title: c++ namespace problem
body: <p>hi 
  can any one help me with this code. i'm new to c++</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include&lt;iostream&gt;
  using namespace std;
  
  namespace manish
  {
    double mean(double a, double b);
  }
  
  double mean(double a, double b)
  {
    return (a+b) / 2;
  }
  </code></pre>
  
  <p>i want to make a static libray for this i use the command
   g++ calc_mean.cpp -o mean.o
   it give me following error</p>
  
  <pre><code>/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/crt1.o: In function `_start':
  (.text+0x20): undefined reference to `main'
  </code></pre>
  
  <p>can anyone tell me what i'm doing wrong?
  or how to make a namespace for a function so that it can be used in another place by using its name space</p>