A serialization primer - Part 2

This article is the second of a 3 part tutorial on serialization.

• Part 1 introduces the basics of serialization.
• Part 2 explains how to gracefully handle reading invalid data stores and support versioning.
• Part 3 describes how to serialize complex objects.

In Part 1 we saw how to serialize a simple object via a CArchive using a serialize() method like this:

  int CFoo::serialize
    (CArchive* pArchive)
  {
    int nStatus = SUCCESS;

    // Serialize the object ...
    ASSERT (pArchive != NULL);
    TRY
    {
      if (pArchive->IsStoring()) {
         // Write employee name and id
         (*pArchive) << m_strName;
         (*pArchive) << m_nId;
      }
      else {
         // Read employee name and id
         (*pArchive) >> m_strName;
         (*pArchive) >> m_nId;
      }
    }
    CATCH_ALL (pException)
    {
      nStatus = ERROR;
    }
    END_CATCH_ALL

    return (nStatus);
  }

Object signatures

  class CFoo
  {
    ...

    // Methods
    public:
      ...
      CString getSignature();

    // Data members
      ...
    protected:
      static const CString  Signature;  // object signature
  };

  // Static constants
  const CString CFoo::Signature = "FooObject";

And here's the code:

  int CFoo::serialize
    (CArchive* pArchive)
  {
    int nStatus = SUCCESS;
    bool bSignatureRead = false;

    // Serialize the object ...
    ASSERT (pArchive != NULL);
    TRY
    {
      if (pArchive->IsStoring()) {
         // Write signature
         (*pArchive) << getSignature();

         // Write employee name and id
         (*pArchive) << m_strName;
         (*pArchive) << m_nId;
      }
      else {
         // Read signature - complain if invalid
         CString strSignature;
         (*pArchive) >> strSignature;
         bSignatureRead = true;
         if (strSignature.Compare (getSignature()) != 0) {
            return (INVALID_DATAFILE);
         }

         // Read employee name and id
         (*pArchive) >> m_strName;
         (*pArchive) >> m_nId;
      }
    }
    CATCH_ALL (pException)
    {
      nStatus = bSignatureRead ? ERROR : INVALID_DATAFILE;
    }
    END_CATCH_ALL

    return (nStatus);
  }

Versioning

  class CFoo
  {
    ...

    // Methods
    public:
      ...
      CString getSignature();
      int     getVersion();

    // Data members
      ...
    protected:
      static const CString  Signature;  // object signature
      static const int      Version;    // object version
  };

  // Static constants
  const CString CFoo::Signature = "FooObject";
  const int     CFoo::Version = 1;

  int CFoo::serialize
    (CArchive* pArchive)
  {
    int nStatus = SUCCESS;
    bool bSignatureRead = false;
    bool bVersionRead = false;

    // Serialize the object ...
    ASSERT (pArchive != NULL);
    TRY
    {
      if (pArchive->IsStoring()) {
         // Write signature and version
         (*pArchive) << getSignature();
         (*pArchive) << getVersion();

         // Write employee name and id
         (*pArchive) << m_strName;
         (*pArchive) << m_nId;
      }
      else {
         // Read signature - complain if invalid
         CString strSignature;
         (*pArchive) >> strSignature;
         bSignatureRead = true;
         if (strSignature.Compare (getSignature()) != 0) {
            return (INVALID_DATAFILE);
         }

         // Read version - complain if unsupported
         int nVersion;
         (*pArchive) >> nVersion;
         bVersionRead = true;
         if (nVersion > getVersion()) {
            return (UNSUPPORTED_VERSION);
         }

         // Read employee name and id
         (*pArchive) >> m_strName;
         (*pArchive) >> m_nId;
      }
    }
    CATCH_ALL (pException)
    {
      nStatus = bSignatureRead && bVersionRead ? ERROR : INVALID_DATAFILE;
    }
    END_CATCH_ALL

    return (nStatus);
  }

  class CFoo
  {
    ...
    // Data members
    public:
      CString  m_strName;  // employee name
      int      m_nId;      // employee id
      int      m_nDept;    // department code (-1 = unknown)
  };

  const int CFoo::Version = 2;

  int CFoo::serialize
    (CArchive* pArchive)
  {
    ...
    // Serialize the object ...
    ASSERT (pArchive != NULL);
    TRY
    {
      if (pArchive->IsStoring()) {
         ...
         // Write employee name, id and department code
         (*pArchive) << m_strName;
         (*pArchive) << m_nId;
         (*pArchive) << m_nDept;
      }
      else {
         ...
         // Read employee name and id
         (*pArchive) >> m_strName;
         (*pArchive) >> m_nId;

         // Read department code (new in version 2)
         if (nVersion >= 2) {
            (*pArchive) >> m_nDept;
         }
         else {
            m_nDept = -1; // unknown
         }
      }
    }
    CATCH_ALL (pException)
    {
      nStatus = bSignatureRead && bVersionRead ? ERROR : INVALID_DATAFILE;
    }
    END_CATCH_ALL

    return (nStatus);
  }

Conclusion

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)