What are public, private and protected in object oriented programming?
They are access modifiers and help us implement Encapsulation (or information hiding). They tell the compiler which other classes should have access to the field or method being defined.
private - Only the current class will have access to the field or method.
protected - Only the current class and subclasses (and sometimes also same-package classes) of this class will have access to the field or method.
public - Any class can refer to the field or call the method.
This assumes these keywords are used as part of a field or method declaration within a class definition.
They aren't really concepts but rather specific keywords that tend to occur (with slightly different semantics) in popular languages like C++ and Java.
Essentially, they are meant to allow a class to restrict access to members (fields or functions). The idea is that the less one type is allowed to access in another type, the less dependency can be created. This allows the accessed object to be changed more easily without affecting objects that refer to it.
Broadly speaking, public means everyone is allowed to access, private means that only members of the same class are allowed to access, and protected means that members of subclasses are also allowed. However, each language adds its own things to this. For example, C++ allows you to inherit non-publicly. In Java, there is also a default (package) access level, and there are rules about internal classes, etc.
All the three are access modifiers and keywords which are used in a class.
Anything declared in public can be used by any object within the class or outside the class,variables in private can only be used by the objects within the class and could not be changed through direct access(as it can change through functions like friend function).Anything defined under protected section can be used by the class and their just derived class.
A public item is one that is accessible from any other class. You just have to know what object it is and you can use a dot operator to access it. Protected means that a class and its subclasses have access to the variable, but not any other classes, they need to use a getter/setter to do anything with the variable. A private means that only that class has direct access to the variable, everything else needs a method/function to access or change that data. Hope this helps.
as above, but qualitatively:
private - least access, best encapsulation
protected - some access, moderate encapsulation
public - full access, no encapsulation
the less access you provide the fewer implementation details leak out of your objects. less of this sort of leakage means more flexibility (aka "looser coupling") in terms of changing how an object is implemented without breaking clients of the object. this is a truly fundamental thing to understand.
To sum it up,in object oriented programming, everything is modeled into classes and objects.
Classes contain properties and methods.
Public, private and protected keywords are used to specify access to these members(properties and methods) of a class from other classes or other .dlls or even other applications.
These are access modifiers.
All the data and functions(behaviours) are encapsulated or bounded into a single unit called a class. In order to access the properties and behaviour of the class we use objects. But it's also important to decide which behaviour or property has to be exposed or should remain accessible to all the classes and which behaviour or property has to be private.
So, that's when access modifiers like public, private, protected and protected internal help in doing so. This act of defining privilege to the class or method or property is called as abstraction.
Related
When should I mark the static method of a specific class as private other than public?
What aspects should I consider when making such considerations.
What are the advantages for mark the static method as private?
Any simple example would be appreciated, which could help me fully understand this matter.
UPDATE:
As per this answer, which says that[emphasise mine]:
This function could easily have been made freestanding, since it doesn't require an object of the class to operate within. Making a function a static member of a class rather than a free function gives two advantages:
It gives the function access to private and protected members of any object of the class, if the object is static or is passed to the function;
It associates the function with the class in a similar way to a namespace.
How to fully understand the statements above?
The general rule for methods (static or otherwise) is to make them private if possible — i.e. unless you absolutely need them to be callable from other classes (which is to say, you need them to be part of your class’s public API)
The reason for making as much as possible private is simple: in the future, you’ll be able to change anything that is private, without breaking a bunch of other classes that were written to call the old version of the method. Changing a public method is more problematic, because other classes might be depending on it.
class Person
{
private BankAccount account;
Person(BankAccount account)
{
this.account = account;
}
public Person someMethod(Person person)
{
//Why accessing private field is possible?
BankAccount a = person.account;
}
}
Please forget about the design. I know that OOP specifies that private objects are private to the class. My question is, why was OOP designed such that private fields have class-level access and not object-level access?
I am also a bit curious with the answer.
The most satisfying answer that I find is from Artemix in another post here (I'm renaming the AClass with Person class):
Why have class-level access modifiers instead of object-level?
The private modifier enforces Encapsulation principle.
The idea is that 'outer world' should not make changes to Person internal processes because Person implementation may change over time (and you would have to change the whole outer world to fix the differences in implementation - which is nearly to impossible).
When instance of Person accesses internals of other Person instance - you can be sure that both instances always know the details of implementation of Person. If the logic of internal to Person processes is changed - all you have to do is change the code of Person.
EDIT:
Please vote Artemix' answer. I'm just copy-pasting it.
Good question. It seems that object level access modifier would enforce the Encapsulation principle even further.
But actually it's the other way around. Let's take an example. Suppose you want to deep copy an object in a constructor, if you cannot access the private members of that object. Then the only possible way is to add some public accessors to all of the private members. This will make your objects naked to all other parts of the system.
So encapsulation doesn't mean being closed to all of the rest of the world. It means being selective about whom you want to be open to.
See the Java Language Specification, Section 6.6.1. Determining Accessibility
It states
Otherwise, if the member or constructor is declared private, then
access is permitted if and only if it occurs within the body of the
top level class (§7.6) that encloses the declaration of the member or
constructor.
Click the link above for more details. So the answer is: Because James Gosling and the other authors of Java decided it to be that way.
This works because you are in the class Person - a class is allowed to poke inside it's own type of class. This really helps when you want to write a copy constructor, for example:
class A
{
private:
int x;
int y;
public:
A(int a, int b) x(a), y(b) {}
A(A a) { x = a.x; y = y.x; }
};
Or if we want to write operator+ and operator- for our big number class.
Just my 2 cents on the question of why the semantics of the private visibility in Java is class level rather than object level.
I would say that convenience seems to be the key here. In fact, a private visibility at object level would have forced to expose methods to other classes (e.g. in the same package) in the scenario illustrated by the OP.
In truth I was not able neither to concoct nor to find an example showing that the visibility at class-private level (like offered by Java) creates any issues if compared to visibility at object-private level.
That said, programming languages with a more fine-grained system of visibility policies can afford both object visibility at object level and class level.
For example Eiffel, offers selective export: you can export any class feature to any class of your choice, from {NONE} (object-private) to {ANY} (the equivalent of public, and also the default), to {PERSON} (class-private, see the OP's example), to specific groups of classes {PERSON, BANK}.
It's also interesting to remark that in Eiffel you don't need to make an attribute private and write a getter to prevent other classes from assigning to it. Public attributes in Eiffel are by default accessible in read-only mode, so you don't need a getter just to return their value.
Of course you still need a setter to set an attribute, but you can hide it by defining it as "assigner" for that attribute. This allows you, if you wish, to use the more convenient assignment operator instead of the setter invocation.
Because the private access modifier renders it visible only within the class. This method is still IN the class.
the private field is accessible in the class/object in which the field is declared. It is private to other classes/objects outside of the one it is located in.
First thing here we have to understand is all we have to do is must follow oops principles so encapsulation is say that wrap data within package(i.e. class) and than represent all data as Object and easy to access. so if we make field as non-private than
it's accessed indivisually. and it result into bad paratice.
With reflection concept in Java is possible modify fields and methods privates
Modificando metodos y campos privados con Refleccion en Java
One of the main design principles of C++ as an Object Oriented language is to let each class enforce who can access it's innards and who can't. A base class controls it's access levels even if it is with respect to a base class object embedded within a object of a class derived from Base.
However I read about using declarations that can be allowed to change the access level of a Base class member in a Derived class.
class A{
protected:
int n;
};
class B:protected A{
public:
using A::n;
};
int main(){
B obj;
obj.n=0;
}
In the example above, I am changing the access level of a protected member to public in the derived class. Is this allowed on purpose? Why couldn't it be enforced that the access level should be more stringent or equal to how it is in the base class (this can be controlled by the access qualification in the derivation list as well) but cannot be more relaxed than the access level in the base class? Is this allowed because this allows some interesting design patterns? I am just trying to understood why something like this which breaks the principle of encapsulation enforced by a class is allowed?
One reason is that this way you can even change the visibility modifier even if you are not the owner of the base class.
For example if the base class is included in an external library/framework you can change the visibility modifier.
I just want to make sure I got the idea of public and private right.
Regarding the private access specifier, does it mean:
Only accessed inside the class
Cannot be accessed from the object of the class unless there are public class methods that can be used to access them (Can other objects use those public functions?)
No other object can access them
And for public:
Accessed from the object of the class
Accessed from any other object
Is that right?
private : Only member functions and friends of class can access it.
public : Can be accessed anywhere where the object has scope.
Answering the questions -
private:
Yes
Yes. (Can other objects use those public functions? With out having class relations, one object of class cannot communicate to the other's members. )
Friends has access to private members of a class. So, answer depends upon your class has friends or not.
public:
Yes
Depends whether the object has hierarchical relations to the member's class you are trying to access.
I think there is an issue of vocabulary to begin with.
In C++ (and most languages) a class is a type. You can think about it as a blueprint to actually build something.
it describes the attributes that are held
it describes the methods to operate on those attributes
it describes the restrictions that apply: this is the "accessibility"
An object is produced by actually instantiating a class, that is, building what the blueprint described. It is more or a less a bundle of attributes. You can have several objects of the same class as you can have several houses from the same blueprint: note that their physical location is different for obvious reasons :)
Now, on to the accessibility. There are 3 typical levels of accessibility: public, protected and private.
public, as expected, means that everyone is given access to either attributes or methods
protected is somewhat less trivial. It means that only the object, or its children, may access the attributes (bad idea*) or methods. (Plus, in C++, friends)
private means that only the objects of that class (and not their children) may access the attributes or methods (Plus, in C++, friends)
Note: whatever the level of accessibility, an object has unrestricted access to all the attributes and methods of any object of the same class.
(*) Even though it pops up now and there, it is generally a bad idea to use protected attributes. The point of encapsulation is to hide the details, not only for the sake of it, but because by precisely controlling who can access the data, we can ensure that the class maintains its invariants (simple example, an array where you would store the size separately, you need to ensure that the "size" really represents the number of items in the array, at all times). Note: this restriction does not apply when you can seal a hierarchy, like in C# for example.
Private members can only be accessed by member functions and static functions of the same class and by friends of the class. It does not matter on which object that function is called. So the case
class Foo
{
private:
void bar() {}
public:
void baz(Foo& var)
{
var.bar();
}
}
is perfectly legal.
That seems correct. Data members and functions marked public can be accessed from anywhere by anyone. Data members and functions marked private can only be accessed by the class and its friends. However, a member function of a class can access data with any access specifier, so a public function can read and write private data members (this is used universally in OOP).
In c++ data and fn are encapsulated as 1 unit.
We begin a program by writing
preprocessor directives
Then, class declaration
Followed by function(fn) declaration where we also specify the access modifier ( public, private or protected)
& finally the main () program.
If we declare a fn
Private:the data within an object of a class is only accessed by fn defined within it- (the object which has the data and the private fn)
Public:the data can be accessed by any fn
Protected:similar to private however data can also be accessed by sub-classes that inherit the properties of another class.
Example if class A inherits from class B, thenA is a subclass of B.
Structure's member are public by default ans class's members are private by default. We can access private data members through a proper channel (using member function). If we have access to member functions we can read/write data in private data member, so how it is secure...we are accessing it and we are changing data too.....
Access specifiers, such as private and public have nothing to do with security. In C++, for example, these specifications get compiled away and don't even exist in the compiled binary.
Rather, access specifiers are designed to make types easy to use correctly, and difficult to use incorrectly.
There are only two syntactic differences between class and struct:
In a class, members and base classes are by default private, whereas in a struct, they are public by default.
For historical reasons, class can be used instead of typename to declare a template type parameter.
In fact, there's no difference between
struct X {
void f() {}
private:
int i;
};
and
class Y {
int i;
public:
void f() {}
};
The only way that a class is more "secure" than a struct is that it gently "presses" you towards better encapsulation by defaulting to private members.
There really was no need to introduce a new keyword class, Stroustrup himself said that on a few occasions. (Read his book The Design and Evolution of C++ if you're interested in such things.) Basically, class was introduced to emphasize the fact that a struct with member functions isn't really a "structure" anymore in the way the term was used in C: a collection of loosely coupled objects of built-in types. It's a "class" in the sense the term is used in statically typed object-oriented languages, even though syntactically, it's no real difference from a struct.
primarily because the member functions can validate the values before storing them. eg say you have a field called 'age', which must be between 0 and 150. in a structure (or a class with a public field), you could just do obj.age = 200. whereas if you only had a setAge(int) method, that method could sanity check the value is between 0 and 150 before storing, possibly throwing an exception if necessary or just clamping the value if not.
the public/private/protected keywords are not meant for security but rather for encapsulation. you design your class by separating the implementation (where the private/protected members are used - those actually used to implement the functionality) from the interface (where you expose the public members - which users of the class -other objects- will access/call) and thus you are later able to change the private parts without changing the interface. the outside objects only need to know about the public members and use these safely regardless of the implementation.
if you are speaking about security, think that anyone could change the private members of your class if they really wanted to and knew the internal structure of the class , just by overwriting the appropriate memory locations with new values - but the problem here is not about security, it's at a lower level
Taking that classes and structs are exactly the same besides the default access, the question is how encapsulating the internal representation from the interface helps build more robust code.
The main difference is that you control when and how your class data is modified, and as such you can control your class invariants. Consider a simple vector class that has a pointer and a size. By having them private you can control how they change: if the resize method is called, both the pointer and the internal size variable will be coherently updated, keeping the invariant that accessing any position in the range [0..size) is well defined. If the member attributes were public, user code would update the storage or size without actually updating the other field, and that invariant could be broken.
Many classes have internal invariants for the correct usage. If you write a class that contains a string with user email addresses, by providing an accessor method you can control that the value passed in is a valid email address, while if the email field was public user code could reset it to anything...
The whole thing is that you control how your members are accessed and modified and that reduces the number of places where mistakes can be made and/or your chances of detecting it.