Sams Teach Yourself C in 21 Days

Implementing Structures, Unions, and TypeDefs 281

11

initialize it. print_function()will evaluate the typevariable in order to print a state-

ment with the appropriate variable type. This prevents getting erroneous data such as that

in Listing 11.7.

DOremember which union member is

being used. If you fill in a member of

one type and then try to use a different

type, you can get unexpected results.

DON’Ttry to initialize more than the

first union member.

DON’Tforget that the size of a union is

equal to its largest member.

DO DON’T

Creating Synonyms for Structures with

typedef

You can use the typedefkeyword to create a synonym for a structure or union type. For

example, the following statements define coordas a synonym for the indicated structure:

typedef struct {

int x;

int y;

} coord;

You can then declare instances of this structure using the coordidentifier:

coord topleft, bottomright;

Note that a typedefis different from a structure tag, as described earlier in today’s les-

son. If you write

struct coord {

int x;

int y;

};

the identifier coordis a tag for the structure. You can use the tag to declare instances of

the structure, but unlike using a typedef, you must include the structkeyword:

struct coord topleft, bottomright;

Whether you use typedefor a structure tag to declare structures makes little difference.

Usingtypedefresults in slightly more concise code because the structkeyword doesn’t

have to be used. On the other hand, using a tag and having the structkeyword explicit

makes it clear that it is a structure being declared.

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