What Is a Structure?
1. A collection of variables that are functionally related to each other.
2. Each variable that is a member of the structure has a specific type.
3. Different members of the structure may have either the same or different
types. Cf. the elements of an array, which must all be of one type.
4. A structure is a derived data type, constructed from two or more objects of
one or more individual types.
5. The entire structure may bear a name.
6. Each member of the structure must [also] have a name.
7. The scope of the name of a structure member is limited to the structure itself
and also to any variable declared to be of the structure's type.
8. THEREFORE, different structures may contain members having the same name;
these may be of the same or of different types.
9. A self-referential structure contains a member which is a pointer to the same
structure type.
10. Declaration of the structure merely defines the new data type; space is NOT
reserved in memory as a result of the declaration.
However, declaration of the structure does define how much memory is
needed to store each variable subsequently declared to be of the type of
the defined structure.
Form of Structure Declaration: Alternative 1
(1) Complete definition including assignment of a tag name to the structure.
(2) The tag name is referred to in subsequent declarations of variables of the type
so defined.
(3) Each such declaration MUST include the keyword struct AND the name of the
user-defined structure type AND the variable name(s).
struct nameOfThisStructureType
{
typeOfFirstMember nameOfFirstMember;
typeOfSecondMember nameOfSecondMember;
typeOfThirdMember nameOfThirdMember;
. . .
};
struct nameOfThisStructureType variable1OfThisStructureType,
variable2OfThisStructureType,
. . . ;
Additional variable declarations can subsequently be made for this structure type.
Form of Structure Declaration: Alternative 2
(1) Basic named definition of the structure is effected same as for Alternative 1.
(2) In ADDITION, one or more variables can be declared within the declaration
of the structure type to be of the defined type.
(3) Other variables may also be declared subsequently to be of the same type of
this structure, using the keyword struct together with the tag name and the
variable names.
struct nameOfThisStructureType
{
typeOfFirstMember nameOfFirstMember;
typeOfSecondMember nameOfSecondMember;
typeOfThirdMember nameOfThirdMember;
. . .
} variable1OfThisStructureType, variable2OfThisStructureType, . . .;
struct nameOfThisStructureType variable3OfThisStructureType,
variable4OfThisStructureType,
. . . ;
Form of Structure Declaration: Alternative 3
(1) Tag name is not assigned to the structure when the type is declared.
(2) Variables are specified within the structure declaration to be of the defined
structure type.
(3) Because of the absence of a tag name for the structure type, there is no means
available to ever be able to declare any other variables to be of this same
type.
struct /* NO NAME ASSIGNED TO THE TYPE */
{
typeOfFirstMember nameOfFirstMember;
typeOfSecondMember nameOfSecondMember;
typeOfThirdMember nameOfThirdMember;
. . .
} variable1OfThisStructureType, variable2OfThisStructureType, . . .;
Form of Structure Declaration: Alternative 4
(1) Complete definition of the structure, including assignment to it of a tag name.
(2) Subsequently, the tag name is used in a typedef declaration to assign a second
name (i.e., an alias ) to the structure. The alias can then be used in declaring
a variable the same way as a native C type name is used, that is, without the
keyword struct, i.e., just like int, char, float, etc.
struct nameOfThisStructureType
{
typeOfFirstMember nameOfFirstMember;
typeOfSecondMember nameOfSecondMember;
typeOfThirdMember nameOfThirdMember;
. . .
};
typedef struct nameOfThisStructureType AliasForThisStructureType;
AliasForThisStructureType variable1OfThisStructureType,
variable2OfThisStructureType, . . . ;
Form of Structure Declaration: Alternative 5
(1) Complete definition of the structure without assignment of a tag name.
(2) The keyword typedef is used within the declaration of the structure to assign a
name (i.e., an alias) to the structure. The structure itself is anonymous, and
has only the alias name. The alias can be used in the same way as a native C
type name is used , that is, without the keyword struct, i.e., just like int,
char, float, etc.
typedef struct
{
typeOfFirstMember nameOfFirstMember;
typeOfSecondMember nameOfSecondMember;
typeOfThirdMember nameOfThirdMember;
. . .
} AliasForThisStructureType;
AliasForThisStructureType variable1OfThisStructureType,
variable2OfThisStructureType, . . . ;
Example 1
enum genders {MALE, FEMALE};
enum studentStatus {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, POSTBAC};
struct student
{
char firstName[20];
char lastName[20];
char middleName[20];
long int studentNumber;
short int entranceYear;
genders studentGender;
studentStatus status;
char major[6];
struct student *nextStudent; /* Useful for making a linked list. */
struct student *priorStudent; /* Useful for a doubly linked list. */
};
struct student undergraduateStudent, graduateStudent;
struct student specialStudent;
Example 2
enum genders {MALE, FEMALE};
enum studentStatus {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, POSTBAC};
struct student
{
char firstName[20];
char lastName[20];
char middleName[20];
long int studentNumber;
short int entranceYear;
genders studentGender;
studentStatus status;
char major[6];
struct student *nextStudent;
struct student *priorStudent;
} undergraduateStudent, graduateStudent;
struct student specialStudent;
Example 3
enum genders {MALE, FEMALE};
enum studentStatus {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, POSTBAC};
struct
{
char firstName[20];
char lastName[20];
char middleName[20];
long int studentNumber;
short int entranceYear;
genders studentGender;
studentStatus status;
char major[6];
struct student *nextStudent;
struct student *priorStudent;
} undergraduateStudent, graduateStudent, specialStudent;
Example 4
enum genders {MALE, FEMALE};
enum studentStatus {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, POSTBAC};
struct student
{
char firstName[20];
char lastName[20];
char middleName[20];
long int studentNumber;
short int entranceYear;
genders studentGender;
studentStatus status;
char major[6];
struct student *nextStudent;
struct student *priorStudent;
} undergraduateStudent, graduateStudent;
typedef struct student StudentType;
StudentType specialStudent;
Example 5
enum genders {MALE, FEMALE};
enum studentStatus {FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, POSTBAC};
typedef struct
{
char firstName[20];
char lastName[20];
char middleName[20];
long int studentNumber;
short int entranceYear;
genders studentGender;
studentStatus status;
char major[6];
struct student *nextStudent;
struct student *priorStudent;
} StudentType;
StudentType undergraduateStudent, graduateStudent, specialStudent;
Which Alternative(s) Should YOU Use?
1. Alternative 3 is useful (example 3) because it forces all variables to be
declared at structure definition time.
2. Alternative 5 is useful (example 5) because it enables variable declarations
to be made to the structure type withOUT use of the keyword struct.
3. NONE of the other alternatives should ever be used; they are principally of
historical interest.
Accessing Members of a Variable of a Structure Type
1. Structure Member operator ≡ Dot operator
StudentType undergraduateStudent;
char lastNameOfStudent[20];
lastNameOfStudent = undergraduateStudent.lastName;
1. Structure Pointer operator ≡ Member operator
StudentType *pointerToGraduateStudent;
short int yearOfStudentEntrance;
yearOfStudentEntrance = pointerToGraduateStudent—>entranceYear;
OR
StudentType *pointerToGraduateStudent
short int yearOfStudentEntrance;
yearOfStudentEntrance = (*pointerToGraduateStudent).entranceYear;
/* NOTE: The parentheses are NECESSARY in this example. */
