Function overload resolution with hsize_t fails in hdf5 >= 1.13.0

[jngrad]

The source code currently contains function overloads that match against two size types, std::size_t and hsize_t, e.g.:

h5xx/h5xx/dataspace/dataspace.hpp

Lines 49 to 53 in 459f07a

	/** construct simple dataspace of given extents, boost::array version */
	template <std::size_t N>
	dataspace(boost::array<std::size_t, N> const& dims);
	template <std::size_t N>
	dataspace(boost::array<hsize_t, N> const& dims);

This design decision was probably motivated by the guessing mechanism for integer type sizes in hdf5 versions < 1.13.0, where hsize_t was defined as:

typedef unsigned long long hsize_t;

(sources: v1.10.0 and v1.12.0). In function overload resolution, this type is considered different from std::size_t (typically defined as unsigned long on 64-bit architectures), although they have the same size.

Starting with hdf5 1.13.0 (HDFGroup/hdf5#709), it is now defined as:

typedef uint64_t hsize_t;

(sources: v1.13.0 and v1.14.1), which can be identical to std::size_t on 64-bit implementations, in which case overload resolution would fail.

One could solve this compatibility issue by wrapping all std::size_t overloads inside adequate ifdef guards, for example:

#include <H5public.h>
#if H5_VERSION_GE(1,13,0)
#define H5XX_PROVIDE_SIZE_T_OVERLOADS
#endif

/* ... */

#ifdef H5XX_PROVIDE_SIZE_T_OVERLOADS
 template <std::size_t N> 
 dataspace(boost::array<std::size_t, N> const& dims);
#endif
 template <std::size_t N> 
 dataspace(boost::array<hsize_t, N> const& dims);

The correct solution will likely look a bit more complicated than that, once portability concerns are factored in. For example, on 32-bit architectures std::size_t can be 4 bytes wide, in which case it is not equivalent to uint64_t and the std::size_t overloads are still needed.

Many thanks to William Brown for initially reporting the issue on the ESPResSo mailing list (thread from Fri, 4 Aug 2023, link).