title: fatal error LNK1120: 1 unresolved externals
body: <p>I am compiling the following code and I am receiving an error. I want to practice templates in boost and I do not know how to handle this problem </p>
  
  <pre><code>#include &lt;stdafx.h&gt;
  #include &lt;iostream&gt;
  #include &lt;string&gt;
  #include &lt;boost/function.hpp&gt; 
  #include &lt;boost/array.hpp&gt;
  
  using namespace std;
  template&lt;typename R,typename D&gt; 
  class GenericFunction
  {
  private:
      boost::function&lt;R (D)&gt; f;
  protected:
      GenericFunction();
  public:
      GenericFunction(const boost::function&lt;R (D)&gt;&amp; myFunction);
      R evaluate(const D&amp; value) const ;
      R operator ()(const D&amp; value) const;
  };
  template &lt;typename R, typename D, int N&gt;
  class ScalarValuedFunction:public GenericFunction&lt;R,boost::array&lt;D, N&gt;&gt;
  {
  public:
      ScalarValuedFunction(const boost::function&lt;R (const boost::array&lt;D, N&gt;)&gt; &amp;myF);
  };
  
  template&lt;typename Numeric, std::size_t N&gt;
  Numeric Norm(const boost::array&lt;Numeric , N&gt;&amp; Vec)
  {
      Numeric Result=Vec[0]*Vec[0];
      for (std::size_t i=1; i&lt;Vec.size();i++)
      {
          Result+=Vec[i]*Vec[i];
      }
      return Result;
  }
  
   int main ()
   {
      const int N=4;
      boost::array&lt;double, N&gt; arr={0.2,.3,1.1,4};
      ScalarValuedFunction&lt;double, double, N&gt; myfun(Norm&lt;double,N&gt;);
  }
  </code></pre>
  
  <p>and receiving the following error,</p>
  
  <pre><code>ConsoleApplication2.obj : error LNK2019: unresolved external symbol "public: __thiscall ScalarValuedFunction&lt;double,double,4&gt;::ScalarValuedFunction&lt;double,double,4&gt;(class boost::function&lt;double __cdecl(class boost::array&lt;double,4&gt;)&gt; const &amp;)" (??0?$ScalarValuedFunction@NN$03@@QAE@ABV?$function@$$A6ANV?$array@N$03@boost@@@Z@boost@@@Z) referenced in function _main
  1&gt;c:\users\university\documents\visual studio 2012\Projects\ConsoleApplication2\Debug\ConsoleApplication2.exe : fatal error LNK1120: 1 unresolved externals
  </code></pre>
  
  <p>please someone tell me what is wrong with my code? </p>