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   ► KBTo/From GuidesVB ClassicOOP Details  Print This     

Cross Ref > OOP Details

By Mike Prestwood

VB Classic versus C++/CLI: A side by side comparison between VB Classic and C++/CLI.

 
OOP Details
 

More object oriented (OO) stuff.

Class Helper

[Other Languages] 

A. In Dephi, class helpers allow you to extend a class without using inheritance. With a class helper, you do not have to create and use a new class descending from a class but instead you enhance the class directly and continue using it as you always have (even just with the DCU).

B. In general terms, developers sometimes use the term to refer to any class that helps out another class.

VB Classic:  "Class Helpers" Not Supported
C++/CLI:  "Class Helpers" Not Supported

However, developers sometimes use the term "class helper" to refer to code that helps out a class. Not truly the meaning we are using here, but you should be aware of the term's general usage.





Code Contract

[Other Languages] 

A.k.a. Class Contract and Design by Contracts.

A contract with a method that must be true upon calling (pre) or exiting (post). A pre-condition contract must be true when the method is called. A post-condition contract must be true when exiting. If either are not true, an error is raised. For example, you can use code contracts to check for the validity of input parameters, and results

An invariant is also a code contract which validates the state of the object required by the method.

VB Classic:  "Code Contracts" Not Supported
C++/CLI:  "Code Contracts" Not Supported




Constructor

[Other Languages] 

General Info: Class Constructor

Constructors are called when you instantiate an object from a class. This is where you can initialize variables and put code you wish executed each time the class is created. When you initially set the member fields and properties of an object, you are initializing the state of the object. The state of an object is the values of all it's member fields and properties at a given time.

Languages Focus

What is the syntax? Can you overload constructors? Is a special method name reserved for constructors?

VB Classic:  "Constructors" Class_Initialize

When an object instance is created from a class, VB6 calls a special parameter-less sub named Class_Initialize. Since you cannot specify parameters for this sub, you also cannot overload it.

When a class is destroyed, VB6 calls a special sub called Class_Terminate.

More Info / Comment  
[Not specified yet. Coming...]




Destructor

[Other Languages] 

General Info: Class Destructor

A special class method called when an object instance of a class is destroyed. With some languages they are called when the object instance goes out of scope, with some languages you specifically have to call the destructor in code to destroy the object, and others use a garbage collector to dispose of object instances at specific times.

Desctructors are commonly used to free the object instance but with languages that have a garbage collector object instances are disposed of when appropriate. Either way, destructors or their equivalent are commonly used to free up resources allocated in the class constructor.

Languages Focus

Are object instances freed with a garbage collector? Or, do you have to destroy object instances.

VB Classic: 

When an object instance is destroyed, VB6 calls a special parameter-less sub named Class_Terminate. For example, when the variable falls out of scope. Since you cannot specify parameters for this sub, you also cannot overload it.

To explicitly destroy an object, use Set YourClass = nothing.

When an object instance is created from a class, VB6 calls a special sub called Class_Initialize.

More Info / Comment  
C++/CLI:  "Finalizer" ~ClassName

Unlike standard C++, C++/CLI uses the .Net garbage collector to free managed object instances. Prism does not have nor need a true destructor.

In .Net, a finalizer is used to free non-managed objects such as a file or network resource. Because you don't know when the garbage collector will call your finalizer, Microsoft recommends you implement the IDisposable interface for non-managed resources and call it's Dispose() method at the appropriate time.

More Info / Comment




Inheritance-Multiple

[Other Languages] 
VB Classic:   Not Supported

VB classic supports only a single "level" of inheritance (abstract class to implimentation class).

More Info / Comment
[Not specified yet. Coming...]




Interface

[Other Languages] 

An element of coding where you define a common set of properties and methods for use with the design of two or more classes.

Both interfaces and abstract classes are types of abstraction. With interfaces, like abstract classes, you cannot provide any implementation. However, unlike abstract classes, interfaces are not based on inheritance. You can apply an Interface to any class in your class tree. In a real sense, interfaces are a technique for designing horizontally in a class hierarchy (as opposed to inheritance where you design vertically). Using interfaces in your class design allows your system to evolve without breaking existing code.

VB Classic:  "Interfaces"

VB6 has limited support for interfaces. You can create an interface of abstract methods and properties and then implement them in one or more descendant classes. It's a single level implementation though (you cannot inherit beyond that). The parent interface class is a pure abstract class (all methods and properites are abstract, you cannot implement any of them in the parent class).

In the single level descendant class, you have to implement all methods and properties and you cannot add any. Your first line of code is Implements InterfaceName.

[Not specified yet. Coming...]




Partial Class

[Other Languages] 

A partial class, or partial type, is a class that can be split into two or more source code files and/or two or more locations within the same source file. Each partial class is known as a class part or just a part. Logically, partial classes do not make any difference to the compiler. The compiler puts the class together at compile time and treats the final class or type as a single entity exactly the same as if all the source code was in a single location.

Languages Focus

For languages that have implemented partial classes, you need to know usage details and restrictions. Can you split a class into two or more files? Can you split a class within a source code file into two or more locations? What are the details of inheritance? Does it apply to interfaces as well?

VB Classic:  "Partial Classes" Not Supported
C++/CLI:  "Partial Classes" Not Supported

C++/CLI does not yet support partial classes. My assumption is that it will soon because it is a .Net language´┐Żbut only time will tell.





Prevent Derivation

[Other Languages] 

Languages Focus

How do you prevent another class from inheriting and/or prevent a class from overriding a member.

VB Classic:   Not Supported

VB Classic supports a form of single level inheritance where you, in essence, create an abstract class and then implement it in one or more classes that inherit from the abstract class. However, you cannot have any further descendant classes so "prevent derivation" is implemented by design of this simple inheritance model.

More Info / Comment
C++/CLI:   sealed




Static Member

[Other Languages] 

General Info: Static Class / Static Member

A static member is a member you can have access to without instantiating the class into an object. For example, you can read and write static properties and call static methods without ever creating the class. Static members are also called class members (class methods, class properties, etc.) since they belong to the class and not to a specific object. A static class is a class that contains only static members. In the UML, these classes are described as utility classes.

Languages Focus

Languages that support static members usually at least support static member fields (the data). Some languages also support static methods, properties, etc. in which case the class member is held in memory at one location and shared with all objects. Finally, some languages support static classes which usually means the compiler will make sure a static class contains only static members.

VB Classic:  "Static Members" Not Supported
[Not specified yet. Coming...]




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