2.7. Structures

Structure types

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A structure is a collection of variables.

Define a structure type as in this example.

struct cell
{
  int          n;
  struct cell* next;
};

Creating structures

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C allows you to create a structure either in a stack frame (as an automatic variable) or in the heap. (In Java, you can only create an object in the heap.)

Statement

struct cell c;

struct cell* p = (struct cell*) malloc(sizeof(struct cell));

Naming structure types

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Type definition

  typedef struct cell 
  {
    int n;
    struct cell* next;
  }
  CELL;

  CELL* p = (CELL*) malloc(sizeof(CELL));

Selecting parts of a structure

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To select a variable from a structure of type CELL, use a dot, as in Java. So if c has type CELL then

• c.n is the variable called n in CELL c.

• c.next is the variable called next in CELL c.

To select a variable from the structure pointed to by a given pointer, use -> instead of a dot. If p has type CELL* then

• p->n is the variable called n in the CELL to which p points.

• p->next is the variable called next in the CELL to which p points.

Defining constructors

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Define a constructor as an ordinary function. Do not try to write it inside the structure type definition. For example,

  CELL* newCell(int m, CELL* q)
  {
    CELL* p = (CELL*) malloc(sizeof(CELL));
    p->n    = m;
    p->next = q;
    return p;
  }

defines constructor newCell. Notice that it actually returns a pointer to a CELL. See the next item.

Practical issues

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A structure is similar to a Java class, but with heavy restrictions. There are only variables. There are no instance methods or static methods associated with a structure.

All of the variables are public. There are no static variables.

But, once you accept those restrictions, you can imagine a value of a structure type to be similar to a Java object.

Java stores all objects in the heap. Unless you are sure that an object is only needed during a call to the function that creates the object, you would do well to use the same principle. Allocate objects in the heap. Refer to them by pointers.

Example

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See linkedlist.h and linkedlist.c for a vey small definition of linked lists.