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Typedefs

CS157 Typedef

Horrible Code

Consider this horrible code:

float bal;
long acct;

What sort of values are in those variables? Who knows? Clearly, bal is a floating-point value, and acct is a big integer, but that’s all we know.

Somewhat Less Horrible Code

Now, consider this code:

money bal;
account_number acct;

That tells us more! We now know that bal is an amount of money, and that acct is an account number.

The Whole Picture

typedef float money;         // Make “money” an alias for “float”
typedef long account_number; // Make “account_nbumer” an alias for long
…
money bal;
account_number acct;

typedefs

• Allow you to make a new name, an alias, for an existing data type.
• Syntax is similar to declaring a variable—put typedef in front.
• Makes code more readable.

typedef unsigned short ushort;
typedef char bigstring[256];

ushort shortie;             // same as unsigned short shortie;
bigstring name;             // same as char name[256];

shortie = 42;
strcpy(name, "hello");

Get the order right

1. Write a normal declaration: int count;
2. Add typedef at the front: typedef int count;

    typedef int count;		// Right
    typedef count int;		// WRONG WRONG WRONG!

typedefs

A typedef is a name for some type. Any type can be named with a typedef.

struct person_s {
    int age;
    char name[30];
};

typedef struct person_s person;

int main() {
    person people[20];        // instead of struct person
    return 0;
}

typedefs

• For stuct, enum, and union, the syntax can be simplified.
• We can combine the declaration and the typedef.
• This allows us to leave out the tag name.

typedef struct {
    int age;
    char name[30];
} person;
// person is a data type, NOT a variable name

int main() {
    person people[20];
    return 0;
}

typedef

Data type comes after closing squiggly for typedef

typedef struct {
    // stuff here
} dataTypeName;

Either define a type, or a variable, but not both:

typedef struct {
    // stuff here
} dataTypeName variableName;        // ERROR

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