title: undefined reference error
body: <p>i got undefined reference error from my main function, but i can not find the problem.
  my files are: </p>
  
  <pre><code>//Student.h
  #ifndef STUDENT_H
  #define STUDENT_H
  #include &lt;vector&gt;
  #include &lt;string&gt;
  
  class Student{
  public:
   Student(std::string &amp;line);
  ...
  virtual void evaluateValue(){}
   virtual ~Student(){}
  ....
  };
  #endif
  
  //HeStudent.h
  #ifndef HESTUDENT_H
  #define HESTUDENT_H
  #include "Student.h"
  
  class HeStudent : public Student{
  public:
   HeStudent(std::string line) : Student(line){
    ...
   }
  
   static int AgradesCount;
   static double Aaverage;
  
   virtual void evaluateValue();
   virtual ~HeStudent(){}
  
  };
  #endif
  </code></pre>
  
  <p>for each .h file there is his .cpp file.
  i've got also a main.cpp file which contains the main and in the main i create:
  Student stud = new HeStudent(line);
  i don't know if it's necessary but i included the Student.h and the HeStudent.h
  and i get some long error and it said: </p>
  
  <pre><code>HeStudent::HeStudent(std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;)]+0x22): undefined reference to `Student::Student
  </code></pre>
  
  <p>can anyone tell me what is the problem?</p>