Printing/Debugging libc++ STL with Xcode/LLDB

Question

I'm trying to use LLDB within Xcode 8 to debug very basic STL. I used to be able to print a vector like this:

p myvector[0]

to see whatever was in the first vector index. Now when I do that, I get this error:

error: Couldn't lookup symbols:
  __ZNSt3__16vectorI9my_classNS_9allocatorIS1_EEEixEm

Instead, I have to type this:

p myvector.__begin_[0]

in order to get any output.

I tried importing the libcxx.py and unordered_multi.py scripts from the LLDB svn repository but that doesn't seem to change anything.

Has anyone been able to get any useful output from LLDB with libc++?

Answer 1

[] is an operator method on std::vector, so to print the expression you want, lldb would have to be able to call the [] method. The problem here is that the STL on OS X is aggressive about inlining everything it can, and not wasting space producing out of line copies of the same functions. That's great for optimized code, but not so good for debugging because it leaves the debugger with no [] operator to call. That's the error message you are seeing.

If you just want to see the elements in this vector, you can use the lldb "STL data formatters" to do this work for you. They know how most STL types are laid out, and can print the elements of most container types. For instance:

(lldb) expr my_vec[0]
error: Couldn't lookup symbols:
  __ZNSt3__16vectorI3FooNS_9allocatorIS1_EEEixEm

but:

(lldb) expr my_vec
(std::__1::vector<Foo, std::__1::allocator<Foo> >) $0 = size=2 {
  [0] = (var1 = 10, var2 = 20)
  [1] = (var1 = 10, var2 = 20)
}

There is also another command "frame variable" which can inspect static objects, and hooks into the data formatters. It can't call functions and do other more complex expression parser tasks, but it does know how to use the STL data formatters to retrieve individual elements:

(lldb) frame var my_vec[1]
(Foo) my_vec[1] = (var1 = 10, var2 = 20)

You can even use frame var's -L option to locate the elements of the vector, and then you can cast the address to pass it to other functions:

(lldb) frame var -L my_vec[1]
0x0000000100100348: (Foo) my_vec[1] = {
0x0000000100100348:   var1 = 10
0x000000010010034c:   var2 = 20
}
(lldb) expr printf("%d\n", ((class Foo *) 0x0000000100100348)->var1)
10
(int) $3 = 3

Another way to work around this for debugging - if you are using C++11 - is by putting:

template class std::vector<MyClass>

in your code somewhere. That will instruct the compiler to emit out-of-line copies of all the template functions for this specialization. That isn't a great general solution, and you only want to do it for debug builds, but it does let you call these functions and use them in complex expressions.