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 whateve

Answer 1

The similar problem also happens with me: error: Couldn't lookup symbols:

My solution is to explicitly use the questioned function somewhere in a source code.

#include 

template
struct Vector : std::vector
{
    Vector(size_t n)
    : std::vector{n}
    {}

    T& operator[](size_t n)
    { return std::vector::operator[](n); }
};

struct XXX
{
    int x;
};

void func()
{
    std::vector a{10};
    Vector b{10};

    auto x = b[0]; // gcc will produce an assembler code of operator[] for debug purpose
    1;  // as a break point
}

Set a breakpoint on the line of 1; and run it.

(lldb) p a[0]
error: Couldn't lookup symbols:
  __ZNSt3__16vectorI3XXXNS_9allocatorIS1_EEEixEm

(lldb) p b[0]
(XXX) $0 = (x = 0)

Bingo!! Does the function exist in a TEXT block?

(lldb) image lookup -r -n 'XXX.*operator'
1 match found in /Users/xxx/Library/Developer/Xcode/DerivedData/xxx:
        Address: sandbox[0x00000001000011f0] (sandbox.__TEXT.__text + 256)
        Summary: sandbox`Vector::operator[](unsigned long) at main.cpp:19

I am not sure, but I had learned this before. In a debugging stage, instead of production stage. If we set a breakpoint on a line in a function of a template, what would a debugger do? Setting breakpoints, actually replacing some existing assembler code with trap or jump, here and there everywhere the template is applied? Or just setting a single breakpoint in a function? It is written as a template. So it should be inlined in a production stage. In a debugging stage, however, the function is not inlined and written as a normal function. Please do not simply believe what I say here. Please confirm by yourself. Consult documentation of gcc, clang, and lldb.

#include of MacOS 10.13.6, Xcode Version 9.4.1 has a macro _LIBCPP_INLINE_VISIBILITY:

template 
inline _LIBCPP_INLINE_VISIBILITY
typename vector<_Tp, _Allocator>::reference
vector<_Tp, _Allocator>::operator[](size_type __n)
{
    _LIBCPP_ASSERT(__n < size(), "vector[] index out of bounds");
    return this->__begin_[__n];
}

The _LIBCPP_INLINE_VISIBILITY is defined in #include <__config> as:

#define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__visibility__("hidden"), __always_inline__))

Such keywords hidden and __always_inline__ seem to control behavior.

When I added inline _LIBCPP_INLINE_VISIBILITY to the sample solution code above:

    inline _LIBCPP_INLINE_VISIBILITY
    T& operator[](size_t n)
    { return std::vector::operator[](n); }

resulted in:

(lldb) p b[0]
error: Couldn't lookup symbols:
  __ZN6VectorI3XXXEixEm

I hope that help and somebody look into much more deeply.