I was reading a VB.NET code and there was the following:
Structure MyRecord
"some code here"
End Structure
Then he use it as if it is a Class. So, what is the "Structure" ??
IS it only in .NET ,or there are similar things in other language??
Something else,in the same code there are:
Dim num As integer=FreeFile()
what does that mean?,can we put a function in a variable in VB?
If we can, then what does that mean??
1
A structure is used to define a value type, just as a class is used to define a reference type. However, a structure is more complicated to implement correctly than a class, so you should stick to classes until you really need to implement a value type. The structure in the example should probably also be implemented as a class instead.
There are structures in other languages, but they may be handled differently. In C++ for example a structure is used to define a type, and the usage determines if it's a value type or a reference type.
2
Yes, you can put a reference to a function (i.e. a delegate) in a variable, but that's not what that code does. It simply calls the function and puts the return value in the variable.
Via startVBdotnet.com:
Structures can be defined as a tool
for handling a group of logically
related data items. They are
user-defined and provide a method for
packing together data of different
types. Structures are very similar to
Classes. Like Classes, they too can
contain members such as fields and
methods. The main difference between
classes and structures is, classes are
reference types and structures are
value types. In practical terms,
structures are used for smaller
lightweight objects that do not
persist for long and classes are used
for larger objects that are expected
to exist in memory for long periods.
We declare a structure in Visual Basic
.NET with the Structure keyword.
Generally, I would suggest implementing a class instead of a structure. This way you can use inheritance and general Object Oriented Design later, if needed.
Related
In statically typed language, people are able to use algebraic data type to abstract data and also generate constructors, or use class, trait and mixin to deal with data abstraction.
In dynamically typed language, like Python and Ruby, they all provide a class system to users.
But what about scheme, the simplest functional language, the closest one to λ-calculi, how does it abstract data?
Do scheme programmers usually just put data in a list or a lambda abstraction, and write some accessor function to make it look like a tree or something else? like EOPL says: specifying data via interfaces.
And then how does this abstraction technique relate to abstract data type (ADT) and objects? with regard to On understanding data abstraction, revisited.
What SICP (and I guess, EOPL) is advocating is just using functions to access data; then you can always switch one set of functions for another, implementing the same named set of functions to work with another concrete implementation. And that (i.e. the sets of such functions) is what forms the "interfaces", and that's what you put in different source files, and by just loading the appropriate one you can switch the concrete implementation while all the other code is none the wiser. That's what makes it "abstract" datatype.
As for the algebraic data types, the old bare-bones Scheme way is to create closures (that hold and hide the data) which respond to "messages" and thus become "objects" (something about "Scheme mailboxes"). This gives us products, i.e. records, and functions we get for free from Scheme itself. For sum types, just as in C/C++, we can use tagged unions in a disciplined manner (or, again, hide the specifics behind a set of "interface" functions).
EOPL has something called "variant-case" which handles such sum types in a manner similar to pattern matching. Searching brings up e.g. this link saying
I'm using DrScheme w/ the EOPL textbook, which uses define-record and variant-case. I've got the macro definitions from the PLT site, but am now dealing with ...
so seems relevant, as one example.
Hi I have a situation like that;
I have different items in my design and all these items has some specific effect on the Character. There is an apply function in every item so it can use Character object and change its functionalities. But what if I change the Character function, I would have to change all the Item classes in accordance to that.
How can I decouple Item and Character efficiently?
The language I am going to use is C++ and I don't know the other variables and functions inside the Item and Character classes. I just want to decouple them.
You could introduce an interface (abstract class in C++) that Character would inherit. Let's call it ItemUser. The Item#apply signature would be changed so that it would take an object of ItemUser instead of Character. Now you are able to change the implementation of Character freely as long as it respects the ItemUser contract.
Check Decorator design pattern, it seems that this design pattern is what you are looking for. Link :Decorator design pattern
As per what I have understood from reading your question is : You have multiple Item classes each having a effect associated. Effect corressponding to the type of Item object is applied on another entity which is Character. Now your issue is whenever there is a change in Character class your Item classes also needs to change and you want a cleaner way to avoid this.
A good way to handle change is to define the well defined Contract which is less prone to change. For example if we have a functionality to add two integers and later we may have the changes such that we require to add two floating point numbers and later we may need to replace add operation with multiplication. In such a case you can define an abstraction Compute (INum num1, INum num2) : INum as return type. Here INum is an abstraction for type and Compute is abstraction for behaviour of function. Actual implementation defines INum and Compute. Now code using our code only depends on the abstractions and we can freely modify the operation and actual type without affecting the user code.
While implementing the contract you can modify the internal implementation without affecting the outside code using the contract.
You can define an abstract class ICharacter. For certain attributes whose type can change in future you can use Templates and generics or simply create interface for the attribute type as well and let the concrete type implement the interfaces. Refer all your fields with interfaces. Let ICharacter define public abstract methods with parameters of type Interfaces and return type also as Interfaces.
Let Item class use ICharacter and When you need to apply effect as per item class just use the constant abstract functions defined. Your Character internal modifications now can change without affecting the Item class.
I have a data class which encapsulates relevant data items in it. Those data items are set and get by users one by one when needed.
My confusion about the design has to do with which object should be responsible for handling the update of multiple properties of that data object. Sometimes an update operation will be performed which affects many properties at once.
So, which class should have the update() method?. Is it the data class itself or another manager class ? The update() method requires data exchange with many different objects, so I don't want to make it a member of the data class because I believe it should know nothing about the other objects required for update. I want the data class to be only a data-structure. Am I thinking wrong? What would be the right approach?
My code:
class RefData
{
Matrix mX;
Vector mV;
int mA;
bool mB;
getX();
setB();
update(); // which affects almost any member attributes in the class, but requires many relations with many different classes, which makes this class dependant on them.
}
or,
class RefDataUpdater
{
update(RefData*); // something like this ?
}
There is this really great section in the book Clean Code, by Robert C. Martin, that speaks directly to this issue.
And the answer is it depends. It depends on what you are trying to accomplish in your design--and
if you might have more than one data-object that exhibit similar behaviors.
First, your data class could be considered a Data Transfer Object (DTO). As such, its ideal form is simply a class without any public methods--only public properties -- basically a data structure. It will not encapsulate any behavior, it simply groups together related data. Since other objects manipulate these data objects, if you were to add a property to the data object, you'd need to change all the other objects that have functions that now need to access that new property. However, on the flip side, if you added a new function to a manager class, you need to make zero changes to the data object class.
So, I think often you want to think about how many data objects might have an update function that relates directly to the properties of that class. If you have 5 classes that contain 3-4 properties but all have an update function, then I'd lean toward having the update function be part of the "data-class" (which is more of an OO-design). But, if you have one data-class in which it is likely to have properties added to it in the future, then I'd lean toward the DTO design (object as a data structure)--which is more procedural (requiring other functions to manipulate it) but still can be part of an otherwise Object Oriented architecture.
All this being said, as Robert Martin points out in the book:
There are ways around this that are well known to experienced
object-oriented designers: VISITOR, or dual-dispatch, for example.
But these techniques carry costs of their own and generally return the
structure to that of a procedural program.
Now, in the code you show, you have properties with types of Vector, and Matrix, which are probably more complex types than a simple DTO would contain, so you may want to think about what those represent and whether they could be moved to separate classes--with different functions to manipulate--as you typically would not expose a Matrix or a Vector directly as a property, but encapsulate them.
As already written, it depends, but I'd probably go with an external support class that handles the update.
For once, I'd like to know why you'd use such a method? I believe it's safe to assume that the class doesn't only call setter methods for a list of parameters it receives, but I'll consider this case as well
1) the trivial updater method
In this case I mean something like this:
public update(a, b, c)
{
setA(a);
setB(b);
setC(c);
}
In this case I'd probably not use such a method at all, I'd either define a macro for it or I'd call the setter themselves. But if it must be a method, then I'd place it inside the data class.
2) the complex updater method
The method in this case doesn't only contain calls to setters, but it also contains logic. If the logic is some sort of simple property update logic I'd try to put that logic inside the setters (that's what they are for in the first place), but if the logic involves multiple properties I'd put this logic inside an external supporting class (or a business logic class if any appropriate already there) since it's not a great idea having logic reside inside data classes.
Developing clear code that can be easily understood is very important and it's my belief that by putting logic of any kind (except for say setter logic) inside data classes won't help you achieving that.
Edit
I just though I'd add something else. Where to put such methods also depend upon your class and what purpose it fulfills. If we're talking for instance about Business/Domain Object classes, and we're not using an Anemic Domain Model these classes are allowed (and should contain) behavior/logic.
On the other hand, if this data class is say an Entity (persistence objects) which is not used in the Domain Model as well (complex Domain Model) I would strongly advice against placing logic inside them. The same goes for data classes which "feel" like pure data objects (more like structs), don't pollute them, keep the logic outside.
I guess like everywhere in software, there's no silver bullet and the right answer is: it depends (upon the classes, what this update method is doing, what's the architecture behind the application and other application specific considerations).
I have a class that is a leaf in the composite pattern. This class has a property that can be either of type A or type B. Their only common interface is of type Object.
How should I support this.
I can
have a add method for each type. That would however mean that I should have two properties of type A and B and should check for null when I want to get the right property.
have one property of type of Object. That would mean I had to check to see which kind of instance it is when I get the property.
What is the best solution for this type of problem? Or any better solutions?
Personally I would choose the single Object property approach. Document what types of objects the property may return, and let the calling code use the available language features to determine the object type, and cast as necessary. Implementing two properties is kinda reinventing the "is-a" operator of your language, and will quickly become unmanageable if you ever need to add more possible types.
Well if you are using a language that supports type abstraction (like Generics in Java or Templates in C++) you can just set that property as a generic type. If not, use Object, Having a method for each type is just an ugly hack (and unmaintanable, if you add more types later).
I have a strong feeling that I do not know what pattern or particular language technique use in this situation.
So, the question itself is how to manage the growing parameter list in class hierarchy in language that has OOP support? I mean if for root class in the hierarchy you have, let's say 3 or 4 parameters, then in it's derived class you need to call base constructor and pass additional parameters for derived part of the object, and so forth... Parameter lists become enormous even if you have depth of inheritance more than two.
I`m pretty sure that many of SOwers faced this problem. And I am interested in ways how to solve it. Many thanks in advance.
Constructors with long parameter lists is an indication that your class is trying to do too much. One approach to resolving that problem is to break it apart, and use a "coordinator" class to manage the pieces. Subclasses that have constructor parameter lists that differ significantly from their superclass is another example of a class doing too much. If a subclass truly is-a superclass, then it shouldn't require significantly more data to do its job.
That said, there are occasional cases where a class needs to work on a large number of related objects. In this situation, I would create a new object to hold the related parameters.
Alternatives:
Use setter injection instead of constructor injection
Encapsulate the parameters in a separate container class, and pass that between constructors instead.
Don't use constructors to initialize the whole object at once. Only have it initialize those things which (1) are absolutely required for the existence of the object and (2) which must be done immediately at its creation. This will dramatically reduce the number of parameters you have to pass (likely to zero).
For a typical hierarchy like SalariedEmployee >> Employee >> Person you will have getters and setters to retrieve and change the various properties of the object.
Seeing the code would help me suggest a solution..
However long parameter lists are a code-smell, so I'd take a careful look at the design which requires this. The suggested refactorings to counter this are
Introduce Parameter Object
Preserve Whole Object
However if you find that you absolutely need this and a long inheritance chain, consider using a hash / property bag like object as the sole parameter
public MyClass(PropertyBag configSettings)
{
// each class extracts properties it needs and applies them
m_Setting1 = configSettings["Setting1"];
}
Possibilities:
Perhaps your class(es) are doing too much if they require so much state to be provided up-front? Aim to adhere to the Single Responsibility Principle.
Perhaps some of these parameters should logically exist in a value object of their own that is itself passed in as a parameter?
For classes whose construction really is complex, consider using the builder or factory pattern to instantiate these objects in a readable way - unlike method names, constructor parameters lack the ability to self document.
Another tip: Keep your class hierarchy shallow and prefer composition to inheritence. That way your constructor parameter list will remain short.