An In-Depth Understanding of Memory and Pointers in C/

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Array Decaying

Array Decaying

Learn about the important mechanism of array decaying.

We'll cover the following...

Introduction

We discovered that arr just like &arr[0] is a pointer to the first element inside the array.

See the following code, where we print their addresses. It’s the same code we used in previous lessons:

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#include <stdio.h>



int main()

{

    int arr[3];



    printf("Address of arr = %u\n", arr);

    printf("Address of arr[0] = %u\n", &(arr[0]));



    //these two pointers are unused, we just want to check if the compiler issues any type mismatch warnings

    int *ptr1 = arr;

    int *ptr2 = &arr[0];



    return 0;

}

Here’s a possible output:

Address of arr = 1865475828
Address of arr[0] = 1865475828

The two addresses are equal.

Moreover, in lines 11 12, we aren’t receiving any warnings from the compiler, which means that both arr and &arr[0] have the same type (pointer to int).

In conclusion, the array name is a pointer to the first element. This conversion happens because of array decaying, which we’ll now explore in depth.

Array decaying

Array decaying is a process in which an array loses its size information and is converted from an array of type T to a pointer to type T. The pointer points to the first element inside the array.

We say that the array decays into a pointer.

This process happens in expressions involving arrays, with the following exceptions:

  • The sizeof operator
  • The address of the & operator
  • With string literals, when they’re used to initialize a char array. We’ll discuss this at a later
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