How to compare pointers in C/C++?

Pointers in C can be directly compared using relational operators to check their memory addresses. This allows us to determine equality, ordering, and relative positions of pointers in memory.

Syntax

pointer1 == pointer2    // Equality comparison
pointer1 != pointer2    // Inequality comparison
pointer1 < pointer2     // Less than comparison
pointer1 > pointer2     // Greater than comparison
pointer1 <= pointer2    // Less than or equal
pointer1 >= pointer2    // Greater than or equal

Pointer Comparison in C

In C, we can compare pointers using relational operators (==, !=, <, >, <=, >=). These operators compare the memory addresses stored in the pointers, not the values they point to. The comparison returns 1 (true) or 0 (false) based on the address relationship.

Example 1: Comparing Pointers to Different Variables

This example demonstrates comparing pointers that point to different variables −

#include <stdio.h>

int main() {
    int a = 10, b = 20;
    int *ptr1 = &a;
    int *ptr2 = &b;
    int *ptr3 = &a;
    
    printf("Address of a: %p\n", (void*)ptr1);
    printf("Address of b: %p\n", (void*)ptr2);
    printf("Address of a (ptr3): %p\n", (void*)ptr3);
    
    printf("ptr1 == ptr2: %s\n", (ptr1 == ptr2) ? "true" : "false");
    printf("ptr1 == ptr3: %s\n", (ptr1 == ptr3) ? "true" : "false");
    printf("ptr1 != ptr2: %s\n", (ptr1 != ptr2) ? "true" : "false");
    
    if (ptr1 < ptr2) {
        printf("ptr1 points to a lower address than ptr2\n");
    } else {
        printf("ptr1 points to a higher address than ptr2\n");
    }
    
    return 0;
}

Address of a: 0x7fff5fbff6bc
Address of b: 0x7fff5fbff6b8
Address of a (ptr3): 0x7fff5fbff6bc
ptr1 == ptr2: false
ptr1 == ptr3: true
ptr1 != ptr2: true
ptr1 points to a higher address than ptr2

Example 2: Comparing Array Element Pointers

When comparing pointers to array elements, the comparison follows the array's memory layout −

#include <stdio.h>

int main() {
    int arr[5] = {1, 2, 3, 4, 5};
    int *ptr1 = &arr[1];
    int *ptr2 = &arr[3];
    
    printf("ptr1 points to arr[1], ptr2 points to arr[3]\n");
    printf("ptr1 == ptr2: %s\n", (ptr1 == ptr2) ? "true" : "false");
    printf("ptr1 < ptr2: %s\n", (ptr1 < ptr2) ? "true" : "false");
    printf("ptr2 > ptr1: %s\n", (ptr2 > ptr1) ? "true" : "false");
    
    /* Calculate distance between pointers */
    printf("Distance between pointers: %ld elements\n", ptr2 - ptr1);
    
    return 0;
}

ptr1 points to arr[1], ptr2 points to arr[3]
ptr1 == ptr2: false
ptr1  ptr1: true
Distance between pointers: 2 elements

Key Points

• Pointer comparison compares memory addresses, not the values being pointed to.
• Comparing pointers to different objects may yield unpredictable results depending on memory layout.
• Comparison is meaningful when pointers reference elements of the same array or object.
• NULL pointer comparison is safe: ptr == NULL or ptr != NULL.

Conclusion

Pointer comparison in C uses standard relational operators to compare memory addresses. It's most useful for array elements and checking for NULL pointers, providing essential functionality for memory management and data structure operations.