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</head>
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<body>
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<h1 class="title toc-ignore">Using later from C++</h1>
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<h4 class="author">Joe Cheng</h4>
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<h4 class="date">2024-11-27</h4>
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<div id="using-later-from-c" class="section level1">
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<h1>Using later from C++</h1>
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<p>You can call <code>later::later</code> from C++ code in your own
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packages, to cause your own C-style functions to be called back. This is
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safe to call from either the main R thread or a different thread; in
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both cases, your callback will be invoked from the main R thread.</p>
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<p>To use the C++ interface, you’ll need to:</p>
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<ul>
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<li>Add <code>later</code> to your <code>DESCRIPTION</code> file, under
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both <code>LinkingTo</code> and <code>Imports</code></li>
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<li>Make sure that your <code>NAMESPACE</code> file has an
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<code>import(later)</code> entry. If your package uses roxygen2, you can
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do this by adding the following lines to any file under
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<code>R/</code>:</li>
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</ul>
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<pre><code>#' @import later
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NULL</code></pre>
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<ul>
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<li>Add <code>#include <later_api.h></code> to the top of each C++
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file that uses the below APIs.</li>
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</ul>
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<div id="executing-a-c-function-later" class="section level2">
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<h2>Executing a C function later</h2>
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<p>The <code>later::later</code> function is accessible from
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<code>later_api.h</code> and its prototype looks like this:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> later<span class="op">(</span><span class="dt">void</span> <span class="op">(*</span>func<span class="op">)(</span><span class="dt">void</span><span class="op">*),</span> <span class="dt">void</span><span class="op">*</span> data<span class="op">,</span> <span class="dt">double</span> secs<span class="op">)</span></span></code></pre></div>
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<p>The first argument is a pointer to a function that takes one
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<code>void*</code> argument and returns void. The second argument is a
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<code>void*</code> that will be passed to the function when it’s called
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back. And the third argument is the number of seconds to wait (at a
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minimum) before invoking. In all cases, the function will be invoked on
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the R thread, when no user R code is executing.</p>
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</div>
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<div id="background-tasks" class="section level2">
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<h2>Background tasks</h2>
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<p>This package also offers a higher-level C++ helper class called
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<code>later::BackgroundTask</code>, to make it easier to execute tasks
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on a background thread. It takes care of launching the background thread
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for you, and returning control back to the R thread at a later point;
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you’re responsible for providing the actual code that executes on the
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background thread, as well as code that executes on the R thread before
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and after the background task completes.</p>
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<p>Its public/protected interface looks like this:</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> BackgroundTask <span class="op">{</span></span>
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<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="kw">public</span><span class="op">:</span></span>
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<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> BackgroundTask<span class="op">();</span></span>
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<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a> <span class="kw">virtual</span> <span class="op">~</span>BackgroundTask<span class="op">();</span></span>
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||
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a></span>
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||
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// Start executing the task </span></span>
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||
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a> <span class="dt">void</span> begin<span class="op">();</span></span>
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||
<span id="cb3-9"><a href="#cb3-9" aria-hidden="true" tabindex="-1"></a></span>
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||
<span id="cb3-10"><a href="#cb3-10" aria-hidden="true" tabindex="-1"></a><span class="kw">protected</span><span class="op">:</span></span>
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||
<span id="cb3-11"><a href="#cb3-11" aria-hidden="true" tabindex="-1"></a> <span class="co">// The task to be executed on the background thread.</span></span>
|
||
<span id="cb3-12"><a href="#cb3-12" aria-hidden="true" tabindex="-1"></a> <span class="co">// Neither the R runtime nor any R data structures may be</span></span>
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||
<span id="cb3-13"><a href="#cb3-13" aria-hidden="true" tabindex="-1"></a> <span class="co">// touched from the background thread; any values that need</span></span>
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||
<span id="cb3-14"><a href="#cb3-14" aria-hidden="true" tabindex="-1"></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" aria-hidden="true" tabindex="-1"></a> <span class="co">// included as fields on the Task subclass object.</span></span>
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<span id="cb3-16"><a href="#cb3-16" aria-hidden="true" tabindex="-1"></a> <span class="kw">virtual</span> <span class="dt">void</span> execute<span class="op">()</span> <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
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||
<span id="cb3-17"><a href="#cb3-17" aria-hidden="true" tabindex="-1"></a> </span>
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||
<span id="cb3-18"><a href="#cb3-18" aria-hidden="true" tabindex="-1"></a> <span class="co">// A short task that runs on the main R thread after the</span></span>
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<span id="cb3-19"><a href="#cb3-19" aria-hidden="true" tabindex="-1"></a> <span class="co">// background task has completed. It's safe to access the</span></span>
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<span id="cb3-20"><a href="#cb3-20" aria-hidden="true" tabindex="-1"></a> <span class="co">// R runtime and R data structures from here.</span></span>
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<span id="cb3-21"><a href="#cb3-21" aria-hidden="true" tabindex="-1"></a> <span class="kw">virtual</span> <span class="dt">void</span> complete<span class="op">()</span> <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
|
||
<span id="cb3-22"><a href="#cb3-22" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></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" aria-hidden="true" tabindex="-1"></a><span class="sc">/</span><span class="er">/</span> [[Rcpp<span class="sc">::</span>export]]</span>
|
||
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>void <span class="fu">asyncMean</span>(Rcpp<span class="sc">::</span>NumericVector data) {</span>
|
||
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a> (new <span class="fu">MyTask</span>(data))<span class="ot">-></span><span class="fu">begin</span>();</span>
|
||
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></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. We’ll soon be introducing a
|
||
complementary R package that provides a suitable “promise” or “future”
|
||
abstraction.</p>
|
||
</div>
|
||
</div>
|
||
|
||
|
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|
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