H5Easy read into existing object

[jjcasmar]

I have an std::vector<Eigen::Vector3f> already allocated and I want to populate the values of that vector with data coming from a dataset. Right now, to populate, I am doing something like this

std::vector<Eigen::Vector3f> v;
v.resize(100);
Eigen::Map<Eigen::Matrix<float, Eigen::Dynamic, 3, Eigen::RowMajor>>(v.data(), v.size(), 3) = H5Easy::load<Eigen::Matrix<float, Eigen::Dynamic, 3, Eigen::RowMajor>(file, path);

This however has a caveat. HighFive is allocating space for the Eigen::Matrix where it has to load the data, and the a copy is made from the Eigen::Matrix to the Eigen::Map.

Would it be possible to add API to H5Easy to be able to pass an existing object where we want the data to be loaded?

std::vector<Eigen::Vector3f> v;
Eigen::Map<Eigen::Matrix<float, Eigen::Dynamic, 3, Eigen::RowMajor>> iface;
H5Easy::load(file, path, iface);

[tdegeus]

You might want to consider HighFive's 'normal' API on this:

file.getDataSet(path).read(iface.data());

If we would want to consider amending H5Easy functionality I do think a different name should be used. Otherwise overloading becomes a mess, and you'd need H5Easy::load<>(...). However, since you can do a one-liner I would say that we should debate if we would like to have another function.

[jjcasmar]

The "normal" API wont work in this case, since it doesn't implement loading something as an Eigen::Map or Eigen::RowMajor, afaik, as discussed in #694

[tdegeus]

I may have looked too quickly ;). Can you get an iterator from Eigen::Map that iterates over the underlying memory as if it were contiguous?

[jjcasmar]

Not sure if that is possible, maybe yes. However, that implies kind of redoing all the logic HighFive already does when loading a dataset into an Eigen::Matrix using H5Easy::load<Eigen::Matrix<...>>, like taking care of Eigen::RowMajor/ColMajor, rows/cols...

[jjcasmar]

Regarding giving the function a different name, I totally agree, maybe load_to(o). nlohmann::json for example uses get and get_to

[tdegeus]

load_to is a good idea. Happy to review a PR!