Use Class Metadata

What Is Class Metadata?

Class metadata is information about class definitions that is available from instances of metaclasses. Use metaclass objects to obtain information about class definitions without the need to create instances of the class itself.

Each block in a class definition has an associated metaclass that defines the attributes for that block. Each attribute corresponds to a property in the metaclass. An instance of a metaclass has values assigned to each property that correspond to the values of the attributes of the associated class block.

Metadata enables the programmatic inspection of classes. Tools such as property inspectors, debuggers, and so on, use these techniques.

The meta Package

The meta package contains metaclasses that MATLAB uses for the definition of classes and class components. The class name indicates the component described by the metaclass:

meta.package
meta.class
meta.property
meta.DynamicProperty
meta.EnumeratedValue
meta.method
meta.event

Each metaclass has properties, methods, and events that contain information about the class or class component. See meta.package, meta.class, meta.property, meta.DynamicProperty, meta.EnumeratedValue, meta.method and meta.event for more information on these metaclasses.

Creating Metaclass Objects

You cannot instantiate metaclasses directly by calling the respective class constructor. You can create metaclass objects from class instances or from the class name.

• ?ClassName — Returns a meta.class object for the named class. Use meta.class.fromName with class names stored as characters in variables.

• meta.class.fromName('ClassName') — returns the meta.class object for the named class (meta.class.fromName is a meta.class method).

• metaclass(obj) — Returns a metaclass object for the class instance (metaclass)

% create metaclass object from class name using the ? operator
mobj = ?classname; 
% create metaclass object from class name using the fromName method
mobj = meta.class.fromName('classname'); 
% create metaclass object from class instance
obj = myClass;
mobj = metaclass(obj); 

The metaclass function returns the meta.class object (that is, an object of the meta.class class). You can obtain other metaclass objects (meta.property, meta.method, and so on) from the meta.class object.

Note   Metaclass is a term used here to refer to all of the classes in the meta package. meta.class is a class in the meta package whose instances contain information about MATLAB classes. Metadata is information about classes contained in metaclasses.

Using Metaclass Objects

Here are ways to access the information in metaclass objects:

• Obtain a meta.class object from a class definition (using ?) or from a class instance (using metaclass).

• Use the meta.class properties, methods, and events to obtain information about the class or class instance from which you obtained the meta.class object. For example, get other metaclass objects, such as the meta.properties objects defined for each of the class properties.

See the following sections for examples that show how to use metadata:

• Use Metadata to Inspect Classes and Objects

• Find Objects Having Specific Settings

• Get Information About Properties

• Get Property Default Values

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Information from Class Metadata

Use Metadata to Inspect Classes and Objects

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