Compare *ptr++, *++ptr and ++*ptr in C++

In C++, both ptr++ and ++ptr are used to increment pointers, but they behave differently in expressions. The difference lies in when the increment happens: before or after the value is used. This is essential when working with loops, arrays, or pointer.

Syntax

Following is the syntax to compare ptr++ vs ++ptr in C++:

ptr++: post-increment; 
++ptr: pre-increment;

Following is the table to compare ptr++ vs ++ptr in C++

We will explore how these operators behave using examples, printing pointer values and dereferenced data before and after incrementing.

• Basic increment difference
• Using in expression
• Loop behavior difference
• Array traversal with ptr++ vs ++ptr

Basic increment difference

The ptr++ increments the pointer after its current value and ++ptr increments the pointer first.

Example

In this example, we demonstrate arithmetic pointer. It starts with a pointer to an array to displays the value at the current pointer position while incrementing it (ptr++), resets the pointer and shows the value at the next position after incrementing the pointer (++ptr).

#include<iostream>
using namespace std;
int main() {
   int arr[] = {6,7,8,9};
   int *ptr = arr;
   cout << "Value at ptr++: " << *ptr++ << endl;
   ptr = arr; // reset pointer
   cout << "Value at ++ptr: " << *++ptr << endl;
   return 0;
}

Output

Following is the output to the above program:

Value at ptr++: 6
Value at ++ptr: 7

Using in expression

When used inside an expression, ptr++ returns the original address, while ++ptr returns the incremented address.

Example

In this example, we use pointer to assign array values to variables 'a' and 'b', while demonstrating how the pointer moves to the next element (ptr++) or before accessing the value (++ptr).

#include<iostream>
using namespace std;
int main() {
   int arr[] = {2,4,6,8};
   int *ptr = arr;
   int a = *ptr++;
   cout<< "a = "<< a << ", ptr points to: "<< *ptr << endl;
   ptr = arr;
   int b = *++ptr;
   cout<< "b = "<< b << ", ptr points to: "<< *ptr << endl;
   return 0;
}

Output

Following is the output to the above program:

a = 2, ptr points to: 4
b = 4, ptr points to: 4

Loop behavior difference

Inside loops, both work, but affect the order of operations differently. Post-increment fetches then moves, pre-increment moves then fetches.

Example

In this example, we demonstrate two pointer traversal methods (ptr++ and ++ptr) to iterate through an array and print the elements while showcasing how the pointer moves different in each case.

#include<iostream>
using namespace std;
int main() {
   int arr[] = {11,22,33,0};
   int *ptr = arr;
   cout<< "Using ptr++: ";
   while (*ptr++)
       cout<< *(ptr - 1) << " ";
   cout<< endl;
   ptr = arr;
   cout<< "Using ++ptr: ";
   while (*++ptr)
       cout<< *ptr << " ";
   return 0;
}

Output

Following is the output to the above program:

Using ptr++: 11 22 33 
Using ++ptr: 22 33 

Array traversal with ptr++ vs ++ptr

Both pre and post-increment can be used to traverse arrays, but depends upon whether the current or next value is needed during the operation.

Example

In this example, we use pointer to traverse and print an array in two ways: ptr++, it prints all elements sequentially. ++ptr, it skips the first element and prints the next two elements.

#include<iostream>
using namespace std;
int main() {
    int arr[] = {9,18,27};
    int *ptr = arr;
    cout<<"Traversal with ptr++: ";
    for (int i = 0; i< 3; i++)
        cout<< *ptr++<< " ";
    cout<< endl;
    ptr = arr;
    cout<<"Traversal with ++ptr: ";
    cout<<*++ptr<< " ";
    cout<<*++ptr<< " ";
    return 0;
}

Output

Following is the output to the above program:

Traversal with ptr++: 9 18 27 
Traversal with ++ptr: 18 27