When/why are libstdc++ and libc++ symbols incompatible? - c++

Question

Setup:
test.cpp
#include <set>
#include <string>
void common_config_file_iterator(const std::set<std::string>& allowed_options) {}
include.cpp
#include <set>
#include <string>
void common_config_file_iterator(const std::set<std::string>&) noexcept;
int main() {
std::set<std::string> set;
common_config_file_iterator(set);
return 0;
}
test.sh
clang++-7 test.cpp -c -O3 -fno-rtti -fno-exceptions -o test.o
g++-8 test.o include.cpp -O3 -fno-rtti -fno-exceptions -o test
Output:
Undefined symbols for architecture x86_64:
"common_config_file_iterator(std::set<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&)", referenced from:
_main in ccWoGgrX.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
So I did nm -g test.o:
0000000000000000 T __Z27common_config_file_iteratorRKNSt3__13setINS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEENS_4lessIS6_EENS4_IS6_EEEE
According to demangler.com, it means:
common_config_file_iterator(std::__1::set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::less<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&)
Libc++ says:
[Features and Goals:] ABI compatibility with gcc's libstdc++ for some low-level features such as exception objects, rtti and memory allocation.
So, is the problem std::allocator<char>?
Note that I use the macOS assembler.
Curiosity caused by this issue and boost/program-options.

Answer 1

So, is the problem std::allocator<char>?
What? No. It's everything in your example.
The doc you quoted clearly says the goal is compatibility for "low-level features such as exception objects, rtti and memory allocation".
std::set and std::string are not "low-level features such as exception objects, rtti and memory allocation". They are very definitely not compatible between libc++ and libstdc++, which are completely different libraries with completely different implementations.
The compatible pieces are things like std::type_info and std::exception (and the derived exception types in <stdexcept>) because those are part of the basic language runtime. Anything above that, such as containers, strings, algorithms, I/O, locales etc. is not compatible.