typedef : A Quick Look

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typedef : A Quick Look

The typedef function is an advanced data feature that enables you to create your own name for a type. It is similar to #define in that respect, but with three differences:

• Unlike #define , typedef is limited to giving symbolic names to types only and not to values.

• The typedef function is performed by the compiler, not the preprocessor.

• Within its limits, typedef is more flexible than #define .

Let's see how typedef works. Suppose you want to use the term BYTE for one-byte numbers . You simply define BYTE as if it were a char variable and precede the definition by the keyword typedef .

 typedef unsigned char BYTE; 

From then on, you can use BYTE to define variables :

 BYTE x, y[10], * z; 

The scope of this definition depends on the location of the typedef statement. If the definition is inside a function, then the scope is local, confined to that function. If the definition is outside a function, then the scope is global.

Often, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you could have used lowercase instead:

 typedef unsigned char BYTE; 

The same rules that govern the valid names of variables govern the name used for a typedef .

Creating a name for an existing type might seem a bit frivolous, but it can be useful. With the preceding example, using BYTE instead of unsigned char helps document that you plan to use BYTE variables to represent numbers rather than character codes. Using typedef also helps increase portability. For example, we've mentioned the size_t type that represents the type returned by the sizeof operator and the time_t type that represents the type of value returned by the time() function. The C standard says sizeof and time() return integer types, but it leaves it up to the implementation to determine which type. The reason for this lack of specificity is that the ANSI C committee feels that no one choice is likely to be the best choice for every computer platform. So they make up a new type name, such as time_t , and let the implementation use a typedef to set that name to some specific type. That way they can provide a general prototype such as the following:

 time_t time(time_t *); 

On one system, time_t can be unsigned int ; on another, it can be unsigned long . As long as you include the time.h header file, your program can access the appropriate definition, and you can declare time_t variables in your code.

Some features of typedef can be duplicated with a #define . For example,

 #define BYTE unsigned char 

cause the preprocessor to replace BYTE with unsigned char . Here is one that can't be duplicated with a #define :

 typedef char * STRING; 

Without the keyword typedef , this example would identify STRING itself as a pointer to char . With the keyword, it makes STRING an identifier for pointers to char . Therefore,

 STRING name, sign; 

means

 char * name, * sign; 

Suppose, instead, you did this:

 #define STRING char * 

Then

 STRING name, sign; 

would translate to the following:

 char * name, sign; 

In this case, only name would be a pointer.

You can use typedef with structures, too:

 typedef struct complex {         float real;         float imag; } COMPLEX; 

You can then use the type COMPLEX instead of the struct complex to represent complex numbers. One reason to use typedef is to create convenient , recognizable names for types that turn up often. For instance, many people prefer to use STRING or its equivalent, as in the earlier example.

A second reason for using typedef is that typedef names are often used for complicated types. For example, the declaration

 typedef char (* FRPTC ()) [5]; 

makes FRPTC announce a type that is a function that returns a pointer to a five-element array of char . (See the upcoming discussion on fancy declarations.)

When using typedef , bear in mind that it does not create new types; it just creates convenient labels. This means, for example, that variables using the STRING type we created can be used as arguments for functions expecting type pointer-to- char .

With structures, unions, and typedef , C gives you the tools for efficient and portable data handling.

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