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C++ Namespaces
2022-05-25 13:54:12

C++ Namespaces

An Overview

In each scope, a name can only represent one entity. As a result, there cannot be two independent variables with the similar names in the same scope, as this may cause compiler issues. Using namespace, we may define two different variables or member functions with the same name within the same scope.

This will result in numerous functions with the same name, and we will be able to access all of them from wherever in the code by referencing the namespace name. The purpose of this article is to explain namespaces and how they are being used in C++.

What is the need for a Namespace in C++ ?

Consider the following scenario:

we're developing some code using the method xyz(), and there's another prepackaged library with the identical function xyz(). The compiler now has no idea which version of the xyz() function we're talking about in our code when it comes time to compile it.

To tackle this challenge, namespace was created and is used as extra information to distinguish comparable functions, variables, classes, and other objects with the same name in various libraries. We may specify the situation in which names are constructed using namespace. A namespace, in essence, specifies a scope.

Since all the types and functions in the C++ standard library are defined in the standard namespace or namespaces contained inside std, it is frequently utilized in most applications.

Example for more clarity :

Assume a circumstance in which an institution has two students with the exactly similar name. Then we have to distinguish them in a new way, and we'll probably have to provide some more information with their name, such as their roll number, parents' names, or email address. The similar scenario might occur in C++ programming when you create code with a function name, such as func(), when another library already exists with the same function name. The compiler will pause as a result of this, and it will be unable to determine which of these two functions to employ within the C++ code. To overcome this problem, namespaces are employed.

What is namespace exactly?

A namespace is a set of blocks that may be used in a code to group together similar objects. Within a namespace, you can relate to all variables, functions or classes.

Namespace declaration syntax :

Since the namespace is a declaratory zone that provides distinct scopes for multiple functions and variables, we may refer to one of these variable functions by its name :

namespace nameofnamespace
{
   int x,y; //declarations of code where 
             // x and y are declared in 
             // name_of_namespace's scope
}

Explanation :

  • namespace - The term namespace is used to define a namespace.
  • nameofnamespace - This is the name that the namespace has been given.
  • Int x,y – These are the variables in the scope of name_of_namespace.

To access a variable in a specified namespace, we can use scope resolution operator :: , as demonstrated below:

name_of_namespace :: nameofvariables
Name_of_namespace :: nameoffunctions

How to create a namespace :

The process of creating a namespace is identical to that of creating a class.

Example :

namespace PersonalSpace
{
    // declaring the namespace
}




int main() 
{
    // main funct
}

Explanation :

The above code will help us to establish a new namespace named PersonalSpace, which we can use to store our member declarations and further which will be used in the main function of the code.

The following are some of the rules for the namespace declarations :

1. The namespace must be defined at the global level or nested within another namespace.

2. In contrast to class definitions, namespace definitions do not end with a semicolon.

3. For convenience, an alias name or nickname can also be used for the name of our namespace.

Alias as an example:

namespace ReadWriteLearn
{
    void read();
    class Study 
    {  
        // class defintion
    };
}




// Rwl is an alias for ReadWriteLearn 
namespace Rwl = ReadWriteLearn;

Explaination :

In this example, we declared a namespace called ReadWriteLearn and a class named Study. Then we created an alias Rwl for namespace ReadWriteLearn and further used it for our convenience. In a way, Rwl is basically another name for ReadWriteLearn namespace.

4. You can't make a namespace instance.

5. Unnamed namespaces are also possible. Each translation unit has its own unnamed namespace. They behave in the same way as named namespaces do.

namespace
{
    class First 
    { 
        // defining a class
    };
    // another class
    class Second 
    { 
        // defining a class
    };
    int x,y,z;
}




int main() 
{ 
    // main funct
}

Explanation:

In this example, we have declared an unnamed namespace and 2 different classes First and Second and three different variable x, y and z. This unnamed namespace is similar to named namespace and will be called using keyword namespace only.

6. A namespace declaration can be carried over and expanded across many files without being redefined or overridden.

Here's an example of a head1.h header file in which we declare a namespace:

namespace PersonalSpace
{
    int i;
    void a();
}

The head1.h header file can then be included in another head2.h header file, expanding an existing namespace:

#include "head1.h";




namespace PersonalSpace
{
    int j;
    void b();
}

Explanation :

In the above example, a head1.h header file was created with namespace Personalspace and its variable i and void function a(). Then another head2.h header file was created which expanded the existing head1.h header file by including that head1.h header file and other variable j and function b().

Working of namespace in C++ :

When a namespace is created using the keyword namespace, any functions, variables, and classes defined inside that namespace are part of that scope and may be referred to with the scope resolution operator:: by using the name of the namespace.

When the scope resolution operator:: is encountered, the compiler sends control to the namespace block. If a namespace is unnamed then the variables and functions provided inside it can be directly accessed in the same way in which global variables are accessed.

Some namespaces may contain nested namespaces. To get access to them, we must utilise the scope resolution operator:: the amount of times that they are available.

Example :

namespace examplex {
  namespace exampley {
    namespace examplez {
      int sample_variable;
    }
  }
}

Explanation :

In the above example, we declared a namespace examplex, which further contains another namespace exampley, which contains another namespace examplez which finally contains a variable called int sample_variable. So in order to access sample_variable, we will have to call examplex::exampley::examplez::sample_variable.

We may utilise this to create individual scopes and reuse variable names that already exist in other namespaces in our software. We may handle files from several libraries by using namespace.

Previously, if we wanted to utilise a namespace, we had to provide the namespace identifier followed by the scope resolution operator when referring to the namespace functions. You may, however, inject a whole namespace into a portion of code by utilizing the using- directive like in the syntax given below:

using namespace nameofnamespace;

While we are in the current scope, this above mentioned way enables developers to call functions from within the namespace without having to declare the function's namespace.

If you're not in the middle of the block of code, before the following closing bracket, or the entire file then using a namespace worldwide can take advantage of this simplicity, which goes against some of the arguments for using a namespace.

Ways to use namespace in C++ :

A namespace may be used in three different ways in a code.

  • Scope resolution operator (::)
  • Using-directive
  • Using-declaration

Scope resolution operator (::) :

Using the name of namespace and the scope resolution operator:: with the identifier, any name (identifier) declared in a namespace may be specifically stated.

Example :

#include<iostream>
using namespace std;
//Declaring namespaces
namespace num1 {
  int func() {
    return 100;
  }
}
namespace num2 {
  //Declaring another var in the namespace
  const double z = 20;
  //Function defined within namespace
  double func() {
    return 2 * z;
  }
}




namespace num3 {
  // a class created in the namespace
  class sample_class {
    public:
      void give() {
        cout << "num3::sample_class::give()\n";
      }
  };
}




int main() {




  // Accessing the function within namespace num1
  cout << num1::func() << '\n';
  // Accessing the func within the namespace num2
  cout << num2::func() << '\n';
  // directly accessing the var z
  cout << num2::z << '\n';
  // Creating an obj of sample_class
  num3::sample_class obj;
  obj.give();
  return 0;
}

Output :

100
4000
20
num3::sample_class::give()

Explanation :

We defined the very similar function name in 2 distinct namespaces, num1 and num2, in the code above. The class sample_class has also been declared and accessed in another namespace, num3.

The scope resolution operator:: is used to obtain the functions of the several namespaces, as previously explained. We may call the function func() using the operator num1::func(), as is shown in the code.

Thus we got our required output using the scope resolution operator:: .

Using-directive:

Using the using namespace directive, we can avoid prepending namespaces. We may import a whole namespace into your application with a global scope by utilising the keyword. It may be used to import a namespace into some other namespace or into any application as needed.

Example :

#include <iostream>
using namespace std;
//Creating nested namespaces
namespace num1 {
  // Declaring a var within namespace num1
  int sample_variable = 100;
  namespace num2 {
    namespace num3 {
      // Declaring var within namespace num3 
      // & initializing to var sample_variable in namespace num1
      int sample_variable1 = sample_variable;
    }
  }
}
//Created a namespace that allows us to use it particularly to obtain a var in nested namespace
namespace newalias = num1::num2::num3;
//Creating namespace exmpl & declaring initialized var
namespace exmpl {
  int varg = 20;
}
//Inserting entire namespace exmpl into the code
using namespace exmpl;




int main() {
  //Accessing var without scope resolution operator :: as it’s a global namespace
  cout << " global variable value in exmpl namespace = " << varg << "\n";
  //Accessing value of sample_variable1 using namespace newalias
  cout << " sample_variable1 value in nested namespace 3 = " << newalias::sample_variable1 << "\n";
  return 0;
}

Output :

global variable value in exmpl namespace = 20
sample_variable1 value in nested namespace 3 = 100

Explanation :

In the above example,it can be seen how nested namespaces like namespace num2, num3 and exmpl are defined and accessed from outside the namespace in the code above. The namespace num1 has variable sample_variable=100 with namespace num2 nested inside it and further namespace num3 nested inside num2 while containing another variable sample_variable1, which has the same value as sample_variable. We also showed how to use the using directive to obtain a variable in the namespace without using the scope resolution operator::. We've also shown how to establish the namespace newalias, which allows us to utilise the alias's name explicitly for some certain use. And finally in the main function the value of global variable in namespace exmpl and value of sample_variable1 are called and printed.

Using-declaration :

We include all the identifiers in the namespace when we use the using-directive, they are accessible throughout the application, thus they have a global scope.

However, when we use using-declaration, we only import one unique name at a time, and it is only available within the current scope.

NOTE: A name imported with a using declaration can override a name imported with a directive.

Example :

#include <iostream>




using namespace std;
// Creating namespaces with similar name
namespace samplespace {
  // Declaring var a within namespace
  int a = 10;
}




namespace samplespace {
  //Declaring var b within namespace
  int b = 25;
}
// Declaring unnamed namespace
namespace {
  //Declared and initialized the z variable
  int z = 30;
  // Func defined within unnamed namespace
  int func() {
    return z;
  }
}




int main() {
  // Accessing var x within namespace samplespace
  cout << "Value of a = " << samplespace::a << '\n';
  // Accessing var y within namespace samplespace
  cout << "Value of b = " << samplespace::b << '\n';
  // Accessing func within unnamed namespace
  cout << "Value returned by unnamed namespace = " << func() << '\n';
  return 0;
}

Output:

Value of a = 10
Value of b = 25
Value returned by unnamed namespace = 30

Explanation :

We can see how to construct many namespaces with the similar name in the example above. Two namespaces as namespace samplespace, each with its own set of variables, have used the same name. The first namespace samplespace has a variable a=10 and the second namespace samplespace has another variable b=25. As a result, within both of them, we have variables with the same scope. We are also able to know how to use the function func() to return the value of the unnamed namespace and construct the unnamed namespace. Finally in the main function the values of a, b and the value returned from func() are called and printed as output.

We never provide the parameter list of a function when importing it with declaration, therefore if a namespace includes overloaded functions, there will be uncertainty.

Benefits of a Namespace in C++ :

  • Namespace can be used to specify distinct scopes in a code, allowing different identifiers defined inside them to have different scopes.
  • The similar variable names can be utilized again in a separate code by utilizing namespace.
  • The usage of collections - Namespaces and named spaces make it much easier to combine the two libraries, and declaring their name using the scope resolution operator (::) helps us figure out which variable we're trying to access.

Conclusion :

With the explanation examples mentioned above, we've seen the advantages of using namespace in this project. We may avoid confrontations in projects with the similar variable and function names inside the same application by utilizing namespace. The usage of namespace to specify identical code in many files and libraries improves the readability of our code.