title: Error : aggregate 'first one' has incomplete type and cannot be defined
body: <p>I have written this header file <code>(header1.h)</code>:</p>
  
  <pre><code>#ifndef HEADER1_H
  #define HEADER1_H
  
  class first ;
  
  //int summ(int a , int b) ;
  
  
  
  #endif
  </code></pre>
  
  <p>and this source files <code>(header1.cpp and main.cpp)</code>:</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include "header1.h"
  
  using namespace std;
  
  
  class first
  {
      public:
    int a,b,c;
    int sum(int a , int b);
  
  };
  
    int first::sum(int a , int b)
  {
  
      return a+b;
  }
  </code></pre>
  
  <p>&nbsp;</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include "header1.h"
  
  
  using namespace std;
  
  
     first one;
  
  int main()
  {
     int j=one.sum(2,4);
      cout &lt;&lt;  j&lt;&lt; endl;
      return 0;
  }
  </code></pre>
  
  <p>But when I run this program in <code>codeblocks</code> , I give this Error :</p>
  
  <blockquote>
    <blockquote>
      <p>aggregate 'first one' has incomplete type and cannot be defined .</p>
    </blockquote>
  </blockquote>