Typedef

typedef is a keyword in the C and C++ programming languages. It is used to give a data type a new name. The intent is to make it easier for programmers to comprehend source code.

Usage examples

Consider this code:

int coxes;int jaffa;...coxes++;...if (jaffa = 10)...

Now consider this:

typedef int Apples;typedef int Oranges;Apples coxes;Oranges jaffa;...coxes++;...if (jaffa = 10)...

Both sections of code do the same thing. The use of typedef in the second example makes it easier to comprehend what is going on, namely, that one variable contains information about apples while the other contains information about oranges.

One more example:

struct var { int data1; int data2; char data3;};

Here a user-defined data type "var" has been defined. To create a variable of the type "var", in C, the following code is required:

struct var a;

Let's add the following line to the end of this example:

typedef struct var newtype;

Now, in order to create a variable of type "var", the following code will suffice:

newtype a;

The same can be accomplished with the following code:typedef struct var { int data1; int data2; char data3;}newtype;

Typedefs can also simplify creating pointers to self-referential structures. Consider this:

struct Node{int a; //Datastruct Node *nextptr;};

Normally, one would need to prefix an asterisk to each variable to assign it a pointer:

Node *startptr, *endptr, *curptr, *prevptr, errptr, *refptr;

A programmer would assume that errptr was indeed a Node *, but a typographical error means that errptr is a Node. This can lead to subtle syntax errors.

It would be better to define a Node * type. The following code does so.

typedef Node *NodePtr;...NodePtr startptr, endptr, curptr, prevptr, errptr, refptr;

Now it is guaranteed that all variables defined are of type Node *, even errptr.

Note that a struct declaration in C++ also defines an implicit typedef — that is, the data type can be referred to as var (rather than struct var) immediately, without any explicit use of the typedef keyword. However, even in C++ it can be worthwhile to define typedefs for more complicated types. For example, a program to manipulate RGB images might find it useful to give the name ImagePointer to the type "pointer to array [3] of int":

typedef int (*ImagePointer) [3] ;

Criticisms

Some people are opposed to the extensive use of typedefs. Most arguments center around the idea that typedefs simply hide the actual data type of a variable. For example, Greg Kroah-Hartman, a Linux kernel hacker and documenter, discourages their use for anything except function prototype declarations. He argues that this practice not only unnecessarily obfuscates code, it can also cause programmers to accidentally misuse large structures thinking them to be simple types [cite web |url=http://www.linuxjournal.com/article/5780 |title=Proper Linux Kernel Coding Style |author=Kroah-Hartman, Greg |authorlink=Greg Kroah-Hartman |date=2002-07-01 |accessdate=2007-09-23 |work=Linux Journal |quote=Using a typedef only hides the real type of a variable.] .

Other languages

In many statically-typed functional languages, like Haskell, Miranda, OCaml, etc., you can define "type synonyms", which are the same as typedefs in C. An example in Haskell:

type pairOfInts = (Int, Int)

We've just defined a type synonym "pairOfInts" which means the same as a pair of Int's.

ee also

*Abstract data type
*C syntax

References

External links

*http://www.cprogramming.com/tutorial/typedef.html - detailed discussion of typedef at "CProgramming.com"