//cword.hpp
//---------
#pragma once
#include <atlstr.h>

class CWord
{
public:
  CWord();
  CWord(const CString word);
  ~CWord();
};

//cword.cpp
//---------
#include "CWord.h"

CWord::CWord() { }

CWord::CWord(const CString word)
{
  m_word = word;
}

//"CWordSearchDoc.h
-------------------
#include <vector>
#include "CWord.h"

class CWordSearchDoc : public CDocument
{
private:
  std::vector< CWord > m_word_list;
public:
  void add_word(CString word);
};

//"CWordSearchDoc.cpp
//-------------------
#include "CWordSearchDoc.h"

void CWordSearchDoc::add_word(CString word)
{
  CWord new_word( word );

  m_word_list.push_back( new_word ); // -- link error LNK2019

  // ......
  m_word_list.push_back( CWord() ); // -- no error
}