title: C++ header file: error: expected unqualified-id before ')' token
body: <p>Sorry for asking such a beginner question, but I'm stuck writing header files. For information, there aren't StackOverflow articles relating to the problem I have -- it seems like I've got a very simple setting wrong but couldn't find it on the internet.</p>
  <p>Error message:</p>
  <pre><code>insertion/insertion.h:16:24: error: expected unqualified-id before ')' token   
     16 |         void insertion();
  </code></pre>
  <p>insertion.cpp:</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;insertion.h&gt;
  // more imports
  
  void insertion() {
      // implementation not shown
  }
  
  int main() {
      insertion();
      return 0;
  }
  </code></pre>
  <p>insertion.h:</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;iostream&gt;
  // exactly the same imports as the file above but didn't import itself
  
  #ifndef insertion
  #define insertion
  
  class Insertion {
      public:
          void insertion();
  };
  
  #endif 
  </code></pre>
  <p>execute.cpp:</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &quot;insertion/insertion.h&quot;
  using namespace std;
  
  int main() {
      // still writing
  }
  </code></pre>
  <p>The file structure looks like this:</p>
  <pre><code>|- execute.cpp
  |- insertion\
  |---- insertion.cpp
  |---- insertion.h
  </code></pre>
  <p>Any help is greatly appreciated.</p>