title: C++ #define directive
body: <p>There is the next code:</p>
  <pre><code>#include &lt;iostream&gt;
  #define F(x) (2*x*x)
  using namespace std;
  
  int main()
  {
      int a = 1, b = 2;
      int res = F(a + b);
  }   
  </code></pre>
  <p>The res value is 6, but why is it?</p>