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<b>Data Structures and Algorithms 
with Object-Oriented Design Patterns in C++</b><br>
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<H3><A NAME="SECTION0018222000000000000000">Constant Parameters</A></H3>
<P>
In general, pass-by-reference parameter passing allows the called function
to modify the actual parameters.
However, 
sometimes it is the case that the programmer
does not want the actual parameters to be modified.
Nevertheless, pass-by-reference may be the preferred method
for performance reasons.
<P>
In C++ we can use the <tt>const</tt> keyword
to achieve the performance of pass-by-reference while at the same time
ensuring that the actual parameter cannot be modified.
Consider the following definition of the function <tt>Two</tt>:
<PRE>void Two (int const&amp; x)
{
    x = 2; // Not allowed.
    cout &lt;&lt; x &lt;&lt; endl; // This is ok.
}</PRE>
The <tt>const</tt> keyword modifies the type <tt>int</tt>.
It says that the <tt>int</tt> to which <tt>x</tt> refers is a constant.
The value of <tt>x</tt> can be used without impunity.
However, <tt>x</tt> cannot be used as the target of an assignment statement.
In fact, the variable <tt>x</tt> cannot be used in any context 
where it might be modified.
Any attempt to do so is an error that is detected by the compiler.
<P>
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