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  <section id="objects-binding-and-lifetime">
<h1>Objects binding and lifetime<a class="headerlink" href="#objects-binding-and-lifetime" title="Permalink to this headline">¶</a></h1>
<p>Crown has a native runtime written in C++. To guarantee good performance and
ease of use, we employ a number of different techniques when binding C-side
objects to the Lua environment, depending on their type and lifetime.</p>
<section id="light-userdata-binding">
<h2>Light userdata binding<a class="headerlink" href="#light-userdata-binding" title="Permalink to this headline">¶</a></h2>
<p>Most objects are exposed to Lua via Light userdata. Light userdata objects are
owned by the C side and do not require garbage collection.</p>
<p>One disadvantage is that individual Light userdata objects cannot have a
metatable, so the regular object notation is not available when calling them:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="n">world</span><span class="p">:</span><span class="n">spawn_unit</span><span class="p">(</span><span class="s2">&quot;units/grass&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>Instead you must explicitly look up the function in the World table and pass
<cite>world</cite> as a parameter:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="n">World</span><span class="p">.</span><span class="n">spawn_unit</span><span class="p">(</span><span class="n">world</span><span class="p">,</span> <span class="s2">&quot;units/grass&quot;</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="full-userdata-binding">
<h2>Full userdata binding<a class="headerlink" href="#full-userdata-binding" title="Permalink to this headline">¶</a></h2>
<p>Full userdata objects are owned by the Lua side, they are heap-allocated and
subject to gargbage collection. They support metatables and thus offer object
notation and operator overloading.</p>
<p>We make sporadic use of full userdata, due to their runtime overhead. One such
example are the “boxed” versions of regular math objects (Vector3Box,
Matrix4x4Box etc).</p>
</section>
<section id="singleton-objects">
<h2>Singleton objects<a class="headerlink" href="#singleton-objects" title="Permalink to this headline">¶</a></h2>
<p>Objects which only have a single instance of them in the runtime are called
singletons. Singletons are typically created at program start and live until the
runtime exits.</p>
<p>Since they always refer to the same instance, you don’t need to specify it when
calling their functions:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="n">Device</span><span class="p">.</span><span class="n">version</span><span class="p">()</span>
</pre></div>
</div>
</section>
<section id="temporary-objects">
<h2>Temporary objects<a class="headerlink" href="#temporary-objects" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you want an object to be owned by the Lua side, but without the
performance implications of a garbage-collected Full userdata object. Vectors
and matrices are a perfect example of that.</p>
<p>Crown uses a fixed-size memory pool to allocate temporary objects from. The pool
is reset at each frame so temporaries are only valid in the frame they are
created.</p>
<p>To store a temporary Vector3 for use in later frames, you need to copy its value
into something else with a longer lifespan, for example, you could “box” it in a
Vector3Box:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="kd">local</span> <span class="n">pos</span> <span class="o">=</span> <span class="n">Vector3Box</span><span class="p">()</span>

<span class="kr">function</span> <span class="nf">update</span><span class="p">(</span><span class="n">dt</span><span class="p">)</span>
        <span class="kd">local</span> <span class="n">p</span> <span class="o">=</span> <span class="n">SceneGraph</span><span class="p">.</span><span class="n">local_position</span><span class="p">(...)</span>
        <span class="n">pos</span><span class="p">:</span><span class="n">store</span><span class="p">(</span><span class="n">p</span><span class="p">)</span>
<span class="kr">end</span>
</pre></div>
</div>
<p>And later retrieve its value with:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="kd">local</span> <span class="n">p</span> <span class="o">=</span> <span class="n">pos</span><span class="p">:</span><span class="n">unbox</span><span class="p">()</span>
</pre></div>
</div>
<p>Crown will report an error when it detects a stale temporary object that is used
across-frames.</p>
<p>If you have particularly long computations, you may run out of temporary
objects. This could happen when doing math inside loops for example. You can use
<code class="docutils literal notranslate"><span class="pre">Device.temp_count()</span></code> and <code class="docutils literal notranslate"><span class="pre">Device.set_temp_count()</span></code> to save and restore the
number of temporary objects used before and after computations:</p>
<div class="highlight-lua notranslate"><div class="highlight"><pre><span></span><span class="kr">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">v</span> <span class="kr">in</span> <span class="nb">pairs</span><span class="p">(</span><span class="n">objects</span><span class="p">)</span> <span class="kr">do</span>
        <span class="kd">local</span> <span class="n">nv</span><span class="p">,</span> <span class="n">nq</span><span class="p">,</span> <span class="n">nm</span> <span class="o">=</span> <span class="n">Device</span><span class="p">.</span><span class="n">temp_count</span><span class="p">()</span>

        <span class="p">...</span>

        <span class="n">Device</span><span class="p">.</span><span class="n">set_temp_count</span><span class="p">(</span><span class="n">nv</span><span class="p">,</span> <span class="n">nq</span><span class="p">,</span> <span class="n">nm</span><span class="p">)</span>
<span class="kr">end</span>
</pre></div>
</div>
</section>
</section>


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