7_Objective-C Structures

Structure is another user-defined data type available allows you to combine data items of different kinds.

Structures are used to represent a record.

Defining a Structure

To define a structure, you must use the struct statement.

The struct statement defines a new data type, with more than one member for your program.

The format of the struct statement

struct [structure tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];
  • The structure tag is optional
  • Each member definition is a normal variable definition,
  • At the end of the structure’s definition, before the final semicolon, you can specify one or more structure variables but it is optional.

Here is the way you would declare the structure for example Book structure:

struct Books
{
   NSString *title;
   NSString *author;
   NSString *subject;
   int book_id;
} book;  

Accessing Structure Members

To access any member of a structure, we use the member access operator (.).

You would use struct keyword to define variables of structure type.

Example

Structures as Function Arguments

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer.

Example

Pointers to Structures

You can define pointers to structures in very similar way as you define pointer to any other variable as follows:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above-defined pointer variable. To find the address of a structure variable, place the & operator before the structure’s name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows:

struct_pointer->title;

Example

Bit Fields

Bit Fields allow the packing of data in a structure.

This is especially useful when memory or data storage is at a premium.

Typical examples:

  • Packing several objects into a machine word. e.g. 1 bit flags can be compacted.
  • Reading external file formats — non-standard file formats could be read in. E.g. 9 bit integers.

Objective-C allows us do this in a structure definition by putting :bit length after the variable. For example:

struct packed_struct {
  unsigned int f1:1;
  unsigned int f2:1;
  unsigned int f3:1;
  unsigned int f4:1;
  unsigned int type:4;
  unsigned int my_int:9;
} pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit my_int.

Objective-C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word.

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