C++ Tutorial index

C++ Tutorial C++ History C++ installation C++ First Program C++ cin and cout C++ Data type C++ Variable C++ operator C++ Keywords C++ OOPS Concepts C++ Comments C++ Data Abstraction C++ Identifier C++ Memory Management C++ Storage Classes C++ Void Pointer C++ Array to function C++ Expressions C++ Features C++ Interfaces C++ Encapsulation

C++ Control Statements

C++ If C++ Nested if C++ If-else C++ If-else-if C++ Switch C++ Break C++ Continue C++ Goto C++ for loop C++ While loop C++ Do while loop

C++ functions

function in C++ C++ Call by Value C++ Call by Reference C++ Recursion function C++ inline function C++ Friend function C++ Friend functions function Pointer in C++ Sizeof() Operators in C++ C++ Math functions swap() function in C++ rand() and srand() in C / C++ wcscpy(), wcslen(), wcscmp() functions in C++ sort() function in C++ C++ vardiac() function System() function in C++ Fread function in C++ Programming Fscanf function in The C++ Gmtime function in C/C++ abs() function in C++ Sleep function in C++ Stoi function in C++ String erase() in C++ String Trim in C++ isprint() function in c++ is_trivial function in C++ div() function in C++ Set insert function in C++ rint(), rintf(), rintl() in C++ String append() function in C++ String push_back() in C++ String replace() in C++ Transform function in C++ exit() and _Exit() in C and C++ Std partition_point in C++ unique() in C++ ctime() function in C++ Gets() and Puts() in C++ Hypot(), hypotf() and hypotl() in C++ Log function in C++ Nearbyint() function in C++ setf() function in C++ Fill Function in C++ Map Find Function in C++ Malloc and Calloc in C++ Operator Function in C++ cin.get() in C++ seekp() Function in C++ eof() Function in C++ Emplace Back in C++ C++ macro function C_str in C++ Clock() in C/C++ round() function in C++ scalbn() function in C++ Stable Sort function in C++ Rotate in C++ Std:: Sort_heap in C++ Std::includes() in C++ STL std::memset() function in C++ Wcscmp() function in C++ Static Member function in C++ atof() function in C++ C++ Sin() function with Example cin.ignore() function in C++ Member function in C++ Pop_front Vector in C++ Vector resize() in C++ Malloc() and new in C++ std::distance in C++ C++ Date and Time fork() in C++ std::min in C++ isprint() in C++ rdbuf() in C++ fixed() Function in C++ setw() in C++ seekg() Function in C++ isdigit() Function in C++

C++ Function Miscellaneous

Precision of floating point numbers Using these functions floor(), ceil(), trunc(), round() and setprecision() in C++ function overloading in C++ Add two numbers using the function in C++ Advantage and disadvantage friend function C++ ATM machine program in C++ using functions Returning Multiple Values from a function using Tuple and Pair in C++ Find the Size of Array in C/C++ without using sizeof() function Pure Virtual function in C++ With Example Program Returning a function Pointer from a function in C/C++ Passing a Vector to a function in C++ functions in C++ With Types and Examples C++ Macro function Example Using Default Arguments with Virtual functions Virtual functions and Runtime Polymorphism Fesetround() and Fegetround() in C++ and their application Object as function Argument in C++ Private Member function in C++ Function Overriding in C++ C++ Program for Addition of Two Numbers Using functions function Object in C++ Elements of User Defined function in C++ Inline Functions in C++ Virtual Function in C++

C++ Arrays

Single dimension array Two dimension array

C++ Strings

C++ Strings

C++ Inheritance

C++ inheritance Single level inheritance Multilevel inheritance Multiple inheritance Hierarchical inheritance Hybrid inheritance

C++ Polymorphism

C++ Polymorphism C++ Overloading C++ Overriding C++ Virtual function

C++ Pointers

C++ Pointers C++ this pointer

C++ Exception Handling

C++ Exception Handling

C++ Constructors

C++ Constructors Default Constructor Parameterize Constructor Copy constructor Constructor Overloading Destructor

C++ File Handling

C++ File Handling C++ Writing to file C++ Reading file C++ Close file

C++ Programs

Structure of C++ Program Hello World Program in C++ C++ Program to Implement Shell Sort C++ Program to Implement Merge Sort C++ program to read string using cin.getline() C ++ Program: Alphabet Triangle and Number Triangle C++ Program: Matrix Multiplication C++ Program to Print Fibonacci Triangle C++ Program to find the product array puzzle C++ Program To Find Largest Subarray With 0 Sum C++ Program To Move All Zeros To The End of The Array C++ Program to find the element that occurs once C++ Program to find the largest number formed from an array Palindrome Number Program in C++ LCM Program in C++ Armstrong Number Program in C++ Factorial Program in C++ C++ Prime number program Program to convert infix to postfix expression in C++ Programs to Print Pyramid Patterns in C++ Array program in C++ Pattern programs in C++ Leap Year Program in C++ C++ Program for FCFS Depth First Search Program to Traverse a Graph in C++ inheritance Program in C++ Program that produces different results in C and C++ Example Program in C++ for input and Output Calculator Program using Switch in C++ Program for COPY Constructor in C++ Bank Program in C++ Anagram Program in C++

C++ Operators

Unary Operators in C++ Delete Operator in C++ C++ Bitwise XOR Operator Binary Operator Overloading in C++ Scope Resolution Operator in C++ C++ Ternary Operator New Operator in C++ Overloading array index operator in C++ Operator overloading in C++ Operators that cannot be overloaded in C++ Default Assignment Operator and References in C++ Conversion Operator in C++ Unary Operator Overloading in C++ Benefits of Operator Overloading in C++ Assignment Operator Overloading in C++ New and Delete Operator in C++ Insertion Operator in C++ Increment Operator in C++ Bitwise Operator in C++

C++ Examples

C++ Programming Examples While Loop Examples in C++ for Loop Examples in C++ Do-While Loop Examples in C++ SDL library in C++ with Examples C++ tcp client server example C++ tcp server example TCP Client Server Example in C++ Example of Parameterized Constructor in C++ Example of User Defined Data Type in C++

C++ Differences

C Vs C++ free() vs delete() in C++ Constructor Vs Destructor Passing by Reference Vs. Passing by the pointer in C++ C++ vs C# Structure Vs Class in C++ Virtual function Vs Pure Virtual function Structure Vs Class in C++ Exception Handling in C++ vs Java Static keyword in C++ vs Java inheritance in C++ vs Java C++ 11 vs C++ 14 vs C++ 17 Templates in C++ vs Generics in Java Bitwise Operator vs Logical Operator Type difference of Character literals in C VS C++ Copy Constructor vs Assignment Operator in C++ Map vs unordered_map in C++ Differences between #define & const in C/C++ Differences Between C Structures and C++ Structures Difference between Two Sets in C++ Difference between Exit and Return Differences between Local and Global Variable Difference between OOP and POP in C++ Difference between exit() and _Exit() in C++ Difference between int main() and int main(void) in C/C++ Difference Between Overloading and Overriding in C++ std::set_symmetric_difference in C++ Difference between Private and Protected in C++ Difference Between Encapsulation and Abstraction in C++ Difference between Template and Macro in C++ Difference between Abstraction and Interface in C++ Differences Between C++ and Javascript Difference between POP and OOP in C++

How To

How to concatenate two strings in C++ How to Reverse a String in C++ using Do-While Loop How to Reverse a String in C++ using for Loop How to Reverse a String in C++ using While Loop How to create a table in C++ How to Setup Environment for C++ Programming on Mac How to call a void function in C++ How to create a directory or folder in C/C++ How to create a library in C++ How to create a stack in C++ How to create the Processes with fork in C++ How to Handle Divide by Zero Exception in C++ How to Declare Unordered Sets in C++ How to Sort an Array in C++ How to run program in turbo c++ How to Use Getline in C++ How is Multiset Implemented in C++ How to Build a Program in C++ How to Declare a 2d Array Dynamically in C++ How to create a button in C++ How the value is passed in C++ How to calculate size of string in C++ How to convert binary string to int in C++ How to convert string to float in C++ How to remove space from string in C++ How to use pair in C++ How to use the string find() in C++ How to Remove an Element From an Array in C++ How to Take Space Separated input in C++ How to Copy in Turbo C++ How to Use Pow in C++ How to Use Set in C++ How to Create Singleton Class in C++ How to Allocate Memory Dynamically in C++ How to Close Turbo C++ in Windows 10 How to Use Modulus in C++

C++ Questions

What does Buffer Flush mean in C++ What is endl in C++ What is Unary Operator Overloading in C++ Which operators cannot be overloaded in C++ What is Static Member function in C++ What is This Pointer in C++ What is Upcasting in C++ What Kind of Exceptions Are Available in C++ What is Memory Allocation for Objects in C++ What is a Linked List in C++ is assignment operator inherited in C++ When should we write own Assignment operator in C++ What is an Object in C++ What is Copy Constructor in C++ What is a Static Member Function in C++

C++ Interview Questions

Advanced C++ interview Questions Oops Interview Questions in C++

Miscellaneous

External merge sort in C++ Remove duplicates from sorted array in C++ C++ References C++ Mutable keyword initialize Array of objects with parameterized constructors in C++ Storage Classes in C goto statement in C and C++ C++ String Concatenation Heap Sort in C++ Swap numbers in C++ input Iterators in C++ Fibonacci Series in C++ Stack in C++ Maps in C++ Queue in C++ C++ Bitset C++ Algorithms Priority Queue in C++ C++ Multimap C++ Deque C++ array of Pointers Timsort Implementation Using C++ CPP Templates C++ Aggregation C++ Enumeration C++ Object Class C++ Queue initialize Vector in C++ Vector in C++ C++ STL Components C++ Maximum index Problem C++ find missing in the second array C++ Namespaces C++ OOPs Concept C++ Static C++ Structs C++ Try-Catch C++ User Defined Exceptions C++ Virtual Destructor Snake Code in C++ Splitting a string in C++ C++ Bidirectional Iterators C++ forward Iterators C++ Iterators C++ Output Iterators C++ Range-based for Loop Converting string into integer in C++ Type conversion in C++ Binary to Decimal in C++ Bit Manipulation in C++ C++ Constructor C++ Dijkstra Algorithm Using the Priority Queue C++ int into String C++ Signal Handling Decimal to Binary in C++ Decimal to Hexadecimal in C++ Decimal to Octal in C++ Hexadecimal to Decimal in C++ Octal to Decimal in C++ Reverse a Number in C++ C++ forward Iterators C++ Output Iterators Char Array to String in C++ Constructor Overloading in C++ Default arguments in C++ Different Ways to Compare Strings in C++ Dynamic Binding in C++ SET Data Structure in C++ Upcasting and Downcasting in C++ Reverse an Array in C++ Fast input and Output in C++ Copy elision in C++ Array of sets in C++ Binary Search in C++ Implementing the sets without C++ STL containers Smart pointers in C++ Types of polymorphism in C++ Const Keyword in C++ Type Casting in C++ Stringstream in C++ and its applications C++ Scope of Variables Star pattern in C++ using for Loops Top 5 IDEs for C++ That You Should Try Once Assertions in C/C++ C++ Convert int to String Continue in C++ While loop Diamond Pattern in C++ using for Loop infinite loop in C++ Loops in C++ Auto keyword in C++ C++ STL (Standard Template Library) Divide by Zero Exception in C++ Dynamic Constructor in C++ Dynamic Memory Allocation in C++ Floating Point Operations and Associativity in C, C++ and Java Implementation of a Falling Matrix in C++ Message Passing in C++ Pointer to Object in C++ Ways to Copy a Vector in C++ Structure Sorting (By Multiple Rules) in C++ Similarities between C++ and Java C++ Tricks for Competitive Programming Desired Capabilities in Selenium Web Driver in C++ Socket Programming in C++ Template Specialization in C++ Classes and Objects in C++ Convex hull Algorithm in C++ DES in C++ Structured Binding in C++ Functors in C++ Lambda Expression in C++ Roadmap to C++ Programming Substring in C++ Virtual base class in C++ Bits stdc++.h in C++ Top 14 Best Free C++ IDE (Editor & Compiler) for Windows in 2022 Bitmasking in C++ Features of OOPS in C++ Hospital Management Project in C++ include Guards in C++ Iostream in C++ Method overriding in C++ Naming Convention in C++ Nullptr in C++ Object Slicing in C++ Principles of Object-Oriented Programming in C++ Processing strings using std string stream in C++ Random Number Generator in C++ Singleton Design Pattern in C++ Size_t Data Type in C++ Skyline Problem in C++ Web Development in C++ Data Hiding in C++ Hashing in C++ Object in C++ Sum of all Elements between k1’th and k2’th Smallest Elements Virtual class in C++ Vector Size in C++ Top best IDEs for C/C++ Developers in 2022 Tensorflow in C++ Sliding Window Technique in C++ Reverse String Word-Wise in C++ RTTI in C++ Pthreads or POSIX Threads in C++ Reserved Keywords in C++ 10 Best C and C++ Books for Beginners & Advanced Programmers Add two numbers represented by two arrays in C++ Array of Object in C++ Containership in C++ Counting Frequencies of Array Elements in C++ Decltype type Specifier in C++ Dynamic _Cast in C++ Features and Use of Pointers in C/C++ int Max and int Min in C/C++ is It Fine to Write Void Main Or Main in C/C++ Compile Time Polymorphism in C++ Division in C++ Factorial of a Number in C++ using while Loop Multiset in C++ 4 Pillars of OOPs Approach in C++ Backtracking Time Complexity in C++ C++ Global Variable C++ Pipe Tutorial Observer Design Pattern in C++ Private inheritance in C++ Pthread in C++ Parameters Pointers in C++ Abstract Factory Design Pattern in C++ Ascending order in C++ Call by Pointer in C++ Constexpr in C++ Deadlock in C++ Design Patterns in C++ Factory Method for Designing Pattern in C++ Name Mangling and extern in C++ Preventing Object Copy in C++ Quick Sort in C++ Single Handling in C++ Use of inheritance in C++ User-defined literals in C++ Vector methods in C++ Void * in C and C++ Zombie and Orphan Process in C++ List and Vector in C++ List iterators in C++ Merging Two Vectors in C++ Early Binding and Late Binding in C++ Factory Design Pattern in C++ Fisher-Yates shuffle algorithm in C++ for Auto in C++ Group anagrams in C++ Dynamic Casting in C++ 2D Vector initialization in C++ C++ GUI Visual Studio C++ IPC C++ Permutation Overloading Stream insertion in C++ Array sum in C++ STL C++ Define Macro C++ Design C++ Factory Pattern Convert String to Uppercase in C++ initializer list in C++ Iterator invalidation in C++ Lower bound in C++ Modulus of Two float numbers or double number Pass by value in C++ Stack size in C++ Unordered_map in C++ Binary String to int C++ Boost C++ Exception handling in constructor and destructor in C++ Explain class template in C++ Kadane Algorithm in C++ Map of Pairs in STL Pair in C++ Access Specifiers in C++ Add Two Numbers in C++ Using Class C++ Socket Programming Windows Cube root in C++ C++ Protected inheritance C++ Return Object Static Data Members in C++ Association in C++ Boost libraries in C++ Automatic Storage Class in C++ C++ Move Constructor Cast in C++ C++ 11 Lambda C++ Multithreading Bottom-up Approach in C++ C++ Static Class C++ Static Member C++ Static Variable C++ Template Class Implementation Kruskals Algorithm in C++ Kth Permutation Sequence in C++ Kth Smallest Element Using Priority Queue in C++ Min Heap in C++ Multilevel inheritance in C++ Playfair Cipher Algorithm in C++ Exception Handling in Java and C++ Sorting in C++ Time and Space Complexity for Loops in C++ Convert Vector To String in C++ Cascading in C++ Control Structure in C++ Passing Object as argument in C++ Vector Pair in C++ String Data Types in C++ Topological Sort in C++ C++ Move Semantics C++ Static Library Linking Call by Value and Call by Reference in C++ Data Structures algorithms and application in C++ Dereferencing a Pointer in C++ Linked List in C++ All Swing Components in C++ anonymous Object in C++ Cerr in C++ Control Flow Statement in C++ cstdlib in C++ Exception Objects in C++ Base Class and Derived Class in C++ Creating A Menu in C++ Escape Sequence in C++ Explicit in C++ int range in C++ Maximum Value in Vector in C++ NPTEL Programming in C++ Shared Pointer in C++ Static Member Function in C++ With Example The Diamond Pattern in C++ Const ptr C++ Const_Cast C++ Area Of Triangle in C++ Array input in C++ Cin String in C++ Coroutine in C++ Gui Design in C++ integer in C++ Advantages and Disadvantages of inheritance in C++ C++ IDE Linux C++ Mini Project With Source Code And Output C++ Variables Types Containers in C++ Dynamic Binding in C++ Explain Access Specifiers in C++ Iomanip in C++ Noexcept in C++ C++ Reference Initializer in C++ Constant Arguments in C++ Vector in C++ Stl Singleton Pattern in C++ Error Handling in C++ C++ Thread Id to Int C Str in C++ C++ Shared Ptr Scope Resolution Operator in C++ C++ Random Number Between 1 and 10 File Opening Mode in C++ New and Delete in C++ Big Integers in C++ Stack Unwinding in C++ C++ Syntax Link Map cbegin() and cend() function in C++ STL Hamming Code Implementation in C++ Files and Streams in C++ Private Destructor in C++ Semaphore in C++ Map Emplace_hint() function in C++ STL Complex Number Program in C++ Meta Classes in C++ C++ Delegates Custom Sort in C++ RAII in C++ Babylonian Square Root Algorithm in C++ File Input and output operation in C++ Sum of Digits of a Number in C++ C++ Type Traits Binary Search Tree Program in C++ Tree in C++ Built in Types in C++ Catch all Exceptions in C++ C++ Template Class Declaration and Definition C++ Generic Programming Introduction Fundamental Data Types in C++ API in C++ Menu Driven Program in C++ Exception Specification in C++ Bit Manipulation in C++ Dynamic Initialization Of Objects in C++ Returning Object from a Function in C++ Class and Structure in C++ Edmonds Karp Algorithm in C++ Flush in C++ Find Substring in C++ Input and Output Operators in C++ Final in C++ Lvalue and Rvalue in C++ C++ Singleton File Stream in C++ Has a Relationship in C++ Character Set in C++ Menu Driven Program in C++ Final Keyword in C++ Associative Containers in C++ Decltype in C++ Insert Function in C++ Metaclass in C++ Move Constructor in C++ Move Semantics in C++

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.