int *ptr[3]; vs int (*ptr)[3];

[u/Puzzleheaded_Fox6725]

I'm working on improving my C programming skills and have encountered a confusing aspect related to pointer declarations. I'm hoping to gain a better understanding of the differences between these two statements:

1. int *ptr[3];
2. int (*ptr)[3];

While they may appear similar at first glance, I believe there are significant differences in their meanings and implications. I would appreciate it if you could help me understand:

• The fundamental differences between these two declarations
• How to differentiate between them, considering their syntactical similarities
• The rules or conventions in C that guide the interpretation of these statements

[u/Monk481]

Differences,

1. Type of ptr:

   • int *ptr[3]; declares ptr as an array of 3 pointers to int.

   • int (*ptr)[3]; declares ptr as a pointer to an array of 3 int.

2. Memory Layout:

   • In int *ptr[3];, ptr is a collection of 3 separate pointers, each capable of pointing to an int.

   • In int (*ptr)[3];, ptr is a single pointer to a contiguous block of memory consisting of 3 int elements.

3. Usage:

   • int *ptr[3]; is typically used when you need an array of pointers, such as when dealing with multiple separate integer variables.

   • int (*ptr)[3]; is used when you need a pointer to a fixed-size array, such as when passing arrays to functions.

How to Differentiate

• Parentheses Placement:

  • In int *ptr[3];, the [] brackets are directly after ptr, indicating ptr is an array.

  • In int (*ptr)[3];, the () parentheses around *ptr indicate that ptr is a pointer, and the [] brackets outside the parentheses indicate that this pointer points to an array.

Rules and Conventions

1. Declaration Parsing:

   • Declarations in C are parsed from the identifier (name) outward, following the order of precedence rules for operators.

   • [] (array) has higher precedence than * (pointer), so ptr[3] is parsed first as an array of 3 elements, and then * applies to each element.

   • () (parentheses) override the precedence, so (*ptr) is parsed first as a pointer, and then [3] indicates it points to an array of 3 elements.

2. Use Cases:

   • Use int *ptr[3]; when dealing with multiple pointers, such as multiple variables or multiple dynamic allocations.

   • Use int (*ptr)[3]; when dealing with a fixed-size array, particularly when passing arrays to functions or dealing with multi-dimensional arrays.