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);
}

There's a problem with this code. What if we mistakenly read a datafile that doesn't contain the expected information? If the datafile doesn't contain a CString followed by an int, our serialize() method would return ERROR. That's nice, but it would be better if we could recognize the situation and return a more specific status code like INVALID_DATAFILE. We can check that we're reading a valid datafile (i.e., one that contains a CFoo object) by using an object signature.

Object Signatures

An object signature is just a character string (e.g.: "FooObject") that identifies an object. We add a signature to CFoo by modifying the class definition:

class CFoo
{
  ...

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

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

The signature is declared in Foo.cpp:

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

Next, we modify the serialize() method to serialize the signature before serializing the object's data members. If an invalid signature is encountered, or if the signature is missing, it's likely that we're attempting to read a data store that doesn't contain a CFoo object. Here's the logic for reading a signed object:

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);
}

You should ensure that all your objects have unique signatures. It's less important what the actual signature is. If you're developing a suite of products, it's helpful to have a process for registering object signatures companywide. That way, developers won't mistakenly use the same signature for different objects. If you want to make it harder to reverse engineer your datafiles, you should use signatures that have no obvious connection to object names.

Versioning

As you upgrade your product during its lifecycle, you may find it necessary to modify the structure of CFoo by adding or removing data members. If you simply released a new version of CFoo, attempts to read old versions of the object from a data store would fail. This is obviously not acceptable. Any version of CFoo should be able to restore itself from an older serialized version. In other words, CFoo's serialization method should always be backward compatible. This is easily accomplished by versioning the object. Just as we added an object signature, we add an integer constant that specifies the object's version number.

class CFoo
{
  ...

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

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

The object's version is declared in Foo.cpp.

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

Next, we modify the serialize() method to serialize the version after serializing the signature, and before serializing the object's data members. If a newer version is encountered, we're attempting to read an unsupported version of the object. In this case, we simply return the status UNSUPPORTED_VERSION.

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);
}

Version 1 of our CFoo contained 2 data members - a CString (m_strName) and an int (m_nId). If we add a third member (e.g.: int m_nDept) in version 2, we need to decide what m_nDept should be initialized to when reading an older version of the object. In this example, we'll initialize m_nDept to -1 implying that the employee's department code is "Unknown".

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

We also need to increase the object's version number in Foo.cpp to 2.

const int CFoo::Version = 2;

Finally, we modify the part of serialize() that reads the object so that m_nDept is initialized to -1 if we're reading an older version of the datafile. Note that the file is always saved as the latest version.

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

So far, we've dealt with providing robust support for serializing simple objects - i.e., those that contain readily serializable data types. In Part 3, we'll see how to serialize any kind of object.