Linking against the correct libc++ with Xcode 9.0 - xcode

Question

I have a library that is written in a mixture of Fortran and C++. It also uses MPI at the Fortran layer. Let's say a user wants to link a new c++ program against my library. From what I've found on the internet, the best way to do this is to have them compile their code with a C++ compiler, and link against my library using the mpif90 fortran wrapper. This is to make sure you link against the correct version of MPI. But in order to make this all work, you also have to link against libc++ explicitly because otherwise your c++ program won't work. So, for example:
mpic++ their_code.cc -c
mpif90 their_code.o -o program.exe -lmylib -lstdc++
This approach has worked fine for me across many platforms and compilers (gnu, intel, etc). It also worked for clang, until the release of Xcode 9.0, which broke this process. The compiler errors appear to be related to incompatibility in the c++ standard library, because it only breaks when I call functions that pass std::string. In the Xcode release notes it says:
Projects created using this Xcode release use the new libc++ implementation of the standard C++ library. The libc++ library is available only on iOS 5.0 and later and OS X 10.7 and later. 12221787
To enable deployment on earlier releases of iOS and OS X in your project, set the C++ Standard Library build setting to libstdc++ (Gnu C++ standard library).
This leads me to believe I need to modify how I link against libc++. You can reproduce this issue with the following code (assuming you have upgraded Xcode to version 9).
main.cc:
#include<string>
using namespace std;
void fun_str(string s);
void fun_num(int n);
int main() {
string my_str = "test";
int my_num = 1;
fun_str(my_str);
fun_num(my_num);
return 0;
}
fun.cc:
#include <string>
using namespace std;
void fun_str(string s) {}
void fun_num(int n) {}
Makefile:
test: main.o fun.o
mpif90 main.o fun.o -o program.exe -lstdc++
main.o: main.cc
mpic++ main.cc -c
fun.o: fun.cc
mpic++ fun.cc -c
Notice how if you remove all the references to strings, it does compile correctly. But if not, you get the following error:
mpic++ main.cc -c
mpic++ fun.cc -c
mpif90 main.o fun.o -o program.exe -lstdc++
Undefined symbols for architecture x86_64:
"std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__init(char const*, unsigned long)", referenced from:
_main in main.o
"std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::basic_string(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
_main in main.o
"std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
make: *** [test] Error 1
Additionally, if you switch to GNU by setting the environment variable OMPI_CXX to g++-7, it also compiles without a hitch. So this is definitely just a Clang issue, that only showed up in the new version. Any help would be greatly appreciated :)!

Answer 1

I fixed this on my own, but using -lc++ instead of -lstdc++. It seems that libstdc++ is depreciated with clang.