Introduction to Union Type in C

1. CSci 160Lecture 44 Martin van Bommel

2. Union Introduction • So far, all variables of a particular structure type have exactly the same components • Sometimes we need a structured type in which some components vary depending on the value of another component • e.g. data stored about a geometric figure depends on the type of figure

3. Union Type • C provides a union data structure typedef union { int num; char ch[2]; } int_ch_t; • Variables of this type represent either an integer or two characters • Need some other value to determine which

4. Union Storage • What if parts of union not same memory size? typedef union { int num1; double num2; } num_t; • When create a variable, how large in memory? • Size of largest part of union (e.g. 8 bytes)

5. Geometric Figures • For problem with geometric figures, first define structures for each type of figure • Then define a union type with a component for each figure type • Finally define structure containing both a component of the union type and a component whose value denotes which is the correct interpretation of union • See program geometry.c

6. Union Problems • Most common error with union types is referencing a component that is not currently valid • Helpful to place in another structure with component whose value indicates correct interpretation • Then all manipulation done within context of if or switch statement