FLINT 3: rational functions with fmpz_mpoly_q and C++ linking

[grea...@gmail.com]

I've been using Nemo for multivariate rational functions, but I'm exploring moving back to C/C++. I just tried the latest github version of FLINT, which has the needed header fmpz_mpoly_q.h, previously part of Calcium. I was able write simple C programs with this. However, I'm not able to compile it under C++, even if I write

extern "C" {
#include "fmpz_mpoly_q.h"
}

This immediately triggers the g++ compiler to complain "error: template with C linkage". More lines of the error message is shown below. Is there a quick solution, or should I wait for the official FLINT 3 release?

In file included from /usr/include/c++/11/bits/stringfwd.h:40,
                 from /usr/include/c++/11/iosfwd:39,
                 from /usr/include/x86_64-linux-gnu/gmp.h:34,
                 from /home/username/flint2/local/include/flint/flint.h:28,
                 from /home/username/flint2/local/include/flint/fmpz_types.h:15,
                 from /home/username/flint2/local/include/flint/fmpz.h:21,
                 from /home/username/flint2/local/include/flint/fmpq.h:22,
                 from /home/username/flint2/local/include/flint/fmpz_mpoly_q.h:25,
                 from try.cpp:3:
/usr/include/c++/11/bits/memoryfwd.h:63:3: error: template with C linkage

[grea...@gmail.com]

More information: the same error appears if I don't use "extern C".

[Fredrik Johansson]

Could you share a minimal failing .cpp program?

The following works for me when compiling with g++ 11.3.0.

#include "fmpz_mpoly.h"
#include "fmpz_mpoly_q.h"

int main()
{
    fmpz_mpoly_q_t a;
    fmpz_mpoly_ctx_t ctx;

    fmpz_mpoly_ctx_init(ctx, 3, ORD_LEX);
    fmpz_mpoly_q_init(a, ctx);

    fmpz_mpoly_q_print_pretty(a, NULL, ctx);

    return 0;
}

Fredrik

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[grea...@gmail.com]

What's funny is that if I comment out the first line of your code,

#include "fmpz_mpoly.h",

then it still compiles and runs without problems with gcc, but breaks under g++. The above line is exactly what's missing from my code that caused the error. (I'm using Ubuntu 22.04 and the gmp version from the distro repo.)