在编译代理类接口定义时获取链接器错误LNK 2019

Question

// Fig. 11.16: Implementation.h
// Implementation class definition.

class Implementation 
{
  private:
       int value;
   public:
       // constructor
       Implementation( int v )  
          : value( v ) // initialize value with v
       { 
          // empty body
       } // end constructor Implementation

       // set value to v
       void setValue( int v )   
       { 
          value = v; // should validate v
       } // end function setValue

       // return value 
       int getValue() const      
       { 
          return value; 
       } // end function getValue
     // data that we would like to hide from the client
    }; // end class Implementation


    // Fig. 11.18: Interface.cpp
    // Implementation of class Interface--client receives this file only 
    // as precompiled object code, keeping the implementation hidden.
    #include "Interface.h" // Interface class definition
    #include "Implementation.h" // Implementation class definition

    // constructor
    Interface::Interface( int v ) 
       : ptr ( new Implementation( v ) ) // initialize ptr to point to
    {                                    // a new Implementation object
       // empty body
    } // end Interface constructor

    // call Implementation's setValue function
    void Interface::setValue( int v ) 
    { 
       ptr->setValue( v ); 
    } // end function setValue

    // call Implementation's getValue function
    int Interface::getValue() const 
    { 
       return ptr->getValue(); 
    } // end function getValue

    // destructor
    Interface::~Interface() 
    { 
       delete ptr; 
    } // end ~Interface destructor

// Fig. 11.17: Interface.h
// Proxy class Interface definition.
// Client sees this source code, but the source code does not reveal 
// the data layout of class Implementation.

class Implementation; // forward class declaration required by line 17

class Interface 
{
public:
   Interface( int ); // constructor
   void setValue( int ); // same public interface as
   int getValue() const; // class Implementation has
   ~Interface(); // destructor
private:
   // requires previous forward declaration (line 6)
   Implementation *ptr;   
}; // end class Interf