Are Arrays and Pointers Inherently the Same in C and C ?

Array Access: Pointer Abstraction or Inherent Property?

In C and C , accessing elements of arrays often involves using pointers. However, the underlying mechanisms behind this behavior can be confusing, leading to the question of whether arrays and pointers are indeed inherently related.

Arrays as Pointers

Contrary to popular belief, arrays and pointers are distinct concepts in C and C . Arrays, in essence, represent contiguous memory blocks that hold multiple elements of the same data type. Pointers, on the other hand, store the memory addresses of variables or array elements.

However, C's language specification introduces a nuance known as "array expression conversions." When arrays are referenced in certain contexts without an address-of operator (&) or when used in a string constant initialization, their expressions transition into pointer expressions. This conversion casts the array expression into a "pointer to the initial element" of the array, blurring the lines between arrays and pointers.

Expression Conversions and Pointer Arithmetic

To illustrate the implications of array expression conversions, consider the following hypothetical memory map:

Object           Address         Data
------           -------         -----
arr              0x10008000      { 0x00, 0x01, 0x02, 0x03 }
parr             0x10008014      0x10008000 (address of arr[0])

arr is an array of four integers, while parr is a pointer pointing to the first element of arr. While both arr and &arr evaluate to the same memory address (0x10008000), they have different types: an array versus a pointer to an array.

Pointer arithmetic becomes relevant here. Incrementing parr by one advances it by the size of an integer (4 bytes), as expected for a pointer. However, incrementing arr increases it by the size of the entire array (4 integers or 16 bytes), highlighting the inherent difference in their underlying types.

Commutative Array Subscripting

Another quirk of C's array expression conversions is their commutativity. The subscript operator [] can be applied to both pointers and arrays, treating them as equivalent. For example, arr[i] and i[arr] both evaluate to the same result when arr is an array and i is an integer.

Conclusion

While array expressions in C may behave like pointers in certain contexts, it's crucial to understand their distinct nature. Arrays represent contiguous memory blocks, while pointers store addresses. Commutative subscripting and array expression conversions provide syntactic shortcuts but do not alter the fundamental distinction between the two concepts.

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