#### 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).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`.