Pointers

Introduction to Pointers in C++

Function Pointers are used to store address of functions, we’ve used pointers to point to data (variables)

Example

#include <iostream>
using std::cout;
using std::endl;


int main()
{
    int a = 10;
    int *p;
    p = &a;

    cout << "Address of p is: " << p << endl;
    cout << "Value of address p is : " << *p << endl;
    cout << "Size of integer is : n = " << sizeof(int) << endl;
    cout << "Address of p+1 is: " << p + 1 << endl;
}

Output

Address of p is: 0x7ffee1b4c60c
Value of address p is : 10
Size of integer is : n = 4
Address of p+1 is: 0x7ffee1b4c610

Working with Pointers

Pointers vs References

#include <iostream>
using std::cout;
using std::endl;

int main()
{
  int *ptr; // Create our Pointer

  int var = 7;

  int foo = 21;

  cout << "prt: " << ptr << endl;
  cout << "var: " << var << endl;
  cout << "foo: " << foo << endl;
  
}

Output:

prt: 0x10e245025
var: 7
foo: 21

• Lets have ptr point to something !

Another Example:

#include <iostream>
using std::cout;
using std::endl;

int main()
{
  int *ptr; // Create our Pointer

  int var = 7;

  int foo = 21;

  ptr = &var;

  cout << "prt: " << ptr << endl;
  cout << "var: " << var << endl;
  cout << "foo: " << foo << endl;
  cout << "ptr value: " << *ptr << endl;
}

Output:

prt: 0x7ffee4997604
var: 7
foo: 21
ptr value: 7

Example:

#include <iostream>
using std::cout;
using std::endl;

int main()
{
  int *ptr; // Create our Pointer

  int var = 7;

  int foo = 21;

  ptr = &var;
  ptr = &foo;

  cout << "prt: " << ptr << endl;
  cout << "var: " << var << endl;
  cout << "foo: " << foo << endl;
  cout << "ptr value: " << *ptr << endl;
}

Ouput:

prt: 0x7ffeee5a5600
var: 7
foo: 21
ptr value: 21

Now what about References?

#include <iostream>
using std::cout;
using std::endl;

int main()
{
  int *ptr; // Create our Pointer

  int var = 7;

  int foo = 21;

  ptr = &var;
  ptr = &foo;

  int& ref = var;

  cout << "prt: " << ptr << endl;
  cout << "var: " << var << endl;
  cout << "foo: " << foo << endl;
  cout << "ptr value: " << *ptr << endl;

  cout << "ref: " << ref << endl;
}

Output:

prt: 0x7ffee9295600
var: 7
foo: 21
ptr value: 21
ref: 7

Difference between Pass By Pointers and Pass By Pointer Reference

Example

#include <iostream>
using std::cout;
using std::endl;

int box1 = 1;
int box2 = 2;

int * gptr;

void passByPtr(int * ptr);
void passByPtrRef(int* & ptrRef); // Integar pointer reference

int main()
{
  int *p = &box1;
  
  gptr = &box2;

  cout << "______________ Current Conditions _____________ " << endl;
  
  if (p == &box1){
      cout << "p is pointing to box1" << endl;
  }

  else if (p == &box2 ){
      cout << "p is pointing to box2" << endl;
  }

  else{
      cout << "p is pointing to another box " << endl;
  }

  if (gptr == &box1)
  {
      cout << "gptr is pointing to box1 " << endl;
  }

  else if (gptr == &box2)
  {
      cout << "gptr is pointing to box2 " << endl;
  }

  else {
      // wont see this though
      cout << "gptr is pointing to another box " << endl;
  }

  cout << "box1 contains the value: " << box1 << endl;
  cout << "box2 contains the value: " << box2 << endl;

}


void passByPtr (int *ptr)
{
    *ptr = 3;
    ptr = gptr;

    *ptr = 4;
    cout << "______passByPtr has been called________" << endl;
}

void passByPtrRef( int* & ptrRef)
{
    *ptrRef = 5;
    ptrRef = gptr;
    *ptrRef = 6;
    cout << "_______passByPtrRef has been called______" << endl;
}

Output:

______________ Current Conditions _____________ 
p is pointing to box1
gptr is pointing to box2 
box1 contains the value: 1
box2 contains the value: 2

Now let’s use PassByPtr

Example

#include <iostream>
using std::cout;
using std::endl;

int box1 = 1;
int box2 = 2;

int * gptr;

void passByPtr(int * ptr);

void passByPtrRef(int* & ptrRef); // Integar pointer reference

int main()
{
  int *p = &box1;
  
  gptr = &box2;
  passByPtr(p);

  cout << "______________ Current Conditions _____________ " << endl;
  
  if (p == &box1){
      cout << "p is pointing to box1" << endl;
  }

  else if (p == &box2 ){
      cout << "p is pointing to box2" << endl;
  }

  else{
      cout << "p is pointing to another box " << endl;
  }

  if (gptr == &box1)
  {
      cout << "gptr is pointing to box1 " << endl;
  }

  else if (gptr == &box2)
  {
      cout << "gptr is pointing to box2 " << endl;
  }

  else {
      // wont see this though
      cout << "gptr is pointing to another box " << endl;
  }

  cout << "box1 contains the value: " << box1 << endl;
  cout << "box2 contains the value: " << box2 << endl;


}


void passByPtr (int *ptr)
{
    *ptr = 3;
    ptr = gptr;

    *ptr = 4;
    cout << "___passByPtr has been called_____" << endl;
}

void passByPtrRef( int* & ptrRef)
{
    *ptrRef = 5;
    ptrRef = gptr;
    *ptrRef = 6;
    cout << "_______passByPtrRef has been called______" << endl;
}

Output:

___passByPtr has been called_____
______________ Current Conditions _____________ 
p is pointing to box1
gptr is pointing to box2 
box1 contains the value: 3
box2 contains the value: 4

Now let’s use PassByRef

#include <iostream>
using std::cout;
using std::endl;

int box1 = 1;
int box2 = 2;

int * gptr;

void passByPtr(int * ptr);

void passByPtrRef(int* & ptrRef); // Integar pointer reference

int main()
{
  int *p = &box1;
  
  gptr = &box2;

  cout << "______________ Current Conditions _____________ " << endl;

  passByPtrRef(p); // Example

  if (p == &box1){
      cout << "p is pointing to box1" << endl;
  }

  else if (p == &box2 ){
      cout << "p is pointing to box2" << endl;
  }

  else{
      cout << "p is pointing to another box " << endl;
  }

  if (gptr == &box1)
  {
      cout << "gptr is pointing to box1 " << endl;
  }

  else if (gptr == &box2)
  {
      cout << "gptr is pointing to box2 " << endl;
  }

  else {
      // wont see this though
      cout << "gptr is pointing to another box " << endl;
  }

  cout << "box1 contains the value: " << box1 << endl;
  cout << "box2 contains the value: " << box2 << endl;


}


void passByPtr (int *ptr)
{
    *ptr = 3;
    ptr = gptr;

    *ptr = 4;
    cout << "___passByPtr has been called_____" << endl;
}

void passByPtrRef( int* & ptrRef)
{
    *ptrRef = 5;
    ptrRef = gptr;
    *ptrRef = 6;
    cout << "_______passByPtrRef has been called______" << endl;
}

Output:

_______passByPtrRef has been called______
______________ Current Conditions _____________ 
p is pointing to box2
gptr is pointing to box2 
box1 contains the value: 5
box2 contains the value: 6

Pointer Types, Pointer Arithmetic, Void Pointers

Pointers to Pointers in C/C++

Code

#include <iostream>
using std::cout;
using std::endl;

int main()
{
  int x = 5;
  int* p = &x;

  *p = 6;

  int** q = &p;
  int*** r = &q;

  cout << "*p = " << *p << endl; 

  cout << "*q = " << *q << endl;

  cout << "*(*q) = " << **q << endl;

  cout << "*(*r) = " << **r << endl;

  cout << "*(*(*r)) = " << ***r << endl;
}

Output:

*p = 6
*q = 0x7ffeee75160c
*(*q) = 6
*(*r) = 0x7ffeee75160c
*(*(*r)) = 6