avoid paying the price of dynamic typing and polymorphism. However, some |
| amount of runtime switching can still be required for parametric types. |
| It is not possible to reify all possible types at compile time, since Arrow |
| data types allows arbitrary nesting.</p> |
| <section id="creating-data-types"> |
| <h2>Creating data types<a class="headerlink" href="#creating-data-types" title="Permalink to this heading">#</a></h2> |
| <p>To instantiate data types, it is recommended to call the provided |
| <a class="reference internal" href="api/datatype.html#api-type-factories"><span class="std std-ref">factory functions</span></a>:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">arrow</span><span class="o">::</span><span class="n">DataType</span><span class="o">></span><span class="w"> </span><span class="n">type</span><span class="p">;</span> |
| |
| <span class="c1">// A 16-bit integer type</span> |
| <span class="n">type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">int16</span><span class="p">();</span> |
| <span class="c1">// A 64-bit timestamp type (with microsecond granularity)</span> |
| <span class="n">type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">timestamp</span><span class="p">(</span><span class="n">arrow</span><span class="o">::</span><span class="n">TimeUnit</span><span class="o">::</span><span class="n">MICRO</span><span class="p">);</span> |
| <span class="c1">// A list type of single-precision floating-point values</span> |
| <span class="n">type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">list</span><span class="p">(</span><span class="n">arrow</span><span class="o">::</span><span class="n">float32</span><span class="p">());</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="type-traits"> |
| <h2>Type Traits<a class="headerlink" href="#type-traits" title="Permalink to this heading">#</a></h2> |
| <p>Writing code that can handle concrete <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a> subclasses would |
| be verbose, if it weren’t for type traits. Arrow’s type traits map the Arrow |
| data types to the specialized array, scalar, builder, and other associated types. |
| For example, the Boolean type has traits:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">template</span><span class="w"> </span><span class="o"><></span> |
| <span class="k">struct</span><span class="w"> </span><span class="nc">TypeTraits</span><span class="o"><</span><span class="n">BooleanType</span><span class="o">></span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">ArrayType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">BooleanArray</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">BuilderType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">BooleanBuilder</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">ScalarType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">BooleanScalar</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">CType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">bool</span><span class="p">;</span> |
| |
| <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">int64_t</span><span class="w"> </span><span class="nf">bytes_required</span><span class="p">(</span><span class="kt">int64_t</span><span class="w"> </span><span class="n">elements</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">bit_util</span><span class="o">::</span><span class="n">BytesForBits</span><span class="p">(</span><span class="n">elements</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="n">is_parameter_free</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">true</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="kr">inline</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">DataType</span><span class="o">></span><span class="w"> </span><span class="n">type_singleton</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">boolean</span><span class="p">();</span><span class="w"> </span><span class="p">}</span> |
| <span class="p">};</span> |
| </pre></div> |
| </div> |
| <p>See the <a class="reference internal" href="api/utilities.html#type-traits"><span class="std std-ref">Type Traits</span></a> for an explanation of each of these fields.</p> |
| <p>Using type traits, one can write template functions that can handle a variety |
| of Arrow types. For example, to write a function that creates an array of |
| Fibonacci values for any Arrow numeric type:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">DataType</span><span class="p">,</span> |
| <span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">BuilderType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">arrow</span><span class="o">::</span><span class="n">TypeTraits</span><span class="o"><</span><span class="n">DataType</span><span class="o">>::</span><span class="n">BuilderType</span><span class="p">,</span> |
| <span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">ArrayType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">arrow</span><span class="o">::</span><span class="n">TypeTraits</span><span class="o"><</span><span class="n">DataType</span><span class="o">>::</span><span class="n">ArrayType</span><span class="p">,</span> |
| <span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">CType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">arrow</span><span class="o">::</span><span class="n">TypeTraits</span><span class="o"><</span><span class="n">DataType</span><span class="o">>::</span><span class="n">CType</span><span class="o">></span> |
| <span class="n">arrow</span><span class="o">::</span><span class="n">Result</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">ArrayType</span><span class="o">>></span><span class="w"> </span><span class="n">MakeFibonacci</span><span class="p">(</span><span class="kt">int32_t</span><span class="w"> </span><span class="n">n</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">BuilderType</span><span class="w"> </span><span class="n">builder</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">CType</span><span class="w"> </span><span class="n">val</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">CType</span><span class="w"> </span><span class="n">next_val</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int32_t</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">builder</span><span class="p">.</span><span class="n">Append</span><span class="p">(</span><span class="n">val</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">CType</span><span class="w"> </span><span class="n">temp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">val</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">next_val</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">val</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">next_val</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">next_val</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">temp</span><span class="p">;</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">ArrayType</span><span class="o">></span><span class="w"> </span><span class="n">out</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">ARROW_RETURN_NOT_OK</span><span class="p">(</span><span class="n">builder</span><span class="p">.</span><span class="n">Finish</span><span class="p">(</span><span class="o">&</span><span class="n">out</span><span class="p">));</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">out</span><span class="p">;</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>For some common cases, there are type associations on the classes themselves. Use:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">Scalar::TypeClass</span></code> to get data type class of a scalar</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">Array::TypeClass</span></code> to get data type class of an array</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">DataType::c_type</span></code> to get associated C type of an Arrow data type</p></li> |
| </ul> |
| <p>Similar to the type traits provided in |
| <a class="reference external" href="https://en.cppreference.com/w/cpp/header/type_traits">std::type_traits</a>, |
| Arrow provides type predicates such as <code class="docutils literal notranslate"><span class="pre">is_number_type</span></code> as well as |
| corresponding templates that wrap <code class="docutils literal notranslate"><span class="pre">std::enable_if_t</span></code> such as <code class="docutils literal notranslate"><span class="pre">enable_if_number</span></code>. |
| These can constrain template functions to only compile for relevant types, which |
| is useful if other overloads need to be implemented. For example, to write a sum |
| function for any numeric (integer or float) array:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">ArrayType</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">DataType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">ArrayType</span><span class="o">::</span><span class="n">TypeClass</span><span class="p">,</span> |
| <span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">CType</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">DataType</span><span class="o">::</span><span class="n">c_type</span><span class="o">></span> |
| <span class="n">arrow</span><span class="o">::</span><span class="n">enable_if_number</span><span class="o"><</span><span class="n">DataType</span><span class="p">,</span><span class="w"> </span><span class="n">CType</span><span class="o">></span><span class="w"> </span><span class="n">SumArray</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">ArrayType</span><span class="o">&</span><span class="w"> </span><span class="n">array</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">CType</span><span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o"><</span><span class="n">CType</span><span class="o">></span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">array</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">value</span><span class="p">.</span><span class="n">has_value</span><span class="p">())</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="n">value</span><span class="p">.</span><span class="n">value</span><span class="p">();</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">sum</span><span class="p">;</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>See <a class="reference internal" href="api/utilities.html#type-predicates-api"><span class="std std-ref">Type Predicates</span></a> for a list of these.</p> |
| </section> |
| <section id="visitor-pattern"> |
| <span id="cpp-visitor-pattern"></span><h2>Visitor Pattern<a class="headerlink" href="#visitor-pattern" title="Permalink to this heading">#</a></h2> |
| <p>In order to process <a class="reference internal" href="api/datatype.html#_CPPv4N5arrow8DataTypeE" title="arrow::DataType"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::DataType</span></code></a>, <a class="reference internal" href="api/scalar.html#_CPPv4N5arrow6ScalarE" title="arrow::Scalar"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Scalar</span></code></a>, or |
| <a class="reference internal" href="api/array.html#_CPPv4N5arrow5ArrayE" title="arrow::Array"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Array</span></code></a>, you may need to write logic that specializes based |
| on the particular Arrow type. In these cases, use the |
| <a class="reference external" href="https://en.wikipedia.org/wiki/Visitor_pattern">visitor pattern</a>. Arrow provides |
| the template functions:</p> |
| <ul class="simple"> |
| <li><p><a class="reference internal" href="api/utilities.html#_CPPv4I0DpEN5arrow15VisitTypeInlineE6StatusRK8DataTypeP7VISITORDpRR4ARGS" title="arrow::VisitTypeInline"><code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">arrow::VisitTypeInline()</span></code></a></p></li> |
| <li><p><a class="reference internal" href="api/utilities.html#_CPPv4I0DpEN5arrow17VisitScalarInlineE6StatusRK6ScalarP7VISITORDpRR4ARGS" title="arrow::VisitScalarInline"><code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">arrow::VisitScalarInline()</span></code></a></p></li> |
| <li><p><a class="reference internal" href="api/utilities.html#_CPPv4I0DpEN5arrow16VisitArrayInlineE6StatusRK5ArrayP7VISITORDpRR4ARGS" title="arrow::VisitArrayInline"><code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">arrow::VisitArrayInline()</span></code></a></p></li> |
| </ul> |
| <p>To use these, implement <code class="docutils literal notranslate"><span class="pre">Status</span> <span class="pre">Visit()</span></code> methods for each specialized type, then |
| pass the class instance to the inline visit function. To avoid repetitive code, |
| use type traits as documented in the previous section. As a brief example, |
| here is how one might sum across columns of arbitrary numeric types:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">TableSummation</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">partial</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0.0</span><span class="p">;</span> |
| <span class="w"> </span><span class="k">public</span><span class="o">:</span> |
| |
| <span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Result</span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="w"> </span><span class="n">Compute</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">arrow</span><span class="o">::</span><span class="n">RecordBatch</span><span class="o">></span><span class="w"> </span><span class="n">batch</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">arrow</span><span class="o">::</span><span class="n">Array</span><span class="o">></span><span class="w"> </span><span class="n">array</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">batch</span><span class="o">-></span><span class="n">columns</span><span class="p">())</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">ARROW_RETURN_NOT_OK</span><span class="p">(</span><span class="n">arrow</span><span class="o">::</span><span class="n">VisitArrayInline</span><span class="p">(</span><span class="o">*</span><span class="n">array</span><span class="p">,</span><span class="w"> </span><span class="k">this</span><span class="p">));</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">partial</span><span class="p">;</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="c1">// Default implementation</span> |
| <span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Status</span><span class="w"> </span><span class="n">Visit</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Array</span><span class="o">&</span><span class="w"> </span><span class="n">array</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Status</span><span class="o">::</span><span class="n">NotImplemented</span><span class="p">(</span><span class="s">"Cannot compute sum for array of type "</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">array</span><span class="p">.</span><span class="n">type</span><span class="p">()</span><span class="o">-></span><span class="n">ToString</span><span class="p">());</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">ArrayType</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">ArrayType</span><span class="o">::</span><span class="n">TypeClass</span><span class="o">></span> |
| <span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">enable_if_number</span><span class="o"><</span><span class="n">T</span><span class="p">,</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Status</span><span class="o">></span><span class="w"> </span><span class="n">Visit</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">ArrayType</span><span class="o">&</span><span class="w"> </span><span class="n">array</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="o">::</span><span class="n">c_type</span><span class="o">></span><span class="w"> </span><span class="n">value</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">array</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">value</span><span class="p">.</span><span class="n">has_value</span><span class="p">())</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">partial</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="k">static_cast</span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">(</span><span class="n">value</span><span class="p">.</span><span class="n">value</span><span class="p">());</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">arrow</span><span class="o">::</span><span class="n">Status</span><span class="o">::</span><span class="n">OK</span><span class="p">();</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">};</span> |
| </pre></div> |
| </div> |
| <p>Arrow also provides abstract visitor classes (<a class="reference internal" href="api/datatype.html#_CPPv4N5arrow11TypeVisitorE" title="arrow::TypeVisitor"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::TypeVisitor</span></code></a>, |
| <a class="reference internal" href="api/scalar.html#_CPPv4N5arrow13ScalarVisitorE" title="arrow::ScalarVisitor"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::ScalarVisitor</span></code></a>, <a class="reference internal" href="api/array.html#_CPPv4N5arrow12ArrayVisitorE" title="arrow::ArrayVisitor"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::ArrayVisitor</span></code></a>) and an <code class="docutils literal notranslate"><span class="pre">Accept()</span></code> |
| method on each of the corresponding base types (e.g. <a class="reference internal" href="api/array.html#_CPPv4NK5arrow5Array6AcceptEP12ArrayVisitor" title="arrow::Array::Accept"><code class="xref cpp cpp-func docutils literal notranslate"><span class="pre">arrow::Array::Accept()</span></code></a>). |
| However, these are not able to be implemented using template functions, so you |
| will typically prefer using the inline type visitors.</p> |
| </section> |
| </section> |
| |
| |
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