(octave.info)Structures in Oct-Files

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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’.