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| <a name="Ambiguous-Expressions"></a> |
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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="prev">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> |
| <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 ‘<samp>></samp>’. |
| The first option is always ‘<samp>[0] cancel</samp>’, 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 ‘<samp>[1] all</samp>’, 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 |
| > 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 "delete" 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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