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<title>Debugging with GDB: Ambiguous Expressions</title>
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<a name="Ambiguous-Expressions"></a>
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<p>
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Next: <a href="Variables.html#Variables" accesskey="n" rel="next">Variables</a>, Previous: <a href="Expressions.html#Expressions" accesskey="p" rel="previous">Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Ambiguous-Expressions-1"></a>
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<h3 class="section">10.2 Ambiguous Expressions</h3>
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<a name="index-ambiguous-expressions"></a>
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<p>Expressions can sometimes contain some ambiguous elements. For instance,
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some programming languages (notably Ada, C<tt>++</tt> and Objective-C) permit
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a single function name to be defined several times, for application in
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different contexts. This is called <em>overloading</em>. Another example
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involving Ada is generics. A <em>generic package</em> is similar to C<tt>++</tt>
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templates and is typically instantiated several times, resulting in
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the same function name being defined in different contexts.
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</p>
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<p>In some cases and depending on the language, it is possible to adjust
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the expression to remove the ambiguity. For instance in C<tt>++</tt>, you
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can specify the signature of the function you want to break on, as in
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<kbd>break <var>function</var>(<var>types</var>)</kbd>. In Ada, using the fully
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qualified name of your function often makes the expression unambiguous
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as well.
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</p>
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<p>When an ambiguity that needs to be resolved is detected, the debugger
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has the capability to display a menu of numbered choices for each
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possibility, and then waits for the selection with the prompt ‘<samp>></samp>’.
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The first option is always ‘<samp>[0] cancel</samp>’, and typing <kbd>0 <span class="key">RET</span></kbd>
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aborts the current command. If the command in which the expression was
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used allows more than one choice to be selected, the next option in the
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menu is ‘<samp>[1] all</samp>’, and typing <kbd>1 <span class="key">RET</span></kbd> selects all possible
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choices.
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</p>
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<p>For example, the following session excerpt shows an attempt to set a
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breakpoint at the overloaded symbol <code>String::after</code>.
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We choose three particular definitions of that function name:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) b String::after
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[0] cancel
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[1] all
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[2] file:String.cc; line number:867
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[3] file:String.cc; line number:860
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[4] file:String.cc; line number:875
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[5] file:String.cc; line number:853
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[6] file:String.cc; line number:846
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[7] file:String.cc; line number:735
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> 2 4 6
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Breakpoint 1 at 0xb26c: file String.cc, line 867.
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Breakpoint 2 at 0xb344: file String.cc, line 875.
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Breakpoint 3 at 0xafcc: file String.cc, line 846.
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Multiple breakpoints were set.
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Use the "delete" command to delete unwanted
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breakpoints.
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(gdb)
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</pre></div>
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<dl compact="compact">
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<dd><a name="index-set-multiple_002dsymbols"></a>
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</dd>
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<dt><code>set multiple-symbols <var>mode</var></code></dt>
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<dd><a name="index-multiple_002dsymbols-menu"></a>
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<p>This option allows you to adjust the debugger behavior when an expression
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is ambiguous.
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</p>
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<p>By default, <var>mode</var> is set to <code>all</code>. If the command with which
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the expression is used allows more than one choice, then <small>GDB</small>
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automatically selects all possible choices. For instance, inserting
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a breakpoint on a function using an ambiguous name results in a breakpoint
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inserted on each possible match. However, if a unique choice must be made,
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then <small>GDB</small> uses the menu to help you disambiguate the expression.
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For instance, printing the address of an overloaded function will result
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in the use of the menu.
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</p>
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<p>When <var>mode</var> is set to <code>ask</code>, the debugger always uses the menu
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when an ambiguity is detected.
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</p>
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<p>Finally, when <var>mode</var> is set to <code>cancel</code>, the debugger reports
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an error due to the ambiguity and the command is aborted.
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</p>
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<a name="index-show-multiple_002dsymbols"></a>
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</dd>
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<dt><code>show multiple-symbols</code></dt>
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<dd><p>Show the current value of the <code>multiple-symbols</code> setting.
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</p></dd>
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</dl>
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Next: <a href="Variables.html#Variables" accesskey="n" rel="next">Variables</a>, Previous: <a href="Expressions.html#Expressions" accesskey="p" rel="previous">Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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