Code Generation for Handle Class Destructors - MATLAB & Simulink (original) (raw)

You can generate code for MATLAB® code that uses delete methods (destructors) for handle classes. To perform clean-up operations, such as closing a previously opened file before an object is destroyed, use a delete method. The generated code calls the delete method at the end of an object's lifetime, even if execution is interrupted by a run-time error. When System objects are destroyed, delete calls the release method, which in turn calls the user-defined releaseImpl. For more information on when to define a delete method in a MATLAB code, see Handle Class Destructor.

Guidelines and Restrictions

When you write the MATLAB code, adhere to these guidelines and restrictions:

• Code generation does not support recursive calls of thedelete method. Do not create an object of a certain class inside the delete method for the same class. This usage might cause a recursive call ofdelete and result in an error message.
• The generated code always calls thedelete method, when an object goes out of scope. Code generation does not support explicit calls of thedelete method.
• Initialize all properties ofMyClass that the delete method of MyClass uses either in the constructor or as the default property value. If delete tries to access a property that has not been initialized in one of these two ways, the code generator produces an error message.
• Suppose a property prop1 ofMyClass1 is itself an object (an instance of another class MyClass2). Initialize all properties ofMyClass2 that the delete method of MyClass1 uses. Perform this initialization either in the constructor of MyClass2 or as the default property value. If delete tries to access a property of MyClass2 that has not been initialized in one of these two ways, the code generator produces an error message. For example, define the two classes MyClass1 andMyClass2:
  classdef MyClass1 < handle
  properties
  prop1
  end
  methods
  function h = MyClass1(index)
  h.prop1 = index;
  end
  function delete(h)
  fprintf('h.prop1.prop2 is: %1.0f\n',h.prop1.prop2);
  end
  end
  end
  classdef MyClass2 < handle
  properties
  prop2
  end
  end
  Suppose you try to generate code for this function:
  function MyFunction
  obj2 = MyClass2;
  obj1 = MyClass1(obj2); % Assign obj1.prop1 to the input (obj2)
  end
  The code generator produces an error message because you have not initialized the property obj2.prop2 that thedelete method displays.

Behavioral Differences of Objects in Generated Code and in MATLAB

The behavior of objects in the generated code can be different from their behavior in MATLAB in these situations:

• The order of destruction of several independent objects might be different in MATLAB than in the generated code.
• The lifetime of objects in the generated code can be different from their lifetime in MATLAB. MATLAB calls the delete method when an object can no longer be reached from any live variable. The generated code calls the delete method when an object goes out of scope. In some situations, this difference causesdelete to be called later on in the generated code than in MATLAB. For example, define the class:
  classdef MyClass < handle
  methods
  function delete(h)
  global g
  % Destructor displays current value of global variable g
  fprintf('The global variable is: %1.0f\n',g);
  end
  end
  end
  Run the function:
  function MyFunction
  global g
  g = 1;
  obj = MyClass;
  obj = MyClass;
  % MATLAB destroys the first object here
  g = 2;
  % MATLAB destroys the second object here
  % Generated code destroys both objects here
  end
  The first object can no longer be reached from any live variable after the second instance of obj = MyClass inMyFunction. MATLAB calls the delete method for the first object after the second instance of obj = MyClass inMyFunction and for the second object at the end of the function. The output is:
  The global variable is: 1
  The global variable is: 2
  In the generated code, both delete method calls happen at the end of the function when the two objects go out of scope. Running MyFunction_mex results in a different output:
  The global variable is: 2
  The global variable is: 2
• In MATLAB, persistent objects are automatically destroyed when they cannot be reached from any live variable. In the generated code, you have to call the terminate function explicitly to destroy the persistent objects.
• The generated code does not destroy partially constructed objects. If a handle object is not fully constructed at run time, the generated code produces an error message but does not call thedelete method for that object. For a System object™, if there is a run-time error in setupImpl, the generated code does not callreleaseImpl for that object.
  MATLAB does call the delete method to destroy a partially constructed object.

Related Topics

• Generate Code for MATLAB Handle Classes and System Objects
• System Objects in MATLAB Code Generation