During your programming experience you may feel the need to define your own type of data. In C this is done using two keywords: struct and typedef. Structures and unions will give you the chance to store non-homogenous data types into a single collection.

Declaring a new data type

typedef struct student_structure{
    char* name;
    char* surname;
    int year_of_birth;

After this little code student will be a new reserved keyword and you will be able to create variables of type student. Please mind that this new kind of variable is going to be structured which means that defines a physically grouped list of variables to be placed under one name in a block of memory.

New data type usage

Let’s now create a new student variable and initialize its attributes:

   student stu;
   strcpy( stu.name, "John");
   strcpy( stu.surname, "Snow"); 
   stu.year_of_birth = 1990;
   printf( "Student name : %s\n", stu.name);
   printf( "Student surname : %s\n", stu.surname);
   printf( "Student year of birth : %d\n", stu.year_of_birth);

As you can see in this example you are required to assign a value to all variables contained in your new data type. To access a structure variable you can use the point like in stu.name. There is also a shorter way to assign values to a structure:

typedef struct{
   int    x;
   int    y;

point image_dimension = {640,480};

Or if you prefer to set it’s values following a different order:

point img_dim = { .y = 480, .x = 640 };

Unions vs Structures

Unions are declared in the same was as structs, but are different because only one item within the union can be used at any time.

typedef union{
      int circle;
      int triangle;
      int ovel;

You should use union in such case where only one condition will be applied and only one variable will be used. Please do not forget that we can use our brand new data type too:

typedef struct{
      char* model;
      int year;

typedef struct{
      char* owner;
      int weight;

typedef union{
  car_type car;
  truck_type truck;

A few more tricks

  • When you create a pointer to a structure using the & operator you can use the special -> infix operator to deference it. This is very used for example when working with linked lists in C
  • The new defined type can be used just as other basic types for almost everything. Try for example to create an array of type student and see how it works.
  • Structs can be copied or assigned but you can not compare them!

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