<p>In <a href="https://github.com/mlpack/mlpack/pull/650#discussion_r64749271">src/mlpack/methods/preprocess/preprocess_split_main.cpp</a>:</p>
<pre style='color:#555'>&gt; +  const string outputTestLabel = CLI::GetParam&lt;string&gt;(&quot;output_test_label&quot;);
&gt; +
&gt; +  // Ratio
&gt; +  const double testRatio = CLI::GetParam&lt;double&gt;(&quot;test_ratio&quot;);
&gt; +
&gt; +  // container for input data and labels
&gt; +  arma::mat data;
&gt; +  arma::Mat&lt;size_t&gt; labels;
&gt; +
&gt; +  // Load Data and Labels
&gt; +  data::Load(inputFile, data, true);
&gt; +  data::Load(inputLabel, labels, true);
&gt; +  arma::Row&lt;size_t&gt; labels_row = labels.row(0); // extract first row
&gt; +
&gt; +  // Split Data
&gt; +  const auto value = data::TrainTestSplit(data, labels_row, testRatio);
</pre>
<p>Does make_tuple call the copy constructor? If that's the case we should avoid this interface in favor of the pass by reference interface.</p>

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