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).

The hdf5_format function is optional and 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.

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);
    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);
    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"
         }
      }
   }
}
}