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| <h3 class="section">10.2 Ambiguous Expressions</h3> |
| |
| <p><a name="index-ambiguous-expressions-632"></a> |
| 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 <dfn>overloading</dfn>. Another example |
| involving Ada is generics. A <dfn>generic package</dfn> 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>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 </kbd><var>function</var><kbd>(</kbd><var>types</var><kbd>)</kbd>. In Ada, using the fully |
| qualified name of your function often makes the expression unambiguous |
| as well. |
| |
| <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><span class="samp">></span></samp>’. |
| The first option is always ‘<samp><span class="samp">[0] cancel</span></samp>’, and typing <kbd>0 <RET></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><span class="samp">[1] all</span></samp>’, and typing <kbd>1 <RET></kbd> selects all possible |
| choices. |
| |
| <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: |
| |
| <!-- FIXME! This is likely to change to show arg type lists, at least --> |
| <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> |
| |
| <a name="index-set-multiple_002dsymbols-633"></a> |
| <dl><dt><code>set multiple-symbols </code><var>mode</var><dd><a name="index-multiple_002dsymbols-menu-634"></a> |
| This option allows you to adjust the debugger behavior when an expression |
| is ambiguous. |
| |
| <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 <span class="sc">gdb</span> |
| 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 <span class="sc">gdb</span> 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>When <var>mode</var> is set to <code>ask</code>, the debugger always uses the menu |
| when an ambiguity is detected. |
| |
| <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-635"></a><br><dt><code>show multiple-symbols</code><dd>Show the current value of the <code>multiple-symbols</code> setting. |
| </dl> |
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