A.1.5 Structures in Oct-Files

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).is_map ())
    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 this single-struct example we 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.