Member access operators

Member access operators allow access to the members of their operands.

Subscript

The array subscrpt expression has the form

where

The subscript operator expression is an lvalue expression whose type is the type of the object pointed to by pointer-expression.

By definition, the subscript operator E1[E2] is exactly identical to *((E1)+(E2)). If pointer-expression is an array expression, it undergoes lvalue-to-rvalue conversion and becomes a pointer to the first element of the array.

Due to the definition of the addition between a pointer and an integer, the result is the element of the array with the index equal to the result of integer-expression (or, if pointer-expression was pointing at ith element of some array, the index of the result is i plus the result of integer-expression)

Note: see array for the details on multidimensional arrays.

#include <stdio.h>
int main(void)
{
    int a[3] = {1,2,3};
    printf("%d %d\n", a[2],  // n == 3
                      2[a]); // same, n == 3
    a[2] = 7; // subscripts are lvalues
 
    int n[2][3] = {1,2,3,4,5,6};
    int (*p)[3] = &n[1]; // elements of n are arrays
    int x = n[1][2]; // applying [] again to the array n[1]
 
    printf("%c %c\n", "abc"[2], 2["abc"]); // string literals are arrays too
}

3 3
c c

Dereference

The dereference or indirection expression has the form

where

If pointer-expression is a pointer to function, the result of the dereference operator is a function designator for that function.

If pointer-expression is a pointer to object, the result is an lvalue expression that designates the pointed-to object.

Dereferencing a null pointer, a pointer to an object outside of its lifetime (a dangling pointer), a misaligned pointer, or a pointer with indeterminate value is undefined behavior, except when the dereference operator is nullified by applying the address-of operator to its result, as in &*E

#include <stdio.h>
int main(void)
{
    int n = 1;
    int* p = &n;
    printf("*p = %d\n", *p); // the value of *p is what's stored in n
    *p = 7; // *p is lvalue
    printf("*p = %d\n", *p);
}

*p = 1
*p = 7

Address of

The address-of expression has the form

The address-of operator produces the non-lvalue address of its operand, suitable for initializing a pointer to the type of the operand. If the operand is a function designator ((1)), the result is a pointer to function. If the operand is an object ((2)), the result is a pointer to object.

If the operand is the dereference operator, no action is taken (so it's okay to apply &* to a null pointer), except that the result is not an lvalue.

If the operand is an array index expression, no action is taken other than the array-to-pointer conversion and the addition, so &a[N] is valid for an array of size N (obtaining a pointer one past the end is okay, dereferencing it is not, but dereference cancels out in this expression).

int f(char c) { return c;}
int main(void)
{
   int n = 1;
   int *p = &n; // address of object n
   int (*fp)(char) = &f; // address of function f
   int a[3] = {1,2,3};
   int *beg=a, *end=&a[3]; // same as end = n+3
}

Member access

The member access expression has the form

where

The member access expression designates the named member of the struct or union designated by its left operand. It has the same value category as its left operand.

If the left operand is const or volatile qualified, the result is also qualified. If the left operand is atomic, the behavior is undefined.

Note: besides identifiers that name objects of struct or union type, the following expressions may have struct or union types: assignment, function call, comma operator, conditional operator, and compound literal.

#include <stdio.h>
struct s {int x;};
struct s f(void) { return (struct s){1}; }
int main(void)
{
    struct s s;
    s.x = 1; // ok, changes the member of s
    int n = f().x; // f() is an expression of type struct s
//  f().x = 1; // Error: this member access expression is not an lvalue
 
    const struct s sc;
//  sc.x = 3;  // Error: sc.x is const, can't be assigned
 
    union { int x; double d; } u = {1};
    u.d = 0.1; // changes the active member of the union
}

Member access through pointer

The member access expression has the form

where

The member access through pointer expression designates the named member of the struct or union type pointed to by its left operand. Its value category is always lvalue

If the type pointed to by the left operand is const or volatile qualified, the result is also qualified. If the type pointed to by the left operand is atomic, the behavior is undefined.

#include <stdio.h>
struct s {int x;};
int main(void)
{
    struct s s={1}, *p = &s;
    p->x = 7; // changes the value of s.x through the pointer
    printf("%d\n", p->x); // prints 7
}

References