 <p>You need to include the appropriate header for <a href="http://en.cppreference.com/w/cpp/algorithm/transform" rel="nofollow"><code>std::transform</code></a>:</p> <pre><code>#include &lt;algorithm&gt; </code></pre> <p>You should also avoid <code>using namespace std;</code> in the global namespace in headers, you pollute the global namespace of any code that includes your header file.</p> <p>See <a href="http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice?rq=1">this post about <code>using namespace std</code></a>.</p>