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Module 10 : creating and destroying objects
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- 2. Overview Using Constructors Initializing Data Objects and Memory Resource Management
- 3. Using Constructors CreatingObjects Using the Default Constructor Overriding the Default Constructor Overloading Constructors
- 4. Creating Objects Step1: Allocating memory Use new keyword to allocate memory from the heap Step 2: Initializing the object by using a constructor Use the name of the class followed by parentheses Date when = new Date( );
- 5. Using the DefaultConstructor Features of a default constructor Public accessibility Same name as the class No return type—not even void Expects no arguments Initializes all fields to zero, false or null Constructor syntax class Date { public Date( ) { ... } }
- 6. Overriding the Default Constructor The default constructor might be inappropriate If so, do not use it; write your own! class Date { public Date( ) { ccyy = 1970; mm = 1; dd = 1; } private int ccyy, mm, dd; }
- 7. Overloading Constructors Constructorsare methods and can be overloaded Same scope, same name, different parameters Allows objects to be initialized in different ways WARNING If you write a constructor for a class, the compiler does not create a default constructor class Date { public Date( ) { ... } public Date(int year, int month, int day) { ... } ... }
- 8. Initializing Data UsingInitializer Lists Declaring Readonly Variables and Constants Initializing Readonly Fields Declaring a Constructor for a Struct Using Private Constructors Using Static Constructors
- 9. Using Initializer Lists Overloaded constructors might contain duplicate code Refactor by making constructors call each other Use the this keyword in an initializer list class Date { ... public Date( ) : this(1970, 1, 1) { } public Date(int year, int month, int day) { ... } }
- 10. Value ofconstant field is obtained at compile time Value of readonly field is obtained at run time Declaring Readonly Variables and Constants
- 11. Initializing Readonly Fields Readonly fields must be initialized Implicitly to zero, false or null Explicitly at their declaration in a variable initializer Explicitly inside an instance constructor class SourceFile { private readonly ArrayList lines; }
- 12. Declaring a Constructorfor a Struct The compiler Always generates a default constructor. Default constructors automatically initialize all fields to zero. The programmer Can declare constructors with one or more arguments. Declared constructors do not automatically initialize fields to zero. Can never declare a default constructor. Can never declare a protected constructor.
- 13. Using Private Constructors A private constructor prevents unwanted objects from being created Instance methods cannot be called Static methods can be called A useful way of implementing procedural functions public class Math { public static double Cos(double x) { ... } public static double Sin(double x) { ... } private Math( ) { } }
- 14. Using Static Constructors Purpose Called by the class loader at run time Can be used to initialize static fields Guaranteed to be called before instance constructor Restrictions Cannot be called Cannot have an access modifier Must be parameterless
- 15. Objects and Memory Object Lifetime Objects and Scope Garbage Collection
- 16. Object Lifetime Creatingobjects You allocate memory by using new You initialize an object in that memory by using a constructor Using objects You call methods Destroying objects The object is converted back into memory The memory is de-allocated
- 17. Objects and Scope The lifetime of a local value is tied to the scope in which it is declared Short lifetime (typically) Deterministic creation and destruction The lifetime of a dynamic object is not tied to its scope A longer lifetime A non-deterministic destruction
- 18. Garbage Collection Youcannot explicitly destroy objects C# does not have an opposite of new (such as delete) This is because an explicit delete function is a prime source of errors in other languages Garbage collection destroys objects for you It finds unreachable objects and destroys them for you It finalizes them back to raw unused heap memory It typically does this when memory becomes low
- 19. Resource Management ObjectCleanup Writing Destructors Warnings About Destructor Timing IDisposable Interface and Dispose Method The using Statement in C#
- 20. Object Cleanup Thefinal actions of different objects will be different They cannot be determined by garbage collection. Objects in .NET Framework have a Finalize method. If present, garbage collection will call destructor before reclaiming the raw memory. In C#, implement a destructor to write cleanup code. You cannot call or override Object.Finalize.
- 21. Writing Destructors Adestructor is the mechanism for cleanup It has its own syntax: - No access modifier - No return type, not even void - Same name as name of class with leading ~ - No parameters class SourceFile { ~SourceFile( ) { ... } }
- 22. Warnings About Destructor Timing The order and timing of destruction is undefined Not necessarily the reverse of construction Destructors are guaranteed to be called Cannot rely on timing Avoid destructors if possible Performance costs Complexity Delay of memory resource release
- 23. IDisposable Interface andDispose Method To reclaim a resource: Inherit from IDisposable Interface and implement Dispose method that releases resources Call GC.SuppressFinalize method Ensure that calling Dispose more than once is benign Ensure that you do not try to use a reclaimed resource
- 24. The using Statementin C# Syntax Dispose is automatically called at the end of the using block using (Resource r1 = new Resource( )) { r1.Method( ); }
- 25. Review Using Constructors Initializing Data Objects and Memory Resource Management