title: Add a method to existing C++ class in other file
body: <p>Is it possible in C++ to extend a class(add a method) in a different source file without editing the original source file where the class is written?</p>
  <p>In obj-c it is possible by writing another <code>@interface AbcClass (ExtCategory) ... @end</code></p>
  <p>I got compile-time error(s) when I tried something like this:</p>
  <pre><code>//Abc.h
  class Abc {            //This class is from a 3rd party library....
                         //  ...I don't want to edit its source file.
      void methodOne();
      void methodTwo();
  
  }
  
  
  //Abc+Ext.h
  class Abc {       // ERROR: Redefinition of 'Abc'
      void methodAdded();
  }
  </code></pre>
  <p>My target is to retain the 'Abc' name and add methods to it. A specific class in a 3rd party library that I used lacks some methods and I want to add those methods but I am keeping the source file unedited.</p>
  <p>Is there a way to do this? I am new in writing C++ codes. I am familiar with some of its syntax but don't know much.</p>