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Nested structures

Structures can contain other structures as members; in other words, structures can nest. Consider the following two structure types:

struct first_structure_type
{
  int integer_member;
  float float_member;
};

struct second_structure_type
{
  double double_member;
  struct first_structure_type struct_member;
};

The first structure type is incorporated as a member of the second structure type. You can initialize a variable of the second type as follows:

struct second_structure_type demo;

demo.double_member = 12345.6789;
demo.struct_member.integer_member = 5;
demo.struct_member.float_member = 1023.17;

The member operator . is used to access members of structures that are themselves members of a larger structure. No parentheses are needed to force a special order of evaluation; a member operator expression is simply evaluated from left to right.

In principle, structures can be nested indefinitely. Statements such as the following are syntactically acceptable, but bad style. (See Style.)

my_structure.member1.member2.member3.member4 = 5;

What happens if a structure contains an instance of its own type, however? For example:

struct regression
{
  int int_member;
  struct regression self_member;
};

In order to compile a statement of this type, your computer would theoretically need an infinite amount of memory. In practice, however, you will simply receive an error message along the following lines:

struct5.c: In function `main':
struct5.c:8: field `self_member' has incomplete type

The compiler is telling you that self_member has been declared before its data type, regression has been fully declared -- naturally, since you're declaring self_member in the middle of declaring its own data type!