Add support for Eigen::Ref<...> function arguments #312
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Eigen::Ref is a common way to pass eigen dense types without needing a template, e.g. the single definition `void func(Eigen::Ref<Eigen::MatrixXd> x)` can be called with any double matrix-like object. The current pybind11 eigen support fails with internal errors if attempting to bind a function with an Eigen::Ref<...> argument because Eigen::Ref<...> satisfies the "is_eigen_dense" requirement, but can't compile if actually used: Eigen::Ref<...> itself is not default constructible, and so the argument std::tuple containing an Eigen::Ref<...> isn't constructible, which results in compilation failure. This commit adds support for Eigen::Ref<...> by giving it its own type_caster implementation which consists of an internal type_caster of the referenced type, load/cast methods that dispatch to the internal type_caster, and a unique_ptr to an Eigen::Ref<> instance that gets set during load(). There is, of course, no performance advantage for pybind11-using code of using Eigen::Ref<...>--we are allocating a matrix of the derived type when loading it--but this has the advantage of allowing pybind11 to bind transparently to C++ methods taking Eigen::Refs.
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Eigen::Ref is a common way to pass eigen dense types without needing a template, e.g. the single definition
void func(const Eigen::Ref<const Eigen::MatrixXd> &x)is a useful signature to accept any double-coefficient matrix-like object/block/etc. (without needing the implementation in a template.)The current pybind11 eigen support fails with internal errors if attempting to bind a function with an Eigen::Ref<...> argument because, while Eigen::Ref<...> satisfies the "is_eigen_dense" requirement, it can't compile if used: Eigen::Ref<...> itself is not default constructible, and so the argument loader can't default construct an std::tuple containing an Eigen::Ref<...>, which results in compilation failure.
This commit adds support for Eigen::Ref<...> arguments by giving it its own type_caster implementation which basically is just a wrapper around the type_caster for the referenced type with a unique_ptr storing the required Ref object (which itself references the wrapped type_caster).
There is, of course, no performance advantage for pybind11-using code of using Eigen::Ref<...>--we are allocating a matrix of the derived type when loading it--this is mainly about allowing pybind11 to bind transparently to C++ methods taking Eigen::Refs.