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[lldb/docs] Fix/improve the gdb command map for dynamic types #138538

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May 6, 2025
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[lldb/docs] Fix/improve the gdb command map for dynamic types #138538

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16 changes: 12 additions & 4 deletions lldb/docs/use/map.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -800,16 +800,24 @@ Print the dynamic type of the result of an expression
(gdb) p someCPPObjectPtrOrReference
(Only works for C++ objects)

LLDB does this automatically if determining the dynamic type does not require
running the target (in C++, running the target is never needed). This default is
controlled by the `target.prefer-dynamic-value` setting. If that is disabled, it
can be re-enabled on a per-command basis:

.. code-block:: shell

(lldb) expr -d 1 -- [SomeClass returnAnObject]
(lldb) expr -d 1 -- someCPPObjectPtrOrReference
(lldb) settings set target.prefer-dynamic-value no-dynamic-values
(lldb) frame variable -d no-run-target someCPPObjectPtrOrReference
(lldb) expr -d no-run-target -- someCPPObjectPtr

or set dynamic type printing to be the default:
Note that printing of the dynamic type of references is not possible with the
`expr` command. The workaround is to take the address of the reference and
instruct lldb to print the children of the resulting pointer.
Comment on lines +814 to +816
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I spent some time trying to make this work, but I came to the conclusion that this would require a relatively big change in how the expression evaluator works. Because we're creating persistent copies of the expression result, it's not just a matter of getting something to recognise the dynamic type of the result -- we would actually need to determine the dynamic type of the object before we create the copy (so that we copy the entire object) -- and then I guess somehow reset the original (non-dynamic) ValueObject to point to the subobject of the result. And that might be too much work to put on the shoulders of the "use-dynamic" setting?

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(If you're wondering why we're making a copy of a reference -- but not of a pointer -- I suspect that's because in the expression evaluator we do not differentiate between an object and a reference to it. I suppose we could do it, and then somehow treat references as pointers, but I don't exactly know what would that entail.)


.. code-block:: shell

(lldb) settings set target.prefer-dynamic run-target
(lldb) expr -P1 -d no-run-target -- &someCPPObjectReference

Call a function so you can stop at a breakpoint in it
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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