Pointers

Pointers are basically a reference to a variable in memory. They're "pointing" to a memory-address where a variable is stored.

Pointers in C++ are very similar to pointers in C. For more details about pointers, visit https://www.learn-c.org/en/Pointers.

C++, however, adds some more features, namely the operators new and delete to allocate and de-allocate memory to a pointer, respectively.

datatype

A pointer in almost all cases needs a datatype that matches the referenced variable. For example an int-pointer can hold the address of an int-variable and a float-pointer can hold the address of a float-variable. A special case is the so called "void-pointer".

void pointer

A so called "void-pointer" is a pointer, which has no data type assigned. It can hold an address of any type and can be typecasted to any other type.

Examples:

Pointermagic explained

// This will explain how pointers are used
#include <iostream> 
using namespace std; 
int main() 
{     
  int a = 5;  //Declare and initialize a variable
  /*
  The following pointer declarations are all valid and they all are doing the same:
  Declaring a new pointer and initializing it with 0 (a non accessible memory address) for safety-reasons.
  int * b = 0;
  int* b = 0                 
  int *b = 0; // This is how its usually written.
  int *b; // <= this, however would cause "b" to point to some random address which might be unsafe.
  */
  int *b = 0; // Create a new pointer of type int and initialize it with 0

  cout << "\"b\" is initialized and now pointing to memory-address:  " << b << endl << endl;

  //Now make the pointer "point" to the address of a
  b = &a; //"b" now points to the address of "a"

  cout << "\"a\" is stored at memory-address:  " << &a << endl;
  cout << "\"b\" is stored at memory-address:  " << &b << endl;
  cout << "\"b\" is pointing to memory-address now:  " << b << endl << endl;

  cout << "The value of \"a\" is: " << a << endl;  // returns the value of a

  //the "*b" will get the value from the address b points to (the value of a)
  cout << "The value of the area \"b\" is pointing to is: " << *b << endl << endl << endl;

  //Now modify the value of the address "b" is pointing to...effectively changing the value of "a"     
  *b=10;

  cout << "\"a\" is still stored at memory-address:  " << &a << endl;
  cout << "\"b\" is still stored at memory-address:  " << &b << endl;
  cout << "\"b\" is still pointing to memory-address:  " << b << endl << endl;

  cout << "The value of \"a\" is now: " << a << endl;  // returns the value of a
  cout << "The value of the area \"b\" is pointing to is now: " << *b << endl << endl;

  return 0;        
}

void pointer in C++ particularity

One of the biggest differences is that pointers in C++ are more secure. In C, we can assign a void pointer to a non-void pointer without using a cast to non-void pointer. In C++, however, you have to explicitly type cast it.

void* p1;
char* p2;
p2 = p1; // Valid in C but not C++
p2 = (char*) p1; // In C++ we have to typecast the pointer!

C++ also introduced 2 new concepts on pointers:

• Constant pointer, in which the address pointed to cannot be changed.
• Pointer to constant, in which the contents pointed to cannot be modified.

Exercise

Indirectly access and modify the value of "n": - Create 2 pointers p1 and p2 for the given integer "n", - with p1 being the void pointer, - and p2 the int pointer. - p2 must be assigned to p1. - indirectly increase the value of n's content by 1 and print the value.