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<head>
<title>The const keyword.</title>
<script language="JavaScript">
</script>
</head>
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<hr>
<center>
<h1>The const keyword.</h1>
</center>
<hr>
The const keyword is used to create a read only variable. Once initialised,
the value of the variable cannot be changed but can be used just like
any other variable.
<h2>const syntax</h2>
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
main()
{
const float pi = 3.14;
}
</pre>
</td></tr>
</table>
</center>
<p>
The const keyword is used as a qualifier to the following data types -
int float char double struct.
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
const int degrees = 360;
const float pi = 3.14;
const char quit = 'q';
</pre>
</td></tr>
</table>
</center>
<p>
<h2>const and pointers.</h2>
Consider the following example.
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
void Func(const char *Str);
main()
{
char *Word;
Word = (char *) malloc(20);
strcpy(Word, "Sulphate");
Func(Word);
}
void Func(const char *Str)
{
}
</pre>
</td></tr>
</table>
</center>
<p>
The <code>const char *Str</code> tells the compiler that the DATA the
pointer points too is <code>const</code>. This means, Str can be changed
within Func, but *Str cannot. As a copy of the pointer is passed to Func,
any changes made to Str are not seen by main....
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
--------
| Str | Value can be changed
-----|--
|
|
V
--------
| *Str | Read Only - Cannot be changed.
--------
</pre>
</td></tr>
</table>
</center>
<p>
<h2>Geeky Stuff</h2>
It is still possible to change the contents of a 'const' variable.
<a href=../EXAMPLES/const2.c>Consider this program</a> it creates
a const variable and then changes its value by accessing the data by
another name.
<p>
I am not sure if this applies to all compilers, but,
you can place the 'const' after the datatype, for example:
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
int const degrees = 360;
float const pi = 3.14;
char const quit = 'q';
</pre>
</pre>
</td></tr>
</table>
</center>
<p>
are all valid in 'gcc'.
<p>
<h2>More Geeky Stuff</h2>
What would you expect these to do?
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
main()
{
const char * const Variable1;
char const * const Variable2;
};
</pre>
</td></tr>
</table>
</center>
<p>
These both make the pointer and the data read only.
Here are a few more examples.
<p>
<center>
<table bgcolor=ivory width="80%" border=1>
<tr><td>
<pre>
const int Var; /* Var is constant */
int const Var; /* Ditto */
int * const Var; /* The pointer is constant,
* the data its self can change. */
const int * Var; /* Var can not be changed. */
</pre>
</td></tr>
</table>
</center>
<p>
<hr>
<h2>See Also.</h2>
<img src="../../GRAPHICS/whiteball.gif" alt="o">
<a href="../SYNTAX/define_preprocessor.html">#define preprocessor</a>
<p>
<font color=brown>
<img src="../../GRAPHICS/whiteball.gif" alt="o">
C++ version of
<a href="../../CPLUSPLUS/SYNTAX/const.html">const</a>
</font>
<hr>
<h2>An Example.</h2>
<img src="../../GRAPHICS/computer.gif" alt="o">
<a href=../EXAMPLES/const.c> const</a> example.
<p>
<hr>
<p>
<center>
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<tr align=center>
<td width="25%">
<a href="../cref.html"> Top</a>
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<a href="../master_index.html"> Master Index</a>
</td><td width="25%">
<a href="keywords.html"> Keywords</a>
</td><td width="25%">
<a href="../FUNCTIONS/funcref.htm"> Functions</a>
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</table>
</center>
<p>
<hr>
<address>Martin Leslie
</address><p>
</body>
</html>
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