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<title>Global and Local variables declaration.</title>
<head>
<script language="JavaScript">
</script>
</head>
<body bgcolor="#ffffcc">
<hr>
<center>
<h1>Global and Local variables.</h1>
</center>
<hr>
<p>
<h2>Local variables</h2>
Local variables must always be
<a href="../glossary.html#definition">defined</a>
at the top of a block.
<p>
<font color="brown">
C++ has changed the rules regarding where you can define a
local variable.
<a href=../../CPLUSPLUS/CONCEPT/local_var.html>Click here</a>
for the low down.
</font>
<p>
When a local variable is
<a href="../glossary.html#definition">defined</a>
- it is not initalised by the system, you
must initalise it yourself.<p>
A local variable is
<a href="../glossary.html#definition">defined</a>
inside a <b>block</b> and is only visable
from within the block.
<p>
<center>
<table border=1 width="80%" bgcolor=ivory>
<tr>
<td>
<pre>
main()
{
int i=4;
i++;
}
</pre>
</td>
</tr>
</table>
</center>
<p>
When execution of the block starts the variable is
available, and when the block ends the variable 'dies'.<p>
A local variable is visible within nested blocks unless a variable with
the same name is
<a href="../glossary.html#definition">defined</a>
within the nested block.
<p>
<center>
<table border=1 bgcolor=ivory width="80%">
<tr>
<td>
<pre>
main()
{
int i=4;
int j=10;
i++;
if (j > 0)
{
printf("i is %d\n",i); /* i defined in 'main' can be seen */
}
if (j > 0)
{
int i=100; /* 'i' is defined and so local to
* this block */
printf("i is %d\n",i);
} /* 'i' (value 100) dies here */
printf("i is %d\n",i); /* 'i' (value 5) is now visable. */
}
</pre>
</td>
</tr>
</table>
</center>
<p>
<a name="global">
<hr width="50%" align=center>
<h2>Global variables</h2>
<font color="brown">
C++ has enhanced the use of
<a href=../../CPLUSPLUS/CONCEPT/scope.html> global variables.</a>
</font>
<p>
Global variables ARE initalised by the system when you
<a href="../glossary.html#definition">define</a>
them!
<p>
<center>
<table border=2 width="50%" bgcolor=ivory>
<tr align=center><td> Data Type </td><td> Initialser </td> </tr>
<tr align=center><td> int </td><td> 0 </td> </tr>
<tr align=center><td> char </td><td> '\0' </td> </tr>
<tr align=center><td> float </td><td> 0 </td> </tr>
<tr align=center><td> pointer </td><td> NULL </td> </tr>
</table>
</center>
<p>
In the next example <b>i</b> is a global variable, it can be seen and modified by
<b> main </b>and any other functions that may reference it.
<p>
<center>
<table border=1 width="80%" bgcolor=ivory>
<tr>
<td>
<pre>
int i=4;
main()
{
i++;
}
</pre>
</td>
</tr>
</table>
</center>
<p>
Now, this example has <b>global</b> and <b>Internal</b> variables.
<p>
<center>
<table border=1 width="80%" bgcolor=ivory>
<tr>
<td>
<pre>
int i=4; /* Global definition */
main()
{
i++; /* global variable */
func
}
func()
{
int i=10; /* Internal declaration */
i++; /* Internal variable */
}
</pre>
</td>
</tr>
</table>
</center>
<p>
<b>i</b> in <b>main</b> is global and will be incremented to 5. <b>i</b> in
<b>func</b> is internal and will be incremented to 11. When control returns
to <b>main</b> the internal variable will die and and any reference to
<b>i</b> will
be to the global.<p>
static variables can be 'seen' within all functions in this source file. At
link time, the static variables defined here will not be seen by the object
modules that are brought in.<p>
<hr>
<h2>Example:</h2>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/global.c">An Example program</a>.
<hr>
<h2>See Also:</h2>
See <a href="../CONCEPT/storage_class.html">Storage classes</a> to see the more powerfull
features of variable declarations.
<p>
<hr>
<p>
<center>
<table border=2 width="80%" bgcolor="ivory">
<tr align=center>
<td width="25%">
<a href="../cref.html"> Top</a>
</td><td width="25%">
<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>
</td>
</tr>
</table>
</center>
<p>
<hr>
<address>Martin Leslie
</address><p>
</body>
</html>
|
