reinterpret_cast in C++ | Type Casting operators

reinterpret_cast is a type of casting operator used in C++. 

• It is used to convert a pointer of some data type into a pointer of another data type, even if the data types before and after conversion are different.
• It does not check if the pointer type and data pointed by the pointer is same or not.

Syntax : 

data_type *var_name = 
       reinterpret_cast <data_type *>(pointer_variable);

Return Type 

• It doesn’t have any return type. It simply converts the pointer type.

Parameters 

• It takes only one parameter i.e., the source pointer variable (p in above example).

CPP

// CPP program to demonstrate working of
// reinterpret_cast
#include <iostream>
using namespace std;

int main()
{
    int* p = new int(65);
    char* ch = reinterpret_cast<char*>(p);
    cout << *p << endl;
    cout << *ch << endl;
    cout << p << endl;
    cout << ch << endl;
    return 0;
}

Output:

65
A
0x1609c20
A

Purpose for using reinterpret_cast 

1. reinterpret_cast is a very special and dangerous type of casting operator. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type).
2. It can typecast any pointer to any other data type.
3. It is used when we want to work with bits.
4. If we use this type of cast then it becomes a non-portable product. So, it is suggested not to use this concept unless required.
5. It is only used to typecast any pointer to its original type.
6. Boolean value will be converted into integer value i.e., 0 for false and 1 for true.

CPP

// CPP code to illustrate using structure
#include <bits/stdc++.h>
using namespace std;

// creating structure mystruct
struct mystruct {
    int x;
    int y;
    char c;
    bool b;
};

int main()
{
    mystruct s;

    // Assigning values
    s.x = 5;
    s.y = 10;
    s.c = 'a';
    s.b = true;

    // data type must be same during casting
    // as that of original

    // converting the pointer of 's' to,
    // pointer of int type in 'p'.
    int* p = reinterpret_cast<int*>(&s);

    cout << sizeof(s) << endl;

    // printing the value currently pointed by *p
    cout << *p << endl;

    // incrementing the pointer by 1
    p++;

    // printing the next integer value
    cout << *p << endl;

    p++;

    // we are casting back char * pointed
    // by p using char *ch.
    char* ch = reinterpret_cast<char*>(p);

    // printing the character value
    // pointed by (*ch)
    cout << *ch << endl;

    ch++;

    /* since, (*ch) now points to boolean value,
    so it is required to access the value using
    same type conversion.so, we have used
    data type of *n to be bool. */

    bool* n = reinterpret_cast<bool*>(ch);
    cout << *n << endl;

    // we can also use this line of code to
    // print the value pointed by (*ch).
    cout << *(reinterpret_cast<bool*>(ch));

    return 0;
}

Output:

12
5
10
a
1
1

Program 2

CPP

// CPP code to illustrate the pointer reinterpret
#include <iostream>
using namespace std;

class A {
public:
    void fun_a()
    {
        cout << " In class A\n";
    }
};

class B {
public:
    void fun_b()
    {
        cout << " In class B\n";
    }
};

int main()
{
    // creating object of class B
    B* x = new B();

    // converting the pointer to object
    // referenced of class B to class A
    A* new_a = reinterpret_cast<A*>(x);

    // accessing the function of class A
    new_a->fun_a();
    return 0;
}

Output:

In class A

Related link : 
https://www.geeksforgeeks.org/casting-operators-in-c-set-1-const_cast/
https://stackoverflow.com/questions/573294/when-to-use-reinterpret-cast” rel=”noopener” target=”_blank
http://forums.codeguru.com/showthread.php?482227-reinterpret_cast-lt-gt-and-where-can-it-be-used
https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/keyword_reinterpret_cast.htm
https://stackoverflow.com/questions/573294/when-to-use-reinterpret-cast