125 lines
5.9 KiB
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125 lines
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<title>Ambiguous Expressions - Debugging with GDB</title>
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<div class="node">
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<a name="Ambiguous-Expressions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Variables.html#Variables">Variables</a>,
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Previous: <a rel="previous" accesskey="p" href="Expressions.html#Expressions">Expressions</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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<hr>
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</div>
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<h3 class="section">10.2 Ambiguous Expressions</h3>
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<p><a name="index-ambiguous-expressions-632"></a>
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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 <dfn>overloading</dfn>. Another example
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involving Ada is generics. A <dfn>generic package</dfn> 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>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 </kbd><var>function</var><kbd>(</kbd><var>types</var><kbd>)</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>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><span class="samp">></span></samp>’.
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The first option is always ‘<samp><span class="samp">[0] cancel</span></samp>’, and typing <kbd>0 <RET></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><span class="samp">[1] all</span></samp>’, and typing <kbd>1 <RET></kbd> selects all possible
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choices.
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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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<!-- FIXME! This is likely to change to show arg type lists, at least -->
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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>
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<a name="index-set-multiple_002dsymbols-633"></a>
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<dl><dt><code>set multiple-symbols </code><var>mode</var><dd><a name="index-multiple_002dsymbols-menu-634"></a>
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This option allows you to adjust the debugger behavior when an expression
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is ambiguous.
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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 <span class="sc">gdb</span>
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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 <span class="sc">gdb</span> 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>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>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><a name="index-show-multiple_002dsymbols-635"></a><br><dt><code>show multiple-symbols</code><dd>Show the current value of the <code>multiple-symbols</code> setting.
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</dl>
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</body></html>
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