Structures

1
Structures Unions

• Chapter 18

2
Structures

• Structs are a way of storing a collection of
  related data together as a single unit, much like
  an object in Java.
• Unlike objects, structs only contain data, they
  do not have member functions to manipulate the
  data.
• Each piece of data in a structure is called a
  field or member.

3
Structure Templates

• A structure template is a type definition that
  defines the fields of a struct.
• typedef struct field declarations type_name

4
An Example

• typedef struct char first_nameMAX_SIZE char
  last_nameMAX_SIZE char middle_initial
• short num_children char ssn12
  person_tint main() person_t someone return
  0

5
Accessing Struct Fields

• To access the data of a structure, use the
  structure variable name followed by the structure
  binding operator, . and then the field name.
• printf(s c. s, someone.first_name,
  someone.middle_initial, someone.last_name)
• someone.num_children2

6
Copying Passing Structures

• You can copy structures just like you would any
  other simple data type
• person_t person1, person2, temptemp
  person1person1person2person2temp
• You can also pass structures by value to your
  functions. Try to avoid passing by value when
  possible, creating a copy of the struct with all
  of its data can be a very time consuming
  operation depending on the size and amount of
  data in the structure.

7
Pointers to Structures

• Creating a pointer to a struct is no more
  difficult than with any other data type.
• person_t ptr, person1ptr person1
• Using the . operator with a struct pointer is a
  little more difficult. The structure binding
  operator has a higher precedence than the
  dereference operator, which means a statement
  like ptr.num_children2 would try to
  dereference the num_children field of the
  structure, rather than the structure itself. You
  need to use this instead (ptr).num_children2
• To eliminate the complexity of the parentheses,
  there is a special operator for structure pointer
  variables -gt which automatically dereferences
  the pointer correctly without the extra
  parentheses.
• ptr-gtnum_children2 //this statement is
  equivalent to // (ptr).num_children2

8
Use of Structs

• While its perfectly ok to declare structs right
  inside your .c file and access their fields
  throughout your code using the binding operator,
  there is a better way to do it.
• If theres a possibility that you may want to use
  your struct again in the future, its a good idea
  to define it in a header file, then include that
  header in any C program you want to use it in.
  Its also a good idea to write a set of member
  functions for setting fields and getting fields
  of the struct. This allows for encapsulation, so
  you (or someone else using your struct) doesnt
  need to know the exact layout of the struct.
  Once you have member functions, you can create a
  simple API to remind yourself or others of how to
  use your struct properly.

9
Nesting Structures

• Your structs can contain other structs as
  members.
• typedef struct char nameMAX_SIZE char
  type_of_animalMAX_SIZE pet_ttypedef
  struct char first_nameMAX_SIZE char
  last_nameMAX_SIZE char middle_initial
• short num_children pet_t pet
  person_tint main() person_t someone
• printf(pet name s, someone.pet.name)
• return 0

10
Self Referencing Structures

• Sometimes its necessary for a structure to
  contain, or have a pointer to a structure of the
  same type. This is useful for things like linked
  lists, graphs, and trees where each node needs to
  reference at least one other node.
• In the way weve defined our structures so far,
  we havent named them until the end of the struct
  definition, thus the compiler doesnt know about
  the type when it encounters it as a field.
  Instead we need to do a typedef first, then
  define our struct.

11
Illustrating the Problem

• typedef struct char first_nameMAX_SIZE char
  last_nameMAX_SIZE char middle_initial
• short num_children char ssn12 person_t
  spouse
• person_t

12
The Solution

• typedef struct person person_t
• struct person char first_nameMAX_SIZE char
  last_nameMAX_SIZE char middle_initial
• short num_children char ssn12 person_t
  spouse

13
Initializing Structures

• You can initialize structures using the
  operators.
• Given our original person struct, you could
  initialize it like soperson_t someone
  Joe, Schmo, D,2,003-11-0002

14
Unions

• A union is a block of memory with multiple names
  and data types. With structures, each field has
  its own memory location, with unions all of the
  fields share the same memory location.
• Can be used to subdivide a large object into
  pieces, or to conserve memory.
• typedef union double number char bytes8
  split_double_t
• In this example, you can access the whole double,
  or you can read it one byte at a time by using
  the bytes field instead.
• To save memory using unions, you could have a
  structure with two mutually exclusive fields one
  of the fields will always be used, but never both
  at the same time. You could create a union for
  those two fields so they only take one segment of
  memory rather than the two that would be required
  if you didnt declare them as a union.