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<html>
<head>
<title>The TYPEDEF keyword.</title>
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<hr>
<center><h1>The TYPEDEF keyword.</h1></center>
<hr>
<p>
Every variable has a
<a href="../CONCEPT/data_types.html">data type</a>. <b>typedef</b> is used to define
new data type names to make a program more readable to the programmer.
<p>
For example:
<p>
<table border=1 width="100%" bgcolor="ivory">
<tr><td>
<pre>
|
main() | main()
{ | {
int money; | typedef int Pounds;
money = 2; | Pounds money = 2
} | }
</pre>
</td></tr></table>
<p>
These examples are EXACTLY the same to the compiler. But the right hand
example tells the programmer the type of money he is dealing with.
<p>
<a name=boolean></a>
A common use for typedef is to define a boolean data type as below.
<font color="brown">
<br>
Note: Recent C++ compilers have introduced a <a href="../../CPLUSPLUS/SYNTAX/bool.html">boolean</a> datatype.
<p>
</font>
<p>
<table border=1 width="100%" bgcolor="ivory">
<tr><td>
<pre>
typedef enum {FALSE=0, TRUE} Boolean
main ()
{
Boolean flag = TRUE;
}
</pre>
</td></tr></table>
<p>
And as a final example, how about creating a string datatype?
<p>
<table border=1 width="100%" bgcolor="ivory">
<tr><td>
<pre>
typedef char *String;
main()
{
String Text = "Thunderbird";
printf("%s\n", Text);
}
</pre>
</td></tr></table>
<p>
The main use for typedef seems to be defining
<a href="struct.html">structures</a>. For example:
<p>
<table border=1 width="100%" bgcolor="ivory">
<tr><td>
<pre>
typedef struct {int age; char *name} person;
person people;
</pre>
</td></tr></table>
<p>
Take care to note that <b>person</b> is now a type specifier and NOT a
variable name.
<p>
As a final note, you can create several data types in one hit.
<table border=1 width="100%" bgcolor=ivory>
<tr><td>
<pre>
typedef int Pounds, Shillings, Pennies, Dollars, Cents;
</pre>
</td></tr></table>
<p>
<p>
<hr>
<h2>Examples:</h2>
<img src="../../GRAPHICS/computer.gif" align=center>
<a href="../EXAMPLES/linklst3.c">Here is a rather heavy example of typedef.</a>
<br clear=left>
<hr>
<h2>Notes:</h2>
I would expect to see 'typedef' in header files.
<p>
<hr>
<p>
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<p>
<hr>
<address>Martin Leslie
</address><p>
</body>
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