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| <div class="section" id="conventions"> |
| <h1>Conventions<a class="headerlink" href="#conventions" title="Permalink to this headline">¶</a></h1> |
| <p>The Arrow C++ API follows a few simple guidelines. As with many rules, |
| there may be exceptions.</p> |
| <div class="section" id="language-version"> |
| <h2>Language version<a class="headerlink" href="#language-version" title="Permalink to this headline">¶</a></h2> |
| <p>Arrow is C++11-compatible. A few backports are used for newer functionality, |
| for example the <code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">std::string_view</span></code> class.</p> |
| </div> |
| <div class="section" id="namespacing"> |
| <h2>Namespacing<a class="headerlink" href="#namespacing" title="Permalink to this headline">¶</a></h2> |
| <p>All the Arrow API (except macros) is namespaced inside a <code class="docutils literal notranslate"><span class="pre">arrow</span></code> namespace, |
| and nested namespaces thereof.</p> |
| </div> |
| <div class="section" id="safe-pointers"> |
| <h2>Safe pointers<a class="headerlink" href="#safe-pointers" title="Permalink to this headline">¶</a></h2> |
| <p>Arrow objects are usually passed and stored using safe pointers – most of |
| the time <code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">std::shared_ptr</span></code> but sometimes also <code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">std::unique_ptr</span></code>.</p> |
| </div> |
| <div class="section" id="immutability"> |
| <h2>Immutability<a class="headerlink" href="#immutability" title="Permalink to this headline">¶</a></h2> |
| <p>Many Arrow objects are immutable: once constructed, their logical properties |
| cannot change anymore. This makes it possible to use them in multi-threaded |
| scenarios without requiring tedious and error-prone synchronization.</p> |
| <p>There are obvious exceptions to this, such as IO objects or mutable data buffers.</p> |
| </div> |
| <div class="section" id="error-reporting"> |
| <h2>Error reporting<a class="headerlink" href="#error-reporting" title="Permalink to this headline">¶</a></h2> |
| <p>Most APIs indicate a successful or erroneous outcome by returning a |
| <a class="reference internal" href="api/support.html#_CPPv4N5arrow6StatusE" title="arrow::Status"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Status</span></code></a> instance. Arrow doesn’t throw exceptions of its |
| own, but third-party exceptions might propagate through, especially |
| <code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">std::bad_alloc</span></code> (but Arrow doesn’t use the standard allocators for |
| large data).</p> |
| <p>When an API can return either an error code or a successful value, it usually |
| does so by returning the template class |
| <a class="reference internal" href="api/support.html#_CPPv4I0EN5arrow6ResultE" title="arrow::Result"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Result</span></code></a>. However, |
| some APIs (usually deprecated) return <a class="reference internal" href="api/support.html#_CPPv4N5arrow6StatusE" title="arrow::Status"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Status</span></code></a> and pass the |
| result value as an out-pointer parameter.</p> |
| <p>Here is an example of checking the outcome of an operation:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">const</span> <span class="kt">int64_t</span> <span class="n">buffer_size</span> <span class="o">=</span> <span class="mi">4096</span><span class="p">;</span> |
| |
| <span class="k">auto</span> <span class="n">maybe_buffer</span> <span class="o">=</span> <span class="n">arrow</span><span class="o">::</span><span class="n">AllocateBuffer</span><span class="p">(</span><span class="n">buffer_size</span><span class="p">,</span> <span class="o">&</span><span class="n">buffer</span><span class="p">);</span> |
| <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">maybe_buffer</span><span class="p">.</span><span class="n">ok</span><span class="p">())</span> <span class="p">{</span> |
| <span class="c1">// ... handle error</span> |
| <span class="p">}</span> <span class="k">else</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">Buffer</span><span class="o">></span> <span class="n">buffer</span> <span class="o">=</span> <span class="o">*</span><span class="n">maybe_buffer</span><span class="p">;</span> |
| <span class="c1">// ... use allocated buffer</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>If the caller function itself returns a <a class="reference internal" href="api/support.html#_CPPv4I0EN5arrow6ResultE" title="arrow::Result"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Result</span></code></a> or |
| <a class="reference internal" href="api/support.html#_CPPv4N5arrow6StatusE" title="arrow::Status"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Status</span></code></a> and wants to propagate any non-successful outcome, two |
| convenience macros are available:</p> |
| <ul class="simple"> |
| <li><p><a class="reference internal" href="api/support.html#c.ARROW_RETURN_NOT_OK" title="ARROW_RETURN_NOT_OK"><code class="xref c c-macro docutils literal notranslate"><span class="pre">ARROW_RETURN_NOT_OK</span></code></a> takes a <a class="reference internal" href="api/support.html#_CPPv4N5arrow6StatusE" title="arrow::Status"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Status</span></code></a> parameter |
| and returns it if not successful.</p></li> |
| <li><p><a class="reference internal" href="api/support.html#c.ARROW_ASSIGN_OR_RAISE" title="ARROW_ASSIGN_OR_RAISE"><code class="xref c c-macro docutils literal notranslate"><span class="pre">ARROW_ASSIGN_OR_RAISE</span></code></a> takes a <a class="reference internal" href="api/support.html#_CPPv4I0EN5arrow6ResultE" title="arrow::Result"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Result</span></code></a> parameter, |
| assigns its result to a <em>lvalue</em> if successful, or returns the corresponding |
| <a class="reference internal" href="api/support.html#_CPPv4N5arrow6StatusE" title="arrow::Status"><code class="xref cpp cpp-class docutils literal notranslate"><span class="pre">arrow::Status</span></code></a> on error.</p></li> |
| </ul> |
| <p>For example:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">arrow</span><span class="o">::</span><span class="n">Status</span> <span class="n">DoSomething</span><span class="p">()</span> <span class="p">{</span> |
| <span class="k">const</span> <span class="kt">int64_t</span> <span class="n">buffer_size</span> <span class="o">=</span> <span class="mi">4096</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">Buffer</span><span class="o">></span> <span class="n">buffer</span><span class="p">;</span> |
| <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="n">arrow</span><span class="o">::</span><span class="n">AllocateBuffer</span><span class="p">(</span><span class="n">buffer_size</span><span class="p">));</span> |
| <span class="c1">// ... allocation successful, do something with buffer below</span> |
| |
| <span class="c1">// return success at the end</span> |
| <span class="k">return</span> <span class="n">Status</span><span class="o">::</span><span class="n">OK</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <div class="admonition seealso"> |
| <p class="admonition-title">See also</p> |
| <p><a class="reference internal" href="api/support.html"><span class="doc">API reference for error reporting</span></a></p> |
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