A structure in Octave is a map between a number of fields represented and their
values.  The Standard Template Library map class is used, with the pair
consisting of a std::string and an Octave Cell variable.
A simple example demonstrating the use of structures within oct-files is
#include <octave/oct.h>
#include <octave/ov-struct.h>
DEFUN_DLD (structdemo, args, , "Struct Demo")
{
  if (args.length () != 2)
    print_usage ();
  if (! args(0).isstruct ())
    error ("structdemo: ARG1 must be a struct");
  octave_scalar_map arg0 = args(0).scalar_map_value ();
  //octave_map arg0 = args(0).map_value ();
  if (! args(1).is_string ())
    error ("structdemo: ARG2 must be a character string");
  std::string arg1 = args(1).string_value ();
  octave_value tmp = arg0.contents (arg1);
  //octave_value tmp = arg0.contents (arg1)(0);
  if (! tmp.is_defined ())
    error ("structdemo: struct does not have a field named '%s'\n",
           arg1.c_str ());
  octave_scalar_map st;
  st.assign ("selected", tmp);
  return octave_value (st);
}
An example of its use is
x.a = 1; x.b = "test"; x.c = [1, 2]; structdemo (x, "b") ⇒ selected = test
The example above specifically uses the octave_scalar_map class which is
for representing a single struct.  For structure arrays, the octave_map
class is used instead.  The commented code shows how the demo could be modified
to handle a structure array.  In that case, the contents method returns
a Cell which may have more than one element.  Therefore, to obtain the
underlying octave_value in the single struct example we would write
octave_value tmp = arg0.contents (arg1)(0);
where the trailing (0) is the () operator on the Cell
object.  If this were a true structure array with multiple elements we could
iterate over the elements using the () operator.
Structures are a relatively complex data container and there are more functions
available in oct-map.h which make coding with them easier than relying
on just contents.