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Structure and Union Members

A "member-selection expression" refers to members of structures and unions. Such an expression has the value and type of the selected member.

postfix-expression . identifier 
postfix-expression –> identifier

This list describes the two forms of the member-selection expressions:

  1. In the first form, postfix-expression represents a value of struct or union type, and identifier names a member of the specified structure or union. The value of the operation is that of identifier and is an l-value if postfix-expression is an l-value. See L-Value and R-Value Expressions for more information.

  2. In the second form, postfix-expression represents a pointer to a structure or union, and identifier names a member of the specified structure or union. The value is that of identifier and is an l-value.

The two forms of member-selection expressions have similar effects.

In fact, an expression involving the member-selection operator (–>) is a shorthand version of an expression using the period (.) if the expression before the period consists of the indirection operator (*) applied to a pointer value. Therefore,

expression –> identifier

is equivalent to

(*expression) . identifier

when expression is a pointer value.

The following examples refer to this structure declaration. For information about the indirection operator (*) used in these examples, see Indirection and Address-of Operators.

struct pair 
    int a;
    int b;
    struct pair *sp;
} item, list[10];

A member-selection expression for the item structure looks like this:

item.sp = &item;

In the example above, the address of the item structure is assigned to the sp member of the structure. This means that item contains a pointer to itself.

(item.sp)–>a = 24;

In this example, the pointer expression item.sp is used with the member-selection operator (–>) to assign a value to the member a.

list[8].b = 12;

This statement shows how to select an individual structure member from an array of structures.