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Modify the Save and Load Process

When to Modify the Save and Load Process

The primary reason for modifying the save and load process is to support backward and forward compatibility of classes. Consider modifying the save and load process when you:

  • Rename a class

  • Remove properties

  • Define a circular reference of handle objects where initialization order is important

  • Must call the constructor with arguments and, therefore, cannot use ConstructOnLoad

How to Modify the Save and Load Process

The most versatile technique for modifying the save and load process is to implement loadobj, and if necessary, saveobj methods for your class. MATLAB® executes these methods when you call save or load on an object of the class.

The save function calls your class saveobj method before performing the save operation. The save function then saves the value returned by the saveobj method. You can use saveobj to return a modified object or a struct that contains property values.

load calls your class loadobj method after loading the object. The load function loads the value returned by the loadobj method into the workspace. A loadobj method can modify the object being loaded or can reconstruct an object from the data saved by the class saveobj method.

Implementing saveobj and loadobj Methods

Implement a saveobj method that modifies the object being saved, then implement a loadobj method to return the object to the correct state when loading it.

Implement the loadobj method as a Static method because MATLAB can call the loadobj method with a struct instead of an object of the class.

Implement the saveobj method as an ordinary method (that is, calling it requires an instance of the class).

MATLAB saves the object class name so that load can determine which loadobj method to call in cases where your saveobj method saves only the object data in a structure. Therefore, the class must be accessible to MATLAB when you load the object.

Use a loadobj method when:

  • The class definition has changed since the object was saved, requiring you to modify the object before loading.

  • A saveobj method modified the object during the save operation, possibly saving data in a struct. Implement the loadobj method to reconstruct the object from the output of saveobj.

Additional Considerations

When you decide to modify the default save and load process, keep the following points in mind:

  • If loading any property value from the MAT-file produces an error, load passes a struct to loadobj. The struct field names correspond to the property names extracted from the file.

  • loadobj must always be able to accept a struct as input and return an object, even if there is no saveobj or saveobj does not return a struct.

  • If saveobj returns a struct, then load always passes that struct to loadobj.

  • Subclass objects inherit superclass loadobj and saveobj methods. Therefore, if you do not implement a loadobj or saveobj method in the subclass, MATLAB calls only the inherited methods.

    If a superclass implements a loadobj or saveobj method, then a subclass can also implement a loadobj or saveobj method that calls the superclass methods. For more information, see Save and Load Objects from Class Hierarchies.

  • The load function does not call the constructor by default. For more information, see Initialize Objects When Loading.

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