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} code > span.fu { color: #900; font-weight: bold; } code > span.er { color: #a61717; background-color: #e3d2d2; } </style> </head> <body> <h1 class="title toc-ignore">Using later from C++</h1> <h4 class="author">Joe Cheng</h4> <h4 class="date">2021-08-18</h4> <div id="using-later-from-c" class="section level1"> <h1>Using later from C++</h1> <p>You can call <code>later::later</code> from C++ code in your own packages, to cause your own C-style functions to be called back. This is safe to call from either the main R thread or a different thread; in both cases, your callback will be invoked from the main R thread.</p> <p>To use the C++ interface, you’ll need to:</p> <ul> <li>Add <code>later</code> to your <code>DESCRIPTION</code> file, under both <code>LinkingTo</code> and <code>Imports</code></li> <li>Make sure that your <code>NAMESPACE</code> file has an <code>import(later)</code> entry. If your package uses roxygen2, you can do this by adding the following lines to any file under <code>R/</code>:</li> </ul> <pre><code>#' @import later NULL</code></pre> <ul> <li>Add <code>#include <later_api.h></code> to the top of each C++ file that uses the below APIs.</li> </ul> <div id="executing-a-c-function-later" class="section level2"> <h2>Executing a C function later</h2> <p>The <code>later::later</code> function is accessible from <code>later_api.h</code> and its prototype looks like this:</p> <div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="dt">void</span> later(<span class="dt">void</span> (*func)(<span class="dt">void</span>*), <span class="dt">void</span>* data, <span class="dt">double</span> secs)</span></code></pre></div> <p>The first argument is a pointer to a function that takes one <code>void*</code> argument and returns void. The second argument is a <code>void*</code> that will be passed to the function when it’s called back. And the third argument is the number of seconds to wait (at a minimum) before invoking. In all cases, the function will be invoked on the R thread, when no user R code is executing.</p> </div> <div id="background-tasks" class="section level2"> <h2>Background tasks</h2> <p>This package also offers a higher-level C++ helper class called <code>later::BackgroundTask</code>, to make it easier to execute tasks on a background thread. It takes care of launching the background thread for you, and returning control back to the R thread at a later point; you’re responsible for providing the actual code that executes on the background thread, as well as code that executes on the R thread before and after the background task completes.</p> <p>Its public/protected interface looks like this:</p> <div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">class</span> BackgroundTask {</span> <span id="cb3-2"><a href="#cb3-2"></a></span> <span id="cb3-3"><a href="#cb3-3"></a><span class="kw">public</span>:</span> <span id="cb3-4"><a href="#cb3-4"></a> BackgroundTask();</span> <span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">virtual</span> ~BackgroundTask();</span> <span id="cb3-6"><a href="#cb3-6"></a></span> <span id="cb3-7"><a href="#cb3-7"></a> <span class="co">// Start executing the task </span></span> <span id="cb3-8"><a href="#cb3-8"></a> <span class="dt">void</span> begin();</span> <span id="cb3-9"><a href="#cb3-9"></a></span> <span id="cb3-10"><a href="#cb3-10"></a><span class="kw">protected</span>:</span> <span id="cb3-11"><a href="#cb3-11"></a> <span class="co">// The task to be executed on the background thread.</span></span> <span id="cb3-12"><a href="#cb3-12"></a> <span class="co">// Neither the R runtime nor any R data structures may be</span></span> <span id="cb3-13"><a href="#cb3-13"></a> <span class="co">// touched from the background thread; any values that need</span></span> <span id="cb3-14"><a href="#cb3-14"></a> <span class="co">// to be passed into or out of the Execute method must be</span></span> <span id="cb3-15"><a href="#cb3-15"></a> <span class="co">// included as fields on the Task subclass object.</span></span> <span id="cb3-16"><a href="#cb3-16"></a> <span class="kw">virtual</span> <span class="dt">void</span> execute() = <span class="dv">0</span>;</span> <span id="cb3-17"><a href="#cb3-17"></a> </span> <span id="cb3-18"><a href="#cb3-18"></a> <span class="co">// A short task that runs on the main R thread after the</span></span> <span id="cb3-19"><a href="#cb3-19"></a> <span class="co">// background task has completed. It's safe to access the</span></span> <span id="cb3-20"><a href="#cb3-20"></a> <span class="co">// R runtime and R data structures from here.</span></span> <span id="cb3-21"><a href="#cb3-21"></a> <span class="kw">virtual</span> <span class="dt">void</span> complete() = <span class="dv">0</span>;</span> <span id="cb3-22"><a href="#cb3-22"></a>}</span></code></pre></div> <p>Create your own subclass, implementing a custom constructor plus the <code>execute</code> and <code>complete</code> methods.</p> <p>It’s critical that the code in your <code>execute</code> method not mutate any R data structures, call any R code, or cause any R allocations, as it will execute in a background thread where such operations are unsafe. You can, however, perform such operations in the constructor (assuming you perform construction only from the main R thread) and <code>complete</code> method. Pass values between the constructor and methods using fields.</p> <pre class="rcpp"><code>#include <Rcpp.h> #include <later_api.h> class MyTask : public later::BackgroundTask { public: MyTask(Rcpp::NumericVector vec) : inputVals(Rcpp::as<std::vector<double> >(vec)) { } protected: void execute() { double sum = 0; for (std::vector<double>::const_iterator it = inputVals.begin(); it != inputVals.end(); it++) { sum += *it; } result = sum / inputVals.size(); } void complete() { Rprintf("Result is %f\n", result); } private: std::vector<double> inputVals; double result; };</code></pre> <p>To run the task, <code>new</code> up your subclass and call <code>begin()</code>, e.g. <code>(new MyTask(vec))->begin()</code>. There’s no need to keep track of the pointer; the task object will delete itself when the task is complete.</p> <div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1"></a><span class="op">/</span><span class="er">/</span><span class="st"> </span>[[Rcpp<span class="op">::</span>export]]</span> <span id="cb5-2"><a href="#cb5-2"></a>void <span class="kw">asyncMean</span>(Rcpp<span class="op">::</span>NumericVector data) {</span> <span id="cb5-3"><a href="#cb5-3"></a> (new <span class="kw">MyTask</span>(data))-><span class="kw">begin</span>();</span> <span id="cb5-4"><a href="#cb5-4"></a>}</span></code></pre></div> <p>It’s not very useful to execute tasks on background threads if you can’t get access to the results back in R. 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