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:
In the first form,
postfix-expressionrepresents a value ofstructoruniontype, andidentifiernames a member of the specified structure or union. The value of the operation is that ofidentifierand is an l-value ifpostfix-expressionis an l-value. For more information, see L-Value and R-Value Expressions.In the second form,
postfix-expressionrepresents a pointer to a structure or union, andidentifiernames a member of the specified structure or union. The value is that ofidentifierand 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.
Examples
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, the address of the item structure is assigned to the sp member of the structure. It 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.