基本结构问题C

Question

for example:

typedef struct {  
    int num;  
    int den;  
} rational_number; 

rational_number a ;

What is the difference between using a.num or a.den
and
a->num or a->den

Thx in advance.

Answer 1

-> is for accessing the members of a pointer to a struct, whereas . is for accessing members of the struct itself. a->num is really just shorthand for (*a).num . Example:

typedef struct {  
    int num;  
    int den;  
} rational_number;

rational_number a;
r.num = 1;
rational_number *a_ptr = &a;
a_ptr->num = 2; /* a.num is now 2 */ 

Answer 2

you use a.num, when it is normal variable, and a->num when it is pointer

For ex.

rational_number a;
a.num; // is correct
a->num; // is wrong

rational_number *b;

b.num;// wrong
b->num; //is correct

Answer 3

The difference is that in the first case you access the structure via a stack variable:

rational_number a;
a.num = 1;
a.den = 1;

in the second case via a heap pointer:

rational_number* a = new rational_number();
a->num = 1;
a->den = 1;

Answer 4

If a were declared as a pointer to your structure, a->num would return the value of num.

rational_number *a;
a->num = 5;
int new_a;
new_a = a->num;

You have a is declared as a structure, so you should use a.num to return the value of num.

rational_number a;
a.num = 5;
int new_a;
new_a = a.num;

Both set the value of new_a to 5.

Also, you can get the address of a if it is a structure and use it like a pointer.

rational_number a;
(&a)->num = 5;
int new_a;
new_a = a.num;