typeid of enum changing when dynamically linked in matlab mex file

Question

I have a C++ library with a particular function that returns a boost::any, whose value type is a particular enum defined in a different included library. This normally works fine.

But when I link to my library dynamically from a Matlab mex file, typeid(the_enum_t) seems to be different for the things made in my library and those made in the caller, in that it doesn't compare ==. Since I'm actually using flann, whose version of boost::any performs a check based on type_info::==, this makes everything break. Static linking works fine, but that's kind of a pain here, and I'd really rather have it work either way.

I thought type_info::== was supposed to work consistently across library boundaries. Does this have to do with how Matlab dynamically loads libraries from mex?

---

Here's some code to reproduce this (also available in easy-to-download form, with a makefile, in this gist).

First, a stand-in for the library that defines the enum (flann):

namespace library {
    enum the_enum_t { el_one, el_two, el_three };
}

Now a proxy for my library, stubby.hpp:

#include <boost/any.hpp>
#include "the_enum.hpp"
boost::any the_function();

And its implementation stubby.cpp:

#include "stubby.hpp"
boost::any the_function() {
    return boost::any(library::el_two);
}

Finally, test code test.cpp, which is compiled with -DNO_MEX for a standalone and not for a mex file:

#include "stubby.hpp"
#include <boost/any.hpp>

#ifdef NO_MEX
#include <cstdio>
using std::printf;
int main() {
#else
#include "mex.h"
void mexFunction(int nlhs, mxArray **plhs, int nrhs, const mxArray **prsh) {
#endif
    boost::any val_any = the_function();
    printf("%s (equal: %d)\n",
            val_any.type().name(),
            val_any.type() == typeid(library::the_enum_t));
}

I get the expected output

N5flann17flann_algorithm_tE (equal: 1)

from each of

$ g++ -o test{,.cpp} -DNO_MEX libstubby.a && ./test      
$ g++ -o test{_s,.cpp} -DNO_MEX libstubby.so && ./test_s
$ ln -sf test{,_s}.cpp && mex test_s.cpp libstubby.a && matlab -r test

But dynamically linking a mex file doesn't work:

$ mex test.cpp libstubby.so && matlab -r test
N5flann17flann_algorithm_tE (equal: 0)

I see this same behavior on

• Matlab R2011b and R2011a, OSX 10.7, Apple gcc 4.2.1
• Matlab R2011b, CentOS 5.7, gcc 4.1.2
• Matlab R2010a, Ubuntu 11.04, gcc 4.4.5

The weird thing is that I could've sworn it worked a few months ago, but maybe I just did a bad job of testing.

Obviously I can get around this by static linking. But why is this happening? Does it have something to do with the way Matlab loads mex files and their libraries?

Answer 1

Boost has a workaround for this issue; see https://svn.boost.org/trac/boost/ticket/754

It's possible that boost is failing to enable the workaround. Try passing compiler flag -DBOOST_AUX_ANY_TYPE_ID_NAME per the patch on that ticket.