 <p>_load_frames is a member function of the struct DataSet.</p> <p>I would presume that given the leading underscore it is a private method.</p> <pre><code>struct DataSet { private: //function declaration std::vector&lt;Frame&gt; _load_frames(const std::string&amp; basepath); }; </code></pre> <p>I would recommend reading a book or taking some classes on c++ as this is a fairly fundamental concept.</p> <p><a href="http://www.tutorialspoint.com/cplusplus/cpp_class_member_functions.htm" rel="nofollow">http://www.tutorialspoint.com/cplusplus/cpp_class_member_functions.htm</a> <a href="https://www.amazon.co.uk/Jumping-into-C-Alex-Allain-ebook/dp/B00F9311YC" rel="nofollow">https://www.amazon.co.uk/Jumping-into-C-Alex-Allain-ebook/dp/B00F9311YC</a> <a href="https://www.amazon.co.uk/C-Programming-Language-Bjarne-Stroustrup-ebook/dp/B00DUW4BMS" rel="nofollow">https://www.amazon.co.uk/C-Programming-Language-Bjarne-Stroustrup-ebook/dp/B00DUW4BMS</a></p>