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A serialization primer - Part 2By Ravi BhavnaniThis tutorial describes how to handle invalid data stores and support versioning during serialization. |
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This article is the second of a 3 part tutorial on serialization.
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.
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.
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 (eg: 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); }
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Last Updated: 17 Feb 2002 Editor: Sean Ewington |
Copyright 2002 by Ravi Bhavnani Everything else Copyright © CodeProject, 1999-2009 Web10 | Advertise on the Code Project |
Forward compatiblity. Many people may not care, but serializing in this way essentially changes your file format every time the objects change.