How to use (link) debug version of libc++ in macOS? - c++

Question

I want to enable debug version of libc++ in macOS, so I define _LIBCPP_DEBUG=1 in cxx_build_flags [Debug Version of libC++],
but unable to link debug function of libc++. I guess there is only release version of libc++ in my system, so how can I get the debug version of libc++ in macOS
Undefined symbols for architecture x86_64:
"std::__1::__libcpp_db::__decrementable(void const*) const", referenced from:
void std::__1::__nth_element<std::__1::__debug_less<std::__1::__less<float, float> >&, std::__1::__wrap_iter<float*> >(std::__1::__wrap_iter<float*>, std::__1::__wrap_iter<float*>, std::__1::__wrap_iter<float*>, std::__1::__debug_less<std::__1::__less<float, float> >&)

Answer 1

I think I got this to work!
So the steps I took,
Check your toolchains __config file for _LIBCPP_VERSION's value.
Browse the source repo for the commit where that version was set
From that commit, find the debug.cpp file
Include/compile/link the debug.cpp with your project (and remember to set _LIBCPP_DEBUG=1).
For me the intermediate results for the steps were (with Catalina, XCode Version 11.3.1)
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__config was at version 8000
Looking at the history at https://github.com/llvm/llvm-project/commits/main?&path[]=libcxx&path[]=include&path[]=__config I found commit https://github.com/llvm/llvm-project/commit/25977548aa5ad7cfe45a17e4ba136abfa4613b96#diff-0a85f740cd20254bc3bb03975c89d93a
The debug.cpp for that commit is at https://github.com/llvm/llvm-project/blob/25977548aa5ad7cfe45a17e4ba136abfa4613b96/libcxx/src/debug.cpp

Answer 2

I try to reactivate this thread without an answer with more details.
I have the same issue. This is very easy to reproduce:
#include <string>
int main()
{
std::string name;
return 0;
}
and then compile with:
clang++ -D_LIBCPP_DEBUG=1 main.cpp
We obtain:
Undefined symbols for architecture x86_64:
"std::__1::__libcpp_db::__insert_c(void*)", referenced from:
void std::__1::__libcpp_db::__insert_c<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*) in main-7ff3c5.o
"std::__1::__libcpp_db::__erase_c(void*)", referenced from:
std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string() in main-7ff3c5.o
"std::__1::__c_node::~__c_node()", referenced from:
std::__1::_C_node<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >::~_C_node() in main-7ff3c5.o
"std::__1::__get_db()", referenced from:
std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::basic_string() in main-7ff3c5.o
std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string() in main-7ff3c5.o
"typeinfo for std::__1::__c_node", referenced from:
typeinfo for std::__1::_C_node<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > in main-7ff3c5.o
"vtable for std::__1::__c_node", referenced from:
std::__1::__c_node::__c_node(void*, std::__1::__c_node*) in main-7ff3c5.o
NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
And if I use g++-9 providen by brew (a real g++ not a disguised Apple Clang compiler), it compiles and runs perfectly.
Forgetting to use debug mode of libc++, is it the only solution?
NB: I saw the same question in https://forums.developer.apple.com/thread/99194
and still no answer.