Example 2: Write/Read a custom C++ type

This example shows how to make your own C++ type HDF5 serializable.

We first define a class foo that we want to write/read to/from HDF5.

The most important functions are h5_write and h5_read. They are called by the generic h5::write and h5::read implementations using argument-dependent lookup (ADL).

It is recommended to also implement the hdf5_format function which can be used to attach a string attribute to foo objects. When reading from HDF5, it allows us to verify that data that we are reading is actually of type foo using h5::assert_hdf5_format.

In main(), we simply create an HDF5 file, write an object of type foo, read the same object from the file and output the result to stdout.

#include <h5/h5.hpp>
#include <iostream>
#include <string>
 
// define HDF5 serializable type
class foo {
private:
  int i{};
  double d{};
  std::string s{};
 
public:
  // constructors
  foo() = default;
  foo(int i, double d, std::string s) : i(i), d(d), s(s) {}
 
  // print members to stdout
  void print() const { std::cout << i << " " << d << " " << s << std::endl; }
 
  // get hdf5_format tag
  static std::string hdf5_format() { return "foo";}
 
  // write to HDF5
  friend void h5_write(h5::group g, const std::string& subgroup_name, const foo& f) {
    auto sg = g.create_group(subgroup_name);
    h5::write_hdf5_format(sg, f); // NOLINT (cppcoreguidelines-slicing)
    h5::write(sg, "i", f.i);
    h5::write(sg, "d", f.d);
    h5::write(sg, "s", f.s);
  }
 
  // read from HDF5
  friend void h5_read(h5::group g, const std::string& subgroup_name, foo& f) {
    auto sg = g.open_group(subgroup_name);
    h5::assert_hdf5_format(sg, f); // NOLINT (cppcoreguidelines-slicing)
    h5::read(sg, "i", f.i);
    h5::read(sg, "d", f.d);
    h5::read(sg, "s", f.s);
  }
};
 
int main() {
  // create file in read/write mode
  h5::file file("foo.h5", 'w');
 
  // write foo
  h5::write(file, "myfoo", foo(1, 2.2, "three"));
 
  // read foo
  foo f;
  h5::read(file, "myfoo", f);
 
  // output foo
  f.print();
}

Output:

1 2.2 three

Contents of foo.h5:

HDF5 "foo.h5" {
GROUP "/" {
   GROUP "myfoo" {
      ATTRIBUTE "Format" {
         DATATYPE  H5T_STRING {
            STRSIZE H5T_VARIABLE;
            STRPAD H5T_STR_NULLTERM;
            CSET H5T_CSET_UTF8;
            CTYPE H5T_C_S1;
         }
         DATASPACE  SCALAR
         DATA {
         (0): "foo"
         }
      }
      DATASET "d" {
         DATATYPE  H5T_IEEE_F64LE
         DATASPACE  SCALAR
         DATA {
         (0): 2.2
         }
      }
      DATASET "i" {
         DATATYPE  H5T_STD_I32LE
         DATASPACE  SCALAR
         DATA {
         (0): 1
         }
      }
      DATASET "s" {
         DATATYPE  H5T_STRING {
            STRSIZE H5T_VARIABLE;
            STRPAD H5T_STR_NULLTERM;
            CSET H5T_CSET_UTF8;
            CTYPE H5T_C_S1;
         }
         DATASPACE  SCALAR
         DATA {
         (0): "three"
         }
      }
   }
}
}