The variables of Modelica classes are instantiated per object.
Can anyone clarify this statement?
First of all Modelica has only one type of object, the class, and the other objects (model, record, package) are only special types of class with restrictions. That means that every object must follow the same rules, therefore I'll refer to the object class, but what I'll write will apply to every Modelica object.
The variables are instantiated per object means that if you have a Modelica class like the following one:
partial class MySimpleClass
Real variable1;
equation
variable1 = time;
end MySimpleClass;
When you declare a member variable such as MySimpleClass in instanceOfTypeMySimpleClass:
class mySecondClass
MySimpleClass instanceOfTypeMySimpleClass;
MySimpleClass instanceTwoOfTypeMySimpleClass;
Real variable1;
equation
variable1 = instanceOfTypeMySimpleClass.variable1;
instanceTwoOfTypeMySimpleClass.variable1 = 3;
end mySecondClass;
you declare an instance variable. Every time you create an instance of a class, the Modelica compiler creates one copy of each the class's instance variables for the instance and this is done by objects. You can access an object's instance variables from an object using the syntax <instanceName>.<VariableName>.
The Modelica compiler allocates class variables once per instance. The The Modelica compiler allocates memory for class variables every time it encounters an instance of that class. Therefore all instances DID NOT share the same copy of the class's class variables as, for example, for static classes in Java. In other words in this example instanceOfTypeMySimpleClass.variable1 and instanceTwoOfTypeMySimpleClass.variable1 are two different variables.
"The variables of Modelica classes are instantiated per object." meant to describe this difference.
I hope this helps,
Marco
Related
I am having trouble understanding my professor's lecture notes because my brain seem to treat objects, attributes, variables and class instance as interchangeable. I really appreciate any help in distinguishing these 4 terms. Thank you!
this would be helpful for u Visit https://www.quora.com/What-is-the-difference-between-instance-variable-and-class-variable
Class variables are declared with keyword static and Instance variables are declared without static keyword.
Class variables are common to all instances of a class. These variables are shared between the objects of a class. Instance variables are not shared between the objects of a class. Each instance will have their own copy of instance variables.
As class variables are common to all objects of a class, changes made to these variables through one object will reflect in another. As each object will have its own copy of instance variables, changes made to these variables through one object will not reflect in another object.
Class variables can be accessed using either class name or object reference. Instance variables can be accessed only through object reference.
https://qph.fs.quoracdn.net/main-qimg-c4b92e80a8500c11fe705c1bafc3ed26
You don't mention the programming language at question.
Usually a class is a model or template that declares
how a certain category of objects look like.
You give a class a name and you mention if it inherits
members from another class or not.
You define also the class members.
These can be variables that hold data (object state)
and methods (class defined functions) that define
the object behaviour.
When you instantiate a class using the declared model
, you get an object, that is a concrete class instance.
This is a concrete entity, think of it as a new variable in memory,
whose data type is the class (instead of for example
integer or string data types), whose value is its state
in a defined moment in time (the state being the
combination of all of its data member variables values
at that moment). This object has to have an identity,
because it exists in memory and it is a different entity
from the other objects you can instantiate from this or
any other class. The data member variables hold specific
values for each instance. These are not shared between
instances.
Now the member methods can be shared between instances
because they have no state, so they are equal for every object.
They are called with some arguments
and they do some action that changes the object state, or
is at least tightly related with the concrete object.
But they are common to every object. The methods usually
know what concrete object they act upon by means of a special
name like 'this' or 'self', that references to 'itself'.
Objects are usually assigned to variables upon creation,
storing a reference to its identity that allows the
remaining code to manipulate them.
You use these variables to refer to the concrete object
outside the code of the classes, and use 'this' or 'self'
to refer to it from inside the classes.
Frequently you access object members qualifying with the
object name. Like in 'player.run()', or 'player.total_score'.
That is if player is a variable to which you assigned a
class Player instance. This can look like player = new Player
or player = Player().
Attributes is just another name given to data members.
Sometimes attributes and also methods can be public or private,
meaning code outside the class can use them, or only
the class code can have access.
Sometimes you see data members or attributes referred as
properties. When you access an attribute, you are accessing
a property. In some languages like Python, property can mean
something a little different but close related anyway...
Now also depending on the language things can be like described
(C++, Java) or you can have everything being treated as objects,
including the class definitions (Python).
You should also search the internet or SO about
inheritance, overriding, class diagrams, and other things class
related.
This is all no more than the ability of defining your own data types
beoynd the language builtin types.
You can think of variables as names for boxes (memory containers in a certain address) holding values. But sometimes you want to manipulate
not the values but the addresses themselves. This time you say you have
references (to addresses). Sometimes variables are just names for those
references. References are also known as pointers. But you could do math with pointers (increment, decrement, add a fixed value to...) that you usually don't do with references.
After evolving my project code for months, I've finally hit a need to define a new class. Having to romp through my previous class definitions as a refresher of the conventions, I noticed that all constructors and property setters all have an output argument, even though nothing is assigned to it, e.g.:
function o = myConstructor( arg1, arg2, ... )
function o = set.SomeProperty( o, arg1 )
I've been looking through the documentation for upward of an hour without finding the explanation for this. It doesn't look like it depends on whether a function is defined in the class definition file or in its own separate m-file.
Can anyone please explain?
The best place to start is the documentation "Comparison of Handle and Value Classes". From the very top:
A value class constructor returns an object that is associated with the variable to which it is assigned. If you reassign this variable, MATLAB® creates an independent copy of the original object. If you pass this variable to a function to modify it, the function must return the modified object as an output argument.
A handle class constructor returns a handle object that is a reference to the object created. You can assign the handle object to multiple variables or pass it to functions without causing MATLAB to make a copy of the original object. A function that modifies a handle object passed as an input argument does not need to return the object.
In other words, value classes need to return a modified object (which is a new object distinct from the original), while handle classes don't. The constructor of either class will always have to return an object, since it is actually constructing it.
Some good additional reading is "Which Kind of Class to Use", which links to a couple helpful examples of each type of class object. Looking at the DocPolynom value class example, you can see that property set methods have to return the modified object, while the dlnode handle class example only requires an output for its constructor. Note that you could still return an object from a handle class method (if desired), but it's not required.
i have learned class is a blueprint of structurally identical items, and the items created using class are called instances.
please let me know what are the difference between class, object, instance and attribute in object oriented programming concept. is the object, instance, attribute same?
http://en.wikipedia.org/wiki/Class_(computer_programming)
Typically they are used like so:
class - blueprint for creating object instances; defines properties and methods
object - synonymous with instance usually (sometimes improperly equated with class)
instance - an actual manifestation of a class; the class defines what properties and methods the instance has while the instance holds the values of the object attributes
attribute - typically synonymous with "property" (an object member whose value can be set), but in some dynamic languages this can also include "methods" (an object member which can be called)
Yes, you're definition of a class is correct. You can create multiple objects of the same class. Each object is an instance of the class. The term instance can not only mean the object is an instance of the class, but it can also relate to polymorphism. There is a keyword, in java it is called instanceof. With it, you can not only tell if an object is an instance of the class, but if the object is an instance of a superclass. So, instance, can also be more type-oriented. Attributes are members of the class, like its variables.
I am confused about inheritance in .NET and here is why.
I was previously of the understanding that a structure cannot inherit from a class in .NET, so for example, the following won't compile:
struct MyStruct : MyClass
{
}
But today I read that integers (and other value types) are structs and not objects and they inherit from the ValueType class. The ValueType class inherits from System.Object and its purpose is to override certain methods in System.Object to make these methods suitable for Value Types.
So what's the deal? Can a structure inherit from a class in .NET, can it not or can it only in certain circumstances?
Thanks
Within the guts of .net, the definition for a struct containing certain members is the same as a definition for a class which those same fields and members, and which inherits from System.ValueType. Note that compilers will not allow one to declare a class which inherits ValueType, but when one declares a struct, the compiler, "behind the scenes" declares a class which does.
What makes value types special in .net is the way the run-time allocates storage locations (variables, fields, parameters, etc.) When a storage location of a type not inheriting from ValueType is declared, the runtime will allocate space for a heap object reference. By contrast, when a storage location of a type inheriting from ValueType is declared, the runtime will allocate space for all the public and private fields of that type. For a type like int, the system allocates a private field which is of a special primitive type, outside the normal type system.
Note that a storage location of a value type doesn't really hold an instance of that type; instead is an instance of that type, and holds all of the fields of that type. A statement like struct1 = struct2 does not replace the value-type instance struct1 with the instance struct2. Instead, it copies all of the fields from struct2 over the corresponding fields in struct1. Likewise if a value-type storage location is passed as a method to a procedure without using the ref keyword, what is passed is not the struct instance itself, but rather the contents of its fields.
If it is necessary to copy a value-type storage location to a one of type not derived from ValueType (e.g. Object or IComparable), the system will create a new heap-object instance of the value type, copy all the fields from the value type to that new instancen and store a reference to that new instance in the target storage location. This process is called "boxing". Most compilers will do this implicitly, thus attempting to behave as though a value type storage location holds an object which derives from ValueType. It's important to note, though, that this is an illusion. If type X derives from Y, one has an X named xx and a Y named yy, and one performs xx = yy, such a statement should cause xx and yy to refer to the same object instance. That will happen if xx and yy are types not derived from ValueType, even if yy holds an instance of something derived from ValueType. It will not happen, however, if xx and/or yy derives from ValueType. In that case, the system will copy fields from one instance to another (possibly new) instance.
Static variables are variables allocated statically at compile time. My doubt is for what purpose some variables are declared statically? I didn't have used any static variable in my code till now. From the apple code http://developer.apple.com/library/ios/#samplecode/CryptoExercise/Introduction/Intro.html
in securityWrapper.m, there is a line
static SecKeyWrapper * __sharedKeyWrapper = nil;
what is the use of statically allocating __sharedKeyWrapper here?
I have seen the use of static key word in so many codes. So kindly explain the use and benefits when static keyword.
Thanks in advance.
That file global is a singleton, used for sharing an instance of a class.
The reason it's static in Objective-C is internal linkage. In practice, this means the variable can not be seen outside the file it is declared in. If another file declares a variable with the same name, they're two different variables.
Keep in mind that the way Objective-C instances work, the instance won't actually be allocated automatically. Rather, you have a pointer to the instance. The code will still have to do something like:
if ( !_sharedKeyWrapper ) {
_sharedKeyWrapper = [[SecKeyWraper alloc] initBlahBlah];
}
See links for more.
When you define a new class of objects, you can decide what instance
variables they should have. Every instance of the class will have its
own copy of all the variables you declare; each object controls its own
data.
However, you can't prescribe variables for the class object; there are
no "class variable" counterparts to instance variables. Only internal
data structures, initialized from the class definition, are provided for
the class. The class object also has no access to the instance variables
of any instances; it can't initialize, read, or alter them.
Therefore, for all the instances of a class to share data, an external
variable of some sort is required. Some classes declare static variables
and provide class methods to manage them. (Declaring a variable static
in the same file as the class definition limits its scope to just the
class-and to just the part of the class that's implemented in the file.
Unlike instance variables, static variables can't be inherited by
subclasses.)
Static variables help give the class object more functionality than just
that of a "factory" producing instances; it can approach being a
complete and versatile object in its own right. A class object can be
used to coordinate the instances it creates, dispense instances from
lists of objects already created, or manage other processes essential to
the application. In the case when you need only one object of a
particular class, you can put all the object's state into static
variables and use only class methods. This saves the step of allocating
and initializing an instance.
Also static variables are initialized once. You can use static variables in recursive calls. Simple Example Factorial.