lldb: conditional breakpoint on a most derived type - c++

Question

typical debugging pattern:
class Button : public MyBaseViewClass
{
...
};
....
void MyBaseViewClass::Resized()
{
//<---- here I want to stop in case MyBaseViewClass is really a Button, but not a ScrollBar, Checkbox or something else. I.e. I want a breakpoint condition on a dynamic (most derived) type
}
trivial approaches like a breakpoint on strstr(typeid(*this).name(), "Button") don't work because on typeid lldb console tells:
(lldb) p typeid(*this)
error: you need to include <typeinfo> before using the 'typeid' operator
error: 1 errors parsing expression
surely #include in console before making the call doesn't help

Answer 1

You can do this in Python pretty easily. Set the breakpoint - say it is breakpoint 1 - then do:
(lldb) break command add -s python 1
Enter your Python command(s). Type 'DONE' to end.
def function (frame, bp_loc, internal_dict):
"""frame: the lldb.SBFrame for the location at which you stopped
bp_loc: an lldb.SBBreakpointLocation for the breakpoint location information
internal_dict: an LLDB support object not to be used"""
this_value = frame.FindVariable("this", lldb.eDynamicDontRunTarget)
this_type = this_value.GetType().GetPointeeType().GetName()
if this_type == "YourClassNameHere":
return True
return False
DONE
The only tricky bit here is that when calling FindVariable I passed lldb.eDynamicDontRunTarget which told lldb to fetch the "dynamic" type of the variable, as opposed to the static type. As an aside, I could have also used lldb.eDynamicRunTarget but I happen to know lldb doesn't have to run the target go get C++ dynamic types.
This way of solving the problem is nice in that you don't have to have used RTTI for it to work (though then we'll only be able to get the type of classes that have some virtual method - since we use the vtable to do this magic.) It will also be faster than a method that requires running code in the debugee as your expression would have to do.
BTW, if you like this trick, you can also put the breakpoint code into a python function in some python file (just copy the def above), then use:
(lldb) command script import my_functions.py
(lldb) breakpoint command add -F my_functions.function
so you don't have to keep retyping it.