C pointers: what's the difference between stru

问题:

I'm doing some researches to better understand pointers in C, but I'm having a hard time understanding these: Is 'struct* A' a pointer on a structure? Then what is 'struct *A'? And I've seen someone writing 'int const * a', what does this mean?

回答1:

what's the difference between struc* A, struct *A and struct * A?

They're equivalent(ly wrong). C is a free-form language, whitespace doesn't matter.

Is struct* A a pointer on a structure?

No, it's (still) a syntax error (struct is a reserved keyword). If you substitute a valid structure name in there, then it will be one, yes.

int const * a, what does this mean?

This declares a to be a pointer to const int.

回答2:

struct *A, struct* A and struct * A are all the same thing and all eqaully wrong since you're missing the struct's name.

int const *a is a the same as const int *a and it means pointer to a const integer.

Aside: int * const a is different and it means const pointer and a non const integer.

回答3:

They are all identical.

struct *A = struct* A = struct*A = struct * A.

回答4:

As others have already mentioned, struct * A et cetera are incorrect but identical.

However, a structure and a pointer can be created in the following manner:

/* Structure definition. */
struct Date
{
    int month;
    int day;
    int year;
};

/* Declaring the structure of type Date named today. */
struct Date today;

/* Declaring a pointer to a Date structure (named procrastinate). */
struct Date * procrastinate;

/* The pointer 'procrastinate' now points to the structure 'today' */
procrastinate = &today;

Also, for your secondary question about the different ways in which pointers are declared, "What is int const * a?", here is an example that I adapted from Programming in C, by Stephen G. Kochan:

char my_char = 'X';

/* This pointer will always point to my_char. */
char * const constant_pointer_to_char = &my_char;
/* This pointer will never change the value of my_char.   */  
const char * pointer_to_a_constant_char = &my_char;
/* This pointer will always point to my_char and never change its value. */
const char * const constant_ptr_to_constant_char = &my_char; 

When I first got started, I would find it helpful to read the definition from right to left, substituting the word 'read-only' for 'const'. For example, in the last pointer I would simply say, "constant_ptr_to_constant_char is a read-only pointer to a char that is read-only". In your question above for int const * a you can say, "'a' is a pointer to a read-only int". Seems dumb, but it works.

There are some variations but when you run into them, you can find more examples by searching around this site. Hope that helps!