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USING NAME STD IN C++

 Why is 'using namespace std' used

 Sep 3, 2011 at 3:48am

jayahere11 (2)

Hi,

I'm just starting to learn C++.
What I've learnt till date is we use <iostream> which contains everything needed for input/output.
Then also we need to type 'using namespace std' otherwise the code throws an error.

So my question is what is the necessity of using this namespace if everything is there in <iostream> for cout, cin etc..

I searched the net but everywhere the only thing i could find is the definition of 'namespace' and that 'namespace std' is a 'standard namespace'.

I would be glad if any of u Gurus kindly clarify my doubt. Many thanks in advance!

Regards,
Jaya

 Sep 3, 2011 at 4:01am

closed account (Gz64jE8b)

Without using namespace std; when you write for example cout <<; you'd have to put std::cout <<;

Here's a small example for you:

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#include <iostream> int main() { std::cout << "Hello World"; system("pause"); return 0; }

Edit & Run

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#include <iostream> using namespace std; int main() { cout << "Hello World"; system("pause"); return 0; }

Edit & Run

They both do the exact same thing.

 Sep 3, 2011 at 4:04am

Kyon (912)

Let's start with a problem to explain what namespaces are. We all know that we can't have functions, classes or any other kind of data that have the same name. Let's say that we have two libraries that both add a function, let's say print(). They both give a different function, but in naming they are indistinguishable. That's where namespaces come in. A namespace is like adding a new group name to which you can add functions and other data, so that it will become distinguishable.

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namespace bla { void print() { // function bla::print() } } namespace blabla { void print() { // function blabla::print() } }

Now, as you can see, there are two functions with the name print, yet there is no naming conflict, because of namespaces. The first function would be called by using: bla::print() and the second with blabla::print().

std is an abbreviation of standard. std is the standard namespace. cout, cin and a lot of other things are defined in it. (This means that one way to call them is by using std::cout and std::cin.)

The keyword using technically means, use this whenever you can. This refers, in this case, to the std namespace. So whenever the computer comes across cout, cin, endl or anything of that matter, it will read it as std::cout, std::cin or std::endl.

When you don't use the std namespace, the computer will try to call cout or cin as if it weren't defined in a namespace (as most functions in your codes). Since it doesn't exist there, the computer tries to call something that doesn't exist! Hence, an error occurs.

If you have any trouble understanding the above, please tell me and I will try to clarify.

THE VOLLEYBALL KILLER

JAVA-INTERFACE

An interface is a reference type in Java, it is similar to class, it is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Along with abstract methods an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Writing an interface is similar to writing a class. But a class describes the attributes and behaviours of an object. And an interface contains behaviours that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways:

• An interface can contain any number of methods.
• An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
• The byte code of an interface appears in a .class file.
• Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including:

• You cannot instantiate an interface.
• An interface does not contain any constructors.
• All of the methods in an interface are abstract.
• An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
• An interface is not extended by a class; it is implemented by a class.
• An interface can extend multiple interfaces.

Declaring Interfaces:

The interface keyword is used to declare an interface. Here is a simple example to declare an interface:

Example:

Below given is an example of an interface:

/* File name : NameOfInterface.java */
import java.lang.*;
//Any number of import statements

public interface NameOfInterface
{
   //Any number of final, static fields
   //Any number of abstract method declarations\
}

Interfaces have the following properties:

• An interface is implicitly abstract. You do not need to use the abstractkeyword while declaring an interface.
• Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
• Methods in an interface are implicitly public.

Example:

/* File name : Animal.java */
interface Animal {

   public void eat();
   public void travel();
}

Implementing Interfaces:

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */
public class MammalInt implements Animal{

   public void eat(){
      System.out.println("Mammal eats");
   }

   public void travel(){
      System.out.println("Mammal travels");
   } 

   public int noOfLegs(){
      return 0;
   }

   public static void main(String args[]){
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
} 

This would produce the following result:

Mammal eats
Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

• Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
• The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
• An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

• A class can implement more than one interface at a time.
• A class can extend only one class, but implement many interfaces.
• An interface can extend another interface, similarly to the way that a class can extend another class.

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

//Filename: Sports.java
public interface Sports
{
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
}

//Filename: Football.java
public interface Football extends Sports
{
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
}

//Filename: Hockey.java
public interface Hockey extends Sports
{
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

public interface Hockey extends Sports, Event

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ABSTRACT METHODS AND CLASSES

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OVERRIDING IS JUST THE FAKE USE OF A METHOD .....ISN'T IT ?

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