12_pointer_to_array_vs_array_of_pointers.md

12 — Pointer to array vs array of pointers

Goal

Understand the difference between:

• int* (pointer to element)
• int (*)[N] (pointer to entire array)
• int* a[N] (array of pointers)

These three forms are often mixed up, but they have different types and memory behavior.

Concepts

• int* p = nums; points to nums[0].
• int (*pa)[4] = &nums; points to the whole nums array.
• p + 1 moves by sizeof(int).
• pa + 1 moves by sizeof(nums) (the whole array block).
• int* ptr_array[3] is an array where each slot stores an address.

Code explained (step by step)

File: 12_pointer_to_array_vs_array_of_pointers.cpp

1. nums is declared as int[4].
2. elem_ptr and array_ptr are created with different pointer types.
3. The program prints addresses:
   • nums and elem_ptr represent the first element address.
   • &nums is the address of the array object.
4. The program prints elem_ptr + 1 and array_ptr + 1:
   • the first advances by one element,
   • the second advances by one entire array.
5. multiply_all(array_ptr, 2) shows how a function can explicitly receive a pointer to an array of fixed size.
6. Finally, ptr_array demonstrates an array whose elements are pointers.

Common mistakes

• Writing int* p[4] when you actually wanted int (*p)[4].
• Assuming &nums has type int* (it does not).
• Forgetting that pointer arithmetic depends on the pointer type.

Exercises

1. Change nums to size 6 and update the type of array_ptr.
2. Add another function that receives int* + size and compare it with the pointer-to-array version.
3. Replace int* ptr_array[3] with const char* words[3] and print each string.

How to build and run

g++ -std=c++20 -O2 -Wall -Wextra -pedantic -g 12_pointer_to_array_vs_array_of_pointers.cpp -o app
./app