What is CArchive class in serialization routine? Why a CFile is not used here?

Serialization routine

It starts with the CObject class that is the ancestor to most MFC classes, which is equipped with a Serialize() member function. The class should declare with the macros DECLARE_SERIAL(< class name >) and also should add implement macro IMPLEMENT_SERIAL(< class name >, < parent class name >, < Schema version >). Serialize routine has this prototype. 

void Serialize(CArchive& ar)

Let us look into a simple example. Here we declared a class called CStudent to store and retrieve fields of a student. It contains standard, enrollment number, name, marks and age. We are saving or restoring these members Serialize routine.

class CStudent : public CObject
{
public :
  CStudent(void);
  ~CStudent(void);


  WORD    m_wStd;
  WORD    m_wRoll;
  CString m_sName;
  double  m_dMarks;
  WORD    m_wAge;

public :
  void Serialize(CArchive& ar);

  DECLARE_SERIAL(CStudent);
};

IMPLEMENT_SERIAL(CStudent, CObject, 0)

/* Main Serialize routine */
void CStudent::Serialize(CArchive& ar)
{
  CObject::Serialize(ar);

  if( ar.IsStoring() ) {
    /* Serialize STD, ROLL, NAME, MARKs, AGE to file */
    ar << m_wStd << m_wRoll << m_sName << m_dMarks << m_wAge;
  } else {
    /* Read STD, ROLL, NAME, MARKs, AGE from file */
    ar >> m_wStd >> m_wRoll >> m_sName >> m_dMarks >> m_wAge;
  }

}

CArchive class

Serialization routine does not directly deal with CFile. It uses CArchive object instead of CFile. CArchive class decouples the file object from Serialize() routine. This is to support any type of stream classes in Serialize routine and make it independent of file stream. Thus Serialize is not limited to disk/storage files. It can be socket file to Serialize in network stream or serial stream file transmitted in serial port.
/* Serialize routine for data saving */
void OnSave() 
{
  UpdateData(TRUE);

  CFile file;
  /* Open file for writing */
  file.Open("student.dat", CFile::modeCreate | CFile::modeWrite);
  /* pass file to CArchive constructor */
  CArchive ar(&file, CArchive::store);
  m_Student.Serialize(ar);
  ar.Close();
  file.Close();
}

/* Serialize routine for data retrieving */
void OnOpen() 
{
  CFile file;
  /* Open file for writing */
  file.Open("student.dat",  CFile::modeRead);
  /* pass file to CArchive constructor */
  CArchive ar(&file, CArchive::load);
  m_Student.Serialize(ar);
  ar.Close();
  file.Close();
  UpdateData(FALSE);
}

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