Undefined Reference to a function

I am new to C++ in terms of large projects distributed over various source files.

When I compile(gcc compiler) I get the following error: undefined reference to 'C::C()'

I have this code:

/src/lib/abstract/A.cpp

#include "src/plugins/geo/C.h"
#include "B.h"

class A:public B
{
 public; 
  A()
   {
     B m_b;
   }
  void speed()
  {
    m_b = new C();
    m_b->cspeed();
  }
}

/src/lib/B.h

class B
{
public:
virtual int cspeed();
}

in /src/pligins/geo/C.h

#include "B.h"
class C:public B
{ private:
   int speed=5;
  public: 
  C()
  {}
  int cspeed()
  {
    return speed;
  }
}