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<a name="Ambiguous-Expressions"></a>
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<a name="Ambiguous-Expressions-1"></a>
<h3 class="section">10.2 Ambiguous Expressions</h3>
<a name="index-ambiguous-expressions"></a>

<p>Expressions can sometimes contain some ambiguous elements.  For instance,
some programming languages (notably Ada, C<tt>++</tt> and Objective-C) permit
a single function name to be defined several times, for application in
different contexts.  This is called <em>overloading</em>.  Another example
involving Ada is generics.  A <em>generic package</em> is similar to C<tt>++</tt>
templates and is typically instantiated several times, resulting in
the same function name being defined in different contexts.
</p>
<p>In some cases and depending on the language, it is possible to adjust
the expression to remove the ambiguity.  For instance in C<tt>++</tt>, you
can specify the signature of the function you want to break on, as in
<kbd>break <var>function</var>(<var>types</var>)</kbd>.  In Ada, using the fully
qualified name of your function often makes the expression unambiguous
as well.
</p>
<p>When an ambiguity that needs to be resolved is detected, the debugger
has the capability to display a menu of numbered choices for each
possibility, and then waits for the selection with the prompt &lsquo;<samp>&gt;</samp>&rsquo;.
The first option is always &lsquo;<samp>[0] cancel</samp>&rsquo;, and typing <kbd>0 <span class="key">RET</span></kbd>
aborts the current command.  If the command in which the expression was
used allows more than one choice to be selected, the next option in the
menu is &lsquo;<samp>[1] all</samp>&rsquo;, and typing <kbd>1 <span class="key">RET</span></kbd> selects all possible
choices.
</p>
<p>For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol <code>String::after</code>.
We choose three particular definitions of that function name:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
&gt; 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the &quot;delete&quot; command to delete unwanted
 breakpoints.
(gdb)
</pre></div>

<dl compact="compact">
<dd><a name="index-set-multiple_002dsymbols"></a>
</dd>
<dt><code>set multiple-symbols <var>mode</var></code></dt>
<dd><a name="index-multiple_002dsymbols-menu"></a>

<p>This option allows you to adjust the debugger behavior when an expression
is ambiguous.
</p>
<p>By default, <var>mode</var> is set to <code>all</code>.  If the command with which
the expression is used allows more than one choice, then <small>GDB</small>
automatically selects all possible choices.  For instance, inserting
a breakpoint on a function using an ambiguous name results in a breakpoint
inserted on each possible match.  However, if a unique choice must be made,
then <small>GDB</small> uses the menu to help you disambiguate the expression.
For instance, printing the address of an overloaded function will result
in the use of the menu.
</p>
<p>When <var>mode</var> is set to <code>ask</code>, the debugger always uses the menu
when an ambiguity is detected.
</p>
<p>Finally, when <var>mode</var> is set to <code>cancel</code>, the debugger reports
an error due to the ambiguity and the command is aborted.
</p>
<a name="index-show-multiple_002dsymbols"></a>
</dd>
<dt><code>show multiple-symbols</code></dt>
<dd><p>Show the current value of the <code>multiple-symbols</code> setting.
</p></dd>
</dl>

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