C++ Classes without class body declared outside of the namespace : <p>I'm really new to C++ Programming and I'm trying to teach it myself. While I was having a look at some code I noticed the following:</p>

<pre><code>#ifndef _someclass_h_
#define _someclass_h_

class A;
class B;
class C;

namespace somenamespace{

class SomeClass 
{
public: 

...
};
}
</code></pre>

<p>I'm confused about the classes A, B and C being declared outside of the namespace while not having any class body. What is done here? 
Does it have something to do with templates? </p>

<p>Thanks in advance!</p>
