C++ Tutorial Index

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C++ Control Statements

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C++ Functions

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C++ Arrays

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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

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C++ File Handling

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

Miscellaneous

C Vs C++ 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 std::min in C++ External merge sort in C++ Remove duplicates from sorted array in C++ Precision of floating point numbers Using these functions floor(), ceil(), trunc(), round() and setprecision() in C++ C++ References C++ Friend Functions C++ Mutable keyword Unary Operators in C++ Initialize Array of objects with parameterized constructors in C++ Differences between #define & const in C/C++ C++ Program to Implement Shell Sort C++ Program to Implement Merge Sort Storage Classes in C Vector resize() in C++ Passing by Reference Vs. Passing by the pointer in C++ Free vs delete() in C++ goto statement in C and C++ C++ program to read string using cin.getline() C++ String Concatenation Heap Sort in C++ Swap numbers in C++ Input Iterators in C++ Fibonacci Series in C++ C ++ Program: Alphabet Triangle and Number Triangle C++ Program: Matrix Multiplication C++ Program to Print Fibonacci Triangle Stack in C++ Maps in C++ Queue in C++ C++ Bitset C++ Algorithms Priority Queue in C++ C++ Multimap C++ Deque Function Pointer in C++ Sizeof() Operators in C++ C++ array of Pointers free() Vs delete in C Timsort Implementation Using C++ CPP Templates C++ Aggregation C++ Enumeration C++ Math Functions C++ Object Class C++ Queue Initialize Vector in C++ Vector in C++ C++ STL Components Function overloading in C++ C++ Maximum Index Problem C++ find missing in the second array 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 Constructor Vs Destructor C++ Namespaces C++ OOPs Concept C++ Static C++ Structs C++ Try-Catch C++ User Defined Exceptions C++ Virtual Destructor C++ vs C# Malloc() and new in C++ Palindrome Number Program in C++ Snake Code in C++ Splitting a string in C++ Structure Vs Class in C++ Virtual Function Vs Pure Virtual Function C++ Bidirectional Iterators C++ Forward Iterators C++ Iterators C++ Output Iterators C++ Range-based For Loop Converting string into integer in C++ LCM Program in C++ Type conversion in C++ Add two numbers using the function in C++ Advantage and disadvantage friend function C++ Armstrong Number Program in C++ ATM machine program in C++ using functions 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++ Factorial Program in C++ Function in C++ Hexadecimal to Decimal in C++ Octal to Decimal in C++ Reverse a Number in C++ Structure Vs Class in C++ C++ Forward Iterators C++ Output Iterators C++ Prime number program 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++ Program to convert infix to postfix expression in C++ SET Data Structure in C++ Upcasting and Downcasting in C++ Reverse an Array in C++ Fast Input and Output in C++ Delete Operator in C++ Copy elision in C++ C++ Date and Time C++ Bitwise XOR Operator Array of sets in C++ Binary Operator Overloading in C++ Binary Search in C++ Implementing the sets without C++ STL containers Scope Resolution Operator in C++ Smart pointers in C++ Types of polymorphism in C++

C++ Mutable keyword

C++ mutable keyword

Mutable class storage specifier in C++ Storage class specifiers in C are auto, register, static, and external. Typedef is also considered as a specifier of the storage class in C. C++ also supports all of those specifiers for storage class. In addition to this C++, adds an absolutely essential specifier for storage class whose name is mutable.

What is the use of mutability?

There is often a need to change one or more class/struct data members by the const method, even though you don't want the method to update other class/struct members. Using a mutable keyword, we can easily accomplish this task.

Consider this example where it may be useful to use the mutable. Suppose you go to the hotel and give an order to the waiter to bring some dishes of food. You suddenly decide to change the order of the food after you give an order. Suppose that the hotel provides the services for changing the ordered food and start ordering new food within ten min after the first order is issued. The order can't be canceled after 10 minutes, and the old order cannot be replaced with the new order. For details, see the code below.

#include <iostream>
#include <string.h>
using std::cout;
using std::endl;
class Customer
{
    char name[25];
    mutable char placedorder[50];
    int tableno;
    mutable int bill;
public:
    Customer(char* q, char* r, int b, int p)
    {
        strcpy(name, q);
        strcpy(placedorder, r);
        tableno = b;
        bill = p;
    }
    void changePlacedOrder(char* p) const
    {
        strcpy(placedorder, p);
    }
    void changeBill(int q) const
    {
        bill = q;
    }
    void display() const
    {
        cout << "Customer name is: " << name << endl;
        cout << "Product placed by customer is: " << placedorder << endl;
        cout << "table no is: " << tableno << endl;
        cout << "Total amount: " << bill << endl;
    }
};
int main()
{
    const Customer cus1("Peter Johenson", "Cold drink", 4, 250);
    cus1.display();
    cus1.changePlacedOrder("chicken fry");
    cus1.changeBill(420);
    cus1.display();
    return 0;
}

Output:

C++ mutable keyword

Watch the output of the above program closely placed order qualities and bill information. The main is modified from const feature since they are decided to declare to be mutable.

The mutable keyword is primarily used to change a given const object member. The feature passed to this pointer has become const when we declare a feature to be const. Adding mutable to a variable allows for a change of members by a const pointer. Mutable is especially useful if most members are constant, but a few need to be updated. Members of the data clarified as mutable may be updated even if they are the part of the object defined const. The mutable specifier cannot be used with names declared static or const or reference.

As an exercise, it predicts the output of two programs that follow.

// PROGRAM 1
#include <iostream>
using std::cout;
class Test {
public:
  int t;
  mutable int p;
  Test() { t = 8; p = 15; }
};
int main()
{                                                                        
    const Test l1;
    l1.p = 50;
    cout << l1.p;
    return 0;
}

Output:

C++ mutable keyword

//PROGRAM2

#include <iostream>
using namespace std;
class Test {
   public:
      int w;
   mutable int k;
   Test(int e=0, int f=0) {
      w=e;
      k=f;
   }
   void setw(int e=0) {
      w = e;
   }
   void setk(int f=0) {
      k = f;
   }
   void disp() {
      cout<<endl<<"w: "<<w<<" k: "<<k<<endl;
   }
};
int main() {
   const Test t(25,30);
   cout<<t.w<<" "<<t.k<<"\n";
   t.k=200; //k still can be changed, because k is mutable.
   cout<<t.w<<" "<<t.k<<"\n";
   return 0;
}

Output:      

C++ mutable keyword



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