I program for few years now and I have a question concerning the internal functioning of methods when we create an object.
So when we create objects in POO each object got its own attributes. but in fact they all share the same methods so how does it work internally ? do methods point on the same address for example in C++ and in this case if we consider a class with tones of methods and no attributes, what is the memory "payload" of creating a lot of object from that class?
In other words do the program duplicate the address points of all methods for each object or is there any other internal functioning to make them reach their methods?
Different languages can implement this differently. In C++ case, imagine the compiler takes your class methods and converts them to free functions. For conversion it add this as the first parameter.
C++
class A
{
public:
void f(bool b);
// and other members
};
A a;
a.f(true);
converts to something like:
struct A
{
// other members
}
void a_f(A* this, bool b);
A a;
a_f(&a, true);
So now we have a C program, no OOP to get confused with.
Related
Is it true that for C++ to work similarly in terms of modern OOP as in Java, Ruby, Python, the function (or methods) must be declared virtual and if not, what "strange" behaviors may occur?
I think it is true that for Java, Ruby, Python, and possibly other OOP languages that are late comers such as PHP and Lua, and even Smalltalk and Objective-C, all methods are just what is known as "virtual functions"?
"Method" is an unfortunately overloaded term that can mean many things. There's a reason C++ prefers different terminology, and that's because not only does it do something different from other languages, but it intends to do something different from what other languages do.
In C++ you call a member function. i.e. you externally make a call to a function associated with an object. Whether that function is virtual or not is secondary; what matters is the intended ordering of your actions - you're reaching into the object's scope, and commanding it to take a specific action. It might be that the object can specialize the action, but if so, it warned you in advance that it would do this.
In Smalltalk and the languages that imitate it (Objective-C most closely), you send a message to an object. A message is constructed on your side of the call consisting of a task name (i.e. method selector), arguments, etc., and the packed up and sent to the object, for the object to deal with as it sees fit. Semantically, it's entirely the object's decision what to do upon receipt of the message - it can examine the task name and decide which implementation to apply dynamically, according to a user-implemented choice process, or even do nothing at all. The outside calling code doesn't get to say what the object will do, and certainly doesn't get any say in which procedure actually runs.
Some languages fall in the middle ground, e.g. Java is inspired by the latter, but doesn't give the user any way to specify unusual dynamic responses - for the sake of simplicity every message does result in a call, but which call is still hidden from the external code, because it's entirely the object's business. C++ was never built on this philosophy of messages in the first place, so it has a different default assumption about how its member functions should operate.
The thing is that C++ is like the great grand father. It has many features, which often requires huge code definition.
Consider an example:
class A
{
virtual void fn() = 0;
};
class B: A
{
void fn();
};
#include "a.hpp"
#include "b.hpp"
int main()
{
A *a = new B();
a->fn();
}
This would implement overriding in C++.
Note that virtual void fn()=0 makes the class A abstract, and a pointer to base class (A) is essential.
In Java, the process is even simpler
abstract class A
{
abstract void fn();
}
class B extends A
{
void fn() {
//Some insane function :)
}
}
public static void main(String[] args) {
B ob = new B();
ob.fn();
}
Well, the effect is same; but the process is largely different. In short, C++ does have many features implemented in languages like Java, Ruby etc. but it is simply implemented using some (often complicated) techniques.
Regarding Php, since it is directly based on C++, there exists some syntax similarities between C++ and Php.
It is true that (for example) in Java all methods are virtual by default. In C++ it is possible to overload a non-virtual function (as opposed to overriding a virtual function) in a subclass, leading to possible counter-intutive behaviour, when only the base function is actually executed via a pointer or reference to the base class (i.e. when polymorphic behavior would normally be expected).
Because C++ is a value-based (as opposed to reference-based) language, then even when a function has been declared as virtual, the well known
object slicing problem can still arise: the superclass method is invoked when the type of a value object of a subclass is `cut down' to that of the base class (e.g. when the subclass is passed to a function which takes a base class argument by value).
For this reason, it is recommended to make all non-leaf classes abstract, something which is often achieved by providing a virtual destructor, even if such would otherwise be gratuitous.
I am developing a compiler for an object oriented language targeted on a virtual machine I wrote that I am using as a cross platform abstraction layer. I am sort of confused about how inherited methods works. Lets say I had the following lines of C# code.
class myObject : Object {
public int aField;
public override string ToString() {
return "Dis be mah object";
}
public void regularMethod() { }
}
Object test = new myObject();
Console.WriteLine(test.ToString());
Now this would output 'Dis be mah object'. If I called regularMethod however the compiled code would in reality do something like this:
struct myObject {
public int aField;
}
public static void regularMethod(ref myObject thisObject)
{
}
How would the inherited method ToString be handled after compilation? The compiler could not do what I did above with regularMethod, because if it did then 'Dis be mah object' would only be returned when creating myObject types and not plain Object types. My guess is that the struct myObject would contain a function pointer/delegate that would get assigned when a new instance is created.
If you are dealing with static overloading, it is really simple: you bind to the correct implementation when processing the code.
But, if you are working with dynamic overloading, you must decide things at runtime. For this you need to use dynamic dispatch, using the real object type. This is the same thign that is done with method overriding.
Dynamic dispatching is not the same as late binding. Here, you are chosing an implementation and not a name for your operation (despite the fact that this binding will occur at compile time, the implementation will only occur at runtime).
Staticly, you would only bind to implementation of the declared type of the object. It is done at compile time.
The are some mechanisms you could use to achieve the dynamic dispathing, it will dictate your language paradigm.
Is your language typed? Weakly typed?
C++, for instance, offers the two types of dispatch I mentioned. For the dynamic one (which I believe is the one you are interested), it uses a virtual table to do the mapping for one class. Each instance of that class will point have a pointer to that vtable.
Implementing
The vtable (one for all objects of same class) will have the addresses of all dynamicly bound methods. One of those addresses will be fetched from this table when a call is made. Type-compatible objects have tables with addresses with the same offset for the methods of all compatible classes.
Hope I've helped.
I have concept of static variables but what are the benefits of static methods in a class. I have worked on some projects but I did not make a method static. Whenever I need to call a method of a class, I create an object of that class and call the desired method.
Q: Static variable in a method holds it's value even when method is executed but accessible only in its containing method but what is the best definition of static method?
Q: Is calling the static method without creating object of that class is the only benefit of static method?
Q: What is the accessible range for static method?
Thanks
Your description of a static variable is more fitting to that found in C. The concept of a static variable in Object Oriented terms is conceptually different. I'm drawing from Java experience here. Static methods and fields are useful when they conceptually don't belong to an instance of something.
Consider a Math class that contains some common values like Pi or e, and some useful functions like sin and cos. It really does not make sense to create separate instances to use this kind of functionality, thus they are better as statics:
// This makes little sense
Math m = new Math();
float answer = m.sin(45);
// This would make more sense
float answer = Math.sin(45);
In OO languages (again, from a Java perspective) functions, or better known as methods, cannot have static local variables. Only classes can have static members, which as I've said, resemble little compared to the idea of static in C.
Static methods don't pass a "this" pointer to an object, so they can't reference non-static variables or methods, but may consequently be more efficient at runtime (fewer parameters and no overhead to create and destroy an object).
They can be used to group cohesive methods into a single class, or to act upon objects of their class, such as in the factory pattern.
Syntax (php) for static methods:
<?php
class Number {
public static function multiply($a, $b) {
return $a * $b;
}
}
?>
Client code:
echo Number::multiply(1, 2);
Which makes more sense than:
$number = new Number();
echo $number->multiply(1, 2);
As the multiply() method does not use any class variables and as such does not require an instance of Number.
Essentially, static methods let you write procedural code in an object oriented language. It lets you call methods without having to create an object first.
The only time you want to use a static method in a class is when a given method does not require an instance of a class to be created. This could be when trying to return a shared data source (eg a Singleton) or performing an operation that doesn't modify the internal state of the object (String.format for example).
This wikipedia entry explains static methods pretty well: http://en.wikipedia.org/wiki/Method_(computer_science)#Static_methods
Static variables and static methods are bound to the class, and not an instance of the class.
Static methods should not contain a "state". Anything related to a state, should be bound to an instantiated object, and not the class.
One common usage of static methods is in the named constructor idiom. See: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.8.
Static Methods in PHP:
Can be called without creating a class object.
Can only call on static methods and function.
Static variable is used when you want to share some info between different objects of the class.As variable is shared each object can update it and the updated value be available for all other objects as well.
As static variable can be shared,these are often called as class variable.
static elements are accessible from any context (i.e. anywhere in your script), so you can access these methods without needing to pass an instance of the class from object to object.
Static elements are available in every instance of a class, so you can set values that you want to be available to all members of a type.
for further reading a link!
Here's a Clone() implementation for my class:
MyClass^ Clone(){
return gcnew MyClass(this->member1, this->member2);
}
Now I have about 10 classes derived from MyClass. The implementation is the same in each case. Owing to the fact that I need to call gcnew with the actual class name in each case, I am required to create 10 nearly identical implementations of Clone().
Is there a way to write one single Clone() method in the base class which will serve all 10 derived classes?
Edit: Is there a way to invoke the constructor of a class via one of it's objects? In a way that will invoke the actual derived class constructor. Something like:
MyClass ^obj2 = obj1->Class->Construct(arg1, arg2);
I'm doing this on C++/CLI but answers from other languages are welcome.
In plain old C++, you can do this with compile-time polymorphism (the curiously-recurring template pattern). Assuming your derived classes are copyable, you can just write:
class Base
{
public:
virtual Base* Clone() const = 0;
//etc.
};
template <typename Derived>
class BaseHelper: public Base
{
//other base code here
//This is a covariant return type, allowed in standard C++
Derived * Clone() const
{
return new Derived(static_cast<Derived *>(*this));
}
};
Then use it like:
class MyClass: public BaseHelper<MyClass>
{
//MyClass automatically gets a Clone method with the right signature
};
Note that you can't derive from a class again and have it work seamlessly - you have to "design in" the option to derive again by templating the intermediate classes, or start re-writing Clone again.
Not in C++ that I'm aware of. As you say, you need to create an object of a different class in each implementation of Clone().
Hm, I think you can use Factory pattern here. I.e.:
MyClass Clone(){
return MyClassFactory.createInstance(this.getClass(), this.member1, this.member2, ...);
}
In the factory, you would have to create instance of subclass based on passed class type. So probably it has the same disadvantages as your approach.
I would suggest using copy constructors instead (as derived classes can call the base implementation's copy constructor as well) -- also handy, as it will be familiar territory for C++ programmers.
You might be able to create a single Clone method that uses reflection to call the copy constructor on itself in this instance.
Possibly also worth noting that Jeffrey Richter said in the Framework Design Guidelines book, "The ICloneable interface is an example of a very simple abstraction with a contract that was never explicitly documented. Some types implement this interface's Clone method so that it performs a shallow copy of the object, whereas some implementations perform a deep copy. Because what this interface's Clone method should do was never fully documented, when using an object with a type that implements ICloneable, you never know what you're going to get. This makes the interface useless" (emphasis mine)
When I first started working with object-oriented programming languages, I was taught the following rule:
When declaring a field in a class, don't initialize it yet. Do that in the constructor.
An example in C#:
public class Test
{
private List<String> l;
public Test()
{
l = new List<String>();
}
}
But when someone recently asked me why to do that, I couldn't come up with a reason.
I'm not really familiar with the internal workings of C# (or other programming languages, for that matter, as I believe this can be done in all OO languages).
So why is this done? Is it security? Properties?
If you have multiple constructors, you might want to initialize a field to different values
When you initialize the field in the constructor, there can be no confusion over when exactly it is initialized in regard to the rest of the constructor. This may seem trivial with a single class, but not so much when you have an inheritance hierarchy with constructor code running at each level and accessing superclass fields.
The C# compiler will take any non-static member intialization that you do inline and move it into the constructor for you. In other words this:
class Test
{
Object o = new Object();
}
gets compiled to this:
class Test
{
Object o;
public Test()
{
this.o = new Object();
}
}
I am not sure how compilers for other languages handle this but as far as C# is concerned it is a matter of style and you are free to do whichever you wish. Please note that static fields are handled differently: read this article for more information on that.
One reason to do it is that it puts all of the initialization code in one place which is convenient for others reading your class. Having said this I don't really do it for two primary reasons. (1) I use TDD/Unit testing to define the behavior of my class. If you want to know what the parameterless constructor does, you should really read the tests I've built on the parameterless constructor. (2) With C# 3.0, I typically use automatic properties and inline initialization with a parameterless constructor to instantiate the object. This is much more flexible and it puts the definition of the properties right in line where the code is being used. This would override any initialization in the constructor so I rarely put any there. Of course, this only applies to C#.
Ex. (of 2)
var foo = new Foo { Bar = "baz" };
public class Foo
{
public string Bar { get; set; }
public Foo() { }
}
sometimes the constructor has parameters that are used for initializing internal variables. For example size of arrays
I haven't heard a compelling reason to not offer both options. I suspect that the real reason has to do with simplifying the language structure from a parsing perspective. This is especially true in C-derivative languages where parsing an assignment statement requires 75% of the language syntax rules. It seems to me that allowing it and defining how it would work precisely would be nice. I agree with Michael's comment about the complexity increase as you insert inheritance and multiple constructors but just because you add a feature doesn't mean that you have to use it. I would vote to support both even though my vote doesn't really add up to much.
I always like to think of the class as a factory for objects, and the constructor as the final stop on the production line. The fields declared in the class are blueprints descirbing the object, but the blueprint won't be realised into an object before such an object is ordered tthrough a call to the constructor... Also, as someone pointed out, doing all your initialisations in your constructor will improve readability, as well as it wil provide for dynamicity in initialisation (it might not be a parameterless constructor you're dealing with).
Also, in some languages the constructor may be used for resetting an object to an original state, which is why it will then be necessary to instatiate the object in the constructor.