Undefined reference error from GCC using a template with a std::vector and an Eigen Matrix?

Question

I am experiencing an undefined reference error, when compiling the following code using GCC 4.7.2 20130108 under x86_64-suse-linux via the command:

g++ main.cpp func.cpp -I/path/to/eigenlibrary/eigen_3.2.1

The error message reads:

     main.cpp:(.text+0x1d): undefined reference to `void f<2>(std::vector<Eigen::Matrix<double, 2, 2, 
((Eigen::._84)0)|((((2)==(1))&&((2)!=(1)))?
    ((Eigen::._84)1) : ((((2)==(1))&&((2)!=(1)))?((Eigen::._84)0) : ((Eigen::._84)0))), 2, 2>,
     std::allocator<Eigen::Matrix<double, 2, 2, ((Eigen::._84)0)|((((2)==(1))&&((2)!=(1)))?
    ((Eigen::._84)1) : ((((2)==(1))&&((2)!=(1)))?((Eigen::._84)0) : ((Eigen::._84)0))), 2, 2> > >&)'

Please note that this has nothing to do with the fact that the template implementation is separated from the header file, because there is no (generic) implementation of the template function, but only a template specialization. The implementation of the template specialization can not be put into the header file, because this yields multiple definition errors.

Another strange thing here is that if I change the order of the first two header inclusions in main.cpp (Eigen/Dense and vector) the error does not occur. I have no understanding for this and any help that goes beyond 'simply change the order of the header inclusions then' would be highly appreciated.

main.cpp:

#include <vector>
#include <Eigen/Dense>

//error does not occur once I change order of header inclusion like so:
//#include <Eigen/Dense>
//#include <vector>

#include "func.h"

int main() {
    std::vector<Eigen::Matrix<double, 2, 2> > m;  
    f<2>(m);
}

func.h

#ifndef FUNC_H
#define FUNC_H

#include <Eigen/Dense>
#include <vector>

template <int N>
void f(std::vector<Eigen::Matrix<double, N, N> >& m);

template <> void f<2>(std::vector<Eigen::Matrix<double, 2, 2> >& m);

#endif

func.cpp

#include "func.h"
#include <vector>

template <>
void f<2>(std::vector<Eigen::Matrix<double, 2, 2> >& m) {}

Would you mind deleting your [old question](http://stackoverflow.com/questions/24723693/why-does-gcc-give-an-undefined-reference-error-when-using-a-template-with-a-std), before asking this again? — πάντα ῥεῖ, Jul 14 '14 at 07:10
@πάνταῥεῖ to be fair, that question is asking about something entirely different. — rubenvb, Jul 14 '14 at 07:14
@rubenvb That's why I didn't close voted this to be a duplicate. The questions are (were) very similar though. — πάντα ῥεῖ, Jul 14 '14 at 07:16
@πάνταῥεῖ but then why ask to delete that question? It was already marked as a duplicate. That's no reason to delete questions... — rubenvb, Jul 14 '14 at 07:20
@rubenvb You might get it, when reading the comment history there. IMHO the OP meant this new question as a replacement. — πάντα ῥεῖ, Jul 14 '14 at 07:22
@πάνταῥεῖ Ah, should have read those indeed. Lazy me :-) — rubenvb, Jul 14 '14 at 07:23
Yes it is meant as a replacement, I think in the old question my mistake was that I did not include a declaration of the template specialization in the header. — user1304680, Jul 14 '14 at 07:25

rubenvb · Answer 1 · 2014-07-15T07:22:21.113

1

In func.h, your template specialization declaration should read:

template <> void f<2>(std::vector<Eigen::Matrix<double, 2, 2> >& m);

So, with the N=2 filled in throughout, as you did with the definition in func.cpp.

Note that you should be able to define your template specialization in func.h if you add inline to the definition.

I can reproduce the failure using GCC 4.6.4, 4.7.4, 4.8.2, 4.9.0, but NOT Clang 3.4.2, all on Arch Linux:

$ echo $'func.h\n---'; cat func.h; echo $'---\nfunc.c++\n---'; cat func.c++; echo $'---\nmain.c++\n---'; cat main.c++; g++-4.6 -I/usr/include/eigen3 main.c++ func.c++; ./a.out
func.h
---
#ifndef FUNC_H
#define FUNC_H

#include <Eigen/Dense>
#include <vector>

template <int N>
void f(std::vector<Eigen::Matrix<double, N, N> >& m);

template <> void f<2>(std::vector<Eigen::Matrix<double, 2, 2> >& m);

#endif
---
func.c++
---
#include "func.h"
#include <vector>

template <>
void f<2>(std::vector<Eigen::Matrix<double, 2, 2> >& m) {}
---
main.c++
---
#include <vector>
#include <Eigen/Dense>

//error does not occur once I change order of header inclusion like so:
//#include <Eigen/Dense>
//#include <vector>

#include "func.h"

int main() {
    std::vector<Eigen::Matrix<double, 2, 2> > m;  
    f<2>(m);
}

I strongly suggest contacting the Eigen developers about this.

edited Jul 15 '14 at 07:22

answered Jul 14 '14 at 07:02

rubenvb

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of course, yes, I typed it incorrectly, the question remains the same – user1304680 Jul 14 '14 at 07:04
@user that makes it compile/link for me without error. – rubenvb Jul 14 '14 at 07:05
are you using the same compiler ? it compiles fine on my machine using intel compiler, but not gcc – user1304680 Jul 14 '14 at 07:06
@user then it might be a compiler bug. Can you try a newer version of GCC, or Clang? – rubenvb Jul 14 '14 at 07:07
not right now, but I will try it with other compilers, which one are you using ? – user1304680 Jul 14 '14 at 07:08
1

GCC 4.9. I'll see if I can get a hold of GCC 4.7.3 and others too. – rubenvb Jul 14 '14 at 07:09
inlining makes it possible to put the definition into header, but I would still like to find out what is going on. – user1304680 Jul 14 '14 at 07:10
I get the same on gcc 4.8.1, 4.6.2 and 4.5.1 – user1304680 Jul 14 '14 at 07:27
It was Eigen3, not Eigen2 that you used. My bad. Edited answer. – rubenvb Jul 15 '14 at 07:23
Very strange. The error goes away with -std=c++11. I guess this rather a GCC issue, but let's see if we can workaround the issue. – ggael Jul 15 '14 at 09:39
This is getting even more strange: in `main.c++` you can replace `#include ` by `enum { fifi = sizeof(int) };` and still reproduce the issue. (there is a similar declaration in `` header file) – ggael Jul 15 '14 at 11:05

score 1 · Answer 2 · answered Apr 27 '16 at 13:26

I know that this is an old question, but we've run into the same problem in a different context, and I thought I would share our solution.

A solution to the unresolved link error, at least for GCC 4.8.2-19ubuntu1, is to replace the following in func.h

template <int N>
void f(std::vector<Eigen::Matrix<double, N, N> >& m);

with

template <int N>
void f(std::vector<Eigen::Matrix<double, N, N, 0> >& m);

Note that the forth template argument is explicitly given as 0, which is the result of the expression shown in the template function declaration in the linker error,

((Eigen::._84)0)|((((2)==(1))&&((2)!=(1)))?
    ((Eigen::._84)1) : ((((2)==(1))&&((2)!=(1)))?((Eigen::._84)0)

The expression comes from the default value for the forth template parameter, which is given in the Eigen ForwardDeclarations.h file as

AutoAlign |
( (_Rows==1 && _Cols!=1) ? RowMajor
: (_Cols==1 && _Rows!=1) ? ColMajor
: EIGEN_DEFAULT_MATRIX_STORAGE_ORDER_OPTION )

There is another way to fix the problem in the Eigen code, which I'll mention in a minute.

The cause of the problem seems to be that the GCC compiler delays the evaluation of this expression and that the expression involves anonymous enum types whose type names are explicitly given in the expression ((Eigen::._84) in this case). Further, the GCC compiler appears to generate anonymous enum type names using a counter, and therefore the type name depends on how many anonymous enum types have appeared before, which can vary for different compilation units. This is why adding #include <vector> or an anonymous enum definition before including Eigen/Dense triggers the problem. The type name in the compiled f<2> is likely (Eigen::._83), and thus the mismatch.

I'm afraid that I'm not an expert on the internals of compilers or the depths of the C++ standard, and can't say if this a bug in the GCC compiler or simply a matter of a difference of interpretation.

Another solution that involves modifying the Eigen code and seems to work is to provide a name for the enum in Constants.h that defines AutoAlign, RowMajor, and ColMajor, as the issue is in using anonymous enum types. For example:

/** \ingroup enums
  * Enum containing possible values for the \p _Options template parameter of
  * Matrix, Array and BandMatrix. */
enum MatrixOptionsType {
  ColMajor = 0,
  RowMajor = 0x1,
  AutoAlign = 0,
  DontAlign = 0x2
};

I'm not sure if that would be acceptable to the Eigen developers, though.

Undefined reference error from GCC using a template with a std::vector and an Eigen Matrix?

2 Answers2

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