Undefined symbols using g++-6 on OS X

[qianyizh]

OS X El Capitan, installed gcc6 and jsoncpp via homebrew:

brew install gcc --without-multilib
brew install jsoncpp

Then build the following code:

#include <json/json.h>
#include <fstream>

int main()
{
    std::ofstream file_out("test.json");
    Json::Value value;
    value["test"] = 1;
    Json::StyledStreamWriter writer;
    writer.write(file_out, value);
    file_out.close();
    return 1;
}

Using command:

g++-6 -g -Wall -I/usr/local/Cellar/jsoncpp/1.8.0/include/ -L/usr/local/Cellar/jsoncpp/1.8.0/lib/ -ljsoncpp ../testjsoncpp.cpp -o test

Error:

Undefined symbols for architecture x86_64:
  "Json::StyledStreamWriter::write(std::basic_ostream<char, std::char_traits<char> >&, Json::Value const&)", referenced from:
      _main in cczWVuKc.o
  "Json::StyledStreamWriter::StyledStreamWriter(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)", referenced from:
      _main in cczWVuKc.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

Note:

• Using Apple LLVM 8.0 works fine, e.g.,

g++ -g -Wall -I/usr/local/Cellar/jsoncpp/1.8.0/include/ -L/usr/local/Cellar/jsoncpp/1.8.0/lib/ -ljsoncpp ../testjsoncpp.cpp -o test

• g++-6 on Ubuntu 16.04, with jconcpp 1.7.2-1 (acquired via apt-get) works fine.
• If Json::StyledStreamWriter related stuff were removed from the code, it works fine.
• Installed gcc-5 on OS X. Same issue.

[cdunn2001]

StyledStreamWriter is deprecated, but not yet removed. This should work. Something else is going on. Can you try a few more versions?

Best guess: some preprocessor macro is different when you compile from when brew compiled the library.

[qianyizh]

Some updates:

A. I replaced Json::StyledStreamWriter with <<.

#include <json/json.h>
#include <fstream>

int main()
{
    std::ofstream file_out("test.json");
    Json::Value value;
    value["test"] = 1;
    file_out << value;
    file_out.close();
    return 1;
}

Got similar error:

Undefined symbols for architecture x86_64:
  "Json::operator<<(std::basic_ostream<char, std::char_traits<char> >&, Json::Value const&)", referenced from:
      _main in ccaQgzGV.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

B. If I build jsoncpp from source with g++-6, then link to the library. It works fine.
But if I build jsoncpp from source with llvm, then link to the library, it breaks.

My current guess is that by default, llvm and g++-6 use different compiler settings. And the iostream related stuff is very suspicious.

But I could not find out the exact thing that causes the conflicts.

I did this (install libraries using brew/compile with llvm, then build code with g++-6 and link to the library) for other libraries include glew and glfw. They all work fine. Jsoncpp is the first library for which I encountered this problem.

[qianyizh]

Update:

I think it is the conflict between libc++ and libstdc++ when using std::ofstream & as a function parameter.

I tried to build a minimal library with only one function:

#include <iostream>

void test_stream(std::ostream& out) {
    out << "test" << std::endl;
}

And an app that uses this library:

#include <iostream>

void test_stream(std::ostream& out);

int main()
{
    test_stream(std::cout);
    return 1;
}

1. Compile both library and app with llvm, no problem.
2. Compile both library and app with g++-6, no problem.
3. Compile library with llvm, compile app with g++-6, link error.
4. Vise versa, link error.
5. Give "-stdlib=libstdc++" to llvm, then it suddenly becomes compatible with g++-6.

However, to solve my problem. I need to either let g++-6 link to libc++, or let brew build with "-stdlib=libstdc++". Both I don't know how to do it.

[cdunn2001]

That's great detective work. But what can we do about it?

[qianyizh]

There are some discussion on stackoverflow too:
http://stackoverflow.com/questions/12542971/using-libstdc-compiled-libraries-with-clang-stdlib-libc

In general, it is bad practice to mix libraries linked against libc++ and libstdc++, since they are implemented differently. As brew and OSX llvm by default links everything to libc++, the easier workaround is to force gcc to link with libc++.

This is done with:
http://libcxx.llvm.org/docs/UsingLibcxx.html#using-libc-with-gcc
It's a lot of compiler flags. But it works for me. So I am okay with this workaround for now.

I think we can close this ticket. The information here should be sufficient for someone to figure out a workaround if he encounters similar issues in the future.

@cdunn2001 thanks for the help!