CGAL users discussion list

[Marc Glisse <>]

On Tue, 23 Mar 2021, aseverino  wrote:

> I'm trying to use Eigen matrices with CGAL::Gmpq:
>
> typedef CGAL::Lazy_exact_nt<CGAL::Gmpq> LazyExact;
> typedef Eigen::Matrix<LazyExact, 3, 1> Vector3Gmpq;
>
> int main(int argc, char* argv[])
> {
>         Vector3Gmpq test;
>         test.normalize();
>         return 0;
> }
>
> I get the error:
> 'return': cannot convert from 'CGAL::Null_functor::result_type' to
> 'CGAL::Gmpq'

With a better compiler, the full message is

In file included from /usr/include/eigen3/Eigen/Core:374,
                 from /usr/include/eigen3/Eigen/Dense:1,
                 from q.cc:1:
/usr/include/eigen3/Eigen/src/Core/MathFunctions.h: In instantiation of ‘T 
Eigen::numext::sqrt(const T&) [with T = CGAL::Lazy_exact_nt<CGAL::Gmpq>]’:
/usr/include/eigen3/Eigen/src/Core/Dot.h:150:30:   required from ‘void 
Eigen::MatrixBase<Derived>::normalize() [with Derived = 
Eigen::Matrix<CGAL::Lazy_exact_nt<CGAL::Gmpq>, 3, 1>]’
q.cc:9:24:   required from here
/usr/include/eigen3/Eigen/src/Core/MathFunctions.h:1034:14: error: could not convert 
‘CGAL::sqrt<CGAL::Lazy_exact_nt<CGAL::Gmpq> >((* & x))’ from 
‘CGAL::Null_functor::result_type’ {aka ‘CGAL::Null_tag’} to ‘CGAL::Lazy_exact_nt<CGAL::Gmpq>’
 1034 |   return sqrt(x);
      |          ~~~~^~~
      |              |
      |              CGAL::Null_functor::result_type {aka CGAL::Null_tag}
In file included from /usr/include/CGAL/double.h:22,
                 from /usr/include/CGAL/number_type_basic.h:51,
                 from /usr/include/CGAL/basic.h:33,
                 from /usr/include/CGAL/Cartesian/Cartesian_base.h:20,
                 from /usr/include/CGAL/Simple_cartesian.h:20,
                 from 
/usr/include/CGAL/Exact_predicates_exact_constructions_kernel.h:20,
                 from q.cc:2:
/usr/include/CGAL/number_utils.h: In instantiation of ‘typename 
CGAL::Algebraic_structure_traits<Type_>::Sqrt::result_type CGAL::sqrt(const AS&) [with AS 
= CGAL::Lazy_exact_nt<CGAL::Gmpq>; typename 
CGAL::Algebraic_structure_traits<Type_>::Sqrt::result_type = CGAL::Null_tag]’:
/usr/include/eigen3/Eigen/src/Core/MathFunctions.h:1034:14:   required from ‘T 
Eigen::numext::sqrt(const T&) [with T = CGAL::Lazy_exact_nt<CGAL::Gmpq>]’
/usr/include/eigen3/Eigen/src/Core/Dot.h:150:30:   required from ‘void 
Eigen::MatrixBase<Derived>::normalize() [with Derived = 
Eigen::Matrix<CGAL::Lazy_exact_nt<CGAL::Gmpq>, 3, 1>]’
q.cc:9:24:   required from here
/usr/include/CGAL/number_utils.h:95:16: error: no match for call to 
‘(CGAL::Algebraic_structure_traits<CGAL::Lazy_exact_nt<CGAL::Gmpq> >::Sqrt {aka 
CGAL::Null_functor}) (const CGAL::Lazy_exact_nt<CGAL::Gmpq>&)’
   95 |     return sqrt( x );
      |            ~~~~^~~~~

And the reason is rather obvious: normalize needs to compute a square 
root, but Gmpq is a rational type, which cannot support square roots. With 
Gmpq, you are restricted to those operations in Eigen that do not use 
square roots. For the others, you need a different type (CORE::Expr?).

--
Marc Glisse