A generic function to store Eigen matrix in a binary file

I know that the following code can be used to write the data of an Eigen Matrix with datatype as double to a binary file.

template<class  T> 
void WriteEigenMatrix(const Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic>& m, const char* fileName)
{
    std::ofstream outFile(fileName, std::ios_base::out | std::ios_base::binary);
    outFile.write(m.data(), m.rows()*m.cols());
    outFile.close();`
}

How can I generalize this function to accept other Eigen data types like Eigen::Vector and Eigen::Array as inputs?
If I use Eigen::EigenBase, I know that the function will accept all such data types. However, the problem is that I can no longer use m.data(), as it is not defined in the base class.
Any suggestion to solve this issue?

Solution

If you want to allow only "Dense" objects, you can to it as follows:

template<class Derived>
void WriteEigen(const Eigen::DenseBase<Derived>& dense, const char* fileName) {
    std::ofstream outFile(fileName, std::ios_base::out | std::ios_base::binary);
    outFile.write(reinterpret_cast<const char*>(dense.derived().data()), dense.rows() * dense.cols());
    outFile.close();
}

All dense Matrices (including vector & array) inherit from DenseBase, calling derived on DenseBase will give you back the concrete matrix type.

Or use any Base you like.

After comments from @chtz and @Homer512 this version uses PlainObjectBase and reshaped in combination with STL algorithm to write to the file, avoiding reinterpret_cast to char* and the possible failure in case innerStride() != 1.

template<class Derived>
void WriteEigen(const Eigen::PlainObjectBase<Derived>& dense, std::string_view fileName) {
    std::ofstream outFile(fileName, std::ios_base::out | std::ios_base::binary);
    const auto& reshaped = dense.reshaped();
    std::copy(reshaped.begin(), reshaped.end(), std::ostream_iterator<typename Derived::Scalar>(outFile));
}

reshaped() allows to iterate over all entries in an STL manner see example, which will take care of all possible strides. C++20 ranges allows for the nicer

std::ranges::copy(reshaped, std::ostream_iterator<typename Derived::Scalar>(outFile));

Using std::ostream_iterator allows to use std::(ranges::)copy instead of ostream::write() which operates on bytes. To get the underlying data type of the Eigen object, Dense::Scalar can be used.