Specializing toml::from for Eigen types

[maspri]

The following code will complain about toml::get being ambiguous.
The cause appears to be that Eigen::VectorXd believes it is constructible from a toml::value (static_assert is true). But in reality it is not.

#include <eigen3/Eigen/Dense>
#include <toml.hpp>

namespace toml {

    template<>
    struct from<Eigen::VectorXd>{
        static Eigen::VectorXd from_toml(const value& v){
            Eigen::VectorXd m;
            int rows = v.size();
            m.resize(rows);
            for (int i = 0; i < rows; ++i) {
                m << v.at(i).as_floating();
            }
            return m;
        }
    };

}

int main() {
    using namespace toml::literals;

    const auto root = u8R"(
        vec = [1,2,3]
    )"_toml;

    static_assert(std::is_constructible<Eigen::VectorXd , const toml::value&>::value, "");

    auto &v = toml::find(root,"vec");

    auto vec = toml::get<Eigen::VectorXd>(v);

    return 0;
}

I am not a complete stranger to template programming but, I can't figure out why Eigen::Vector thinks it is constructible
from a toml::value. I speculate it may be because Eigen tries to allow initializing from things that look like they are iterable?

Is there a workaround, or am i missing something else entirely?
Thank you for your time!

[ToruNiina]

Sorry for the late response and thank you for reporting the issue.

I reproduced the problem with Eigen 3.3.7 in my local environment. I will look into it. I will comment as soon as I figure out the reason.

[ToruNiina]

It seems that, as you suggested, it is because of the constructor of template<typename T> explicit Eigen::Matrix::Matrix(const T& x). It makes std::is_constructible<Eigen::Matrix, toml::value> true and it conflicts with the toml::from. In Eigen 3.3.7, it is defined at ${include}/eigen3/Eigen/src/Core/Matrix.h:291.

I think Eigen has it to make Matrix constructible from any of a scalar, a vector or a matrix. But since template<T> Matrix(const T& x) basically matches to almost anything, hence it makes Matrix constructible from toml::value.

I'm now trying to add precedence over the existing conversion methods, such as from<T> > from_toml > constructor, and it seems to be possible (at least with GCC). Once I have succeeded, I will merge it.

[ToruNiina]

I make toml::from and from_toml() always precede over a constructor. It makes your code compile, I think.

But, in your example, you defined vec elements as integers, so it will report a type error. It is required to be [1.0, 2.0, 3.0] or you need to check the types of elements in from_toml.