### A.3 Standalone Programs ¶

The libraries Octave uses itself can be utilized in standalone applications. These applications then have access, for example, to the array and matrix classes, as well as to all of the Octave algorithms. The following C++ program, uses class Matrix from `liboctave.a` or `liboctave.so`.

    #include <iostream>
    #include <octave/oct.h>

    int
    main ()
    {
      std::cout << "Hello Octave world!\n";

      int n = 2;
      Matrix a_matrix = Matrix (n, n);

      for (octave_idx_type i = 0; i < n; i++)
        for (octave_idx_type j = 0; j < n; j++)
          a_matrix(i,j) = (i + 1) * 10 + (j + 1);

      std::cout << a_matrix;

      return 0;
    }

mkoctfile can be used to build a standalone application with a command like

    $ mkoctfile --link-stand-alone standalone.cc -o standalone
    $ ./standalone
    Hello Octave world!
      11 12
      21 22
    $

Note that the application `standalone` will be dynamically linked against the Octave libraries and any Octave support libraries. The above allows the Octave math libraries to be used by an application. It does not, however, allow the script files, oct-files, or built-in functions of Octave to be used by the application. To do that, the Octave interpreter needs to be initialized first. An example of how to do this can then be seen in the code

    #include <iostream>
    #include <octave/oct.h>
    #include <octave/octave.h>
    #include <octave/parse.h>
    #include <octave/interpreter.h>

    int
    main ()
    {
      // Create interpreter.

      octave::interpreter interpreter;

      try
        {
          // Inhibit reading history file by calling
          //
          //   interpreter.initialize_history (false);

          // Set custom load path here if you wish by calling
          //
          //   interpreter.initialize_load_path (false);

          // Perform final initialization of interpreter, including
          // executing commands from startup files by calling
          //
          //   interpreter.initialize ();
          //
          //   if (! interpreter.is_initialized ())
          //     {
          //       std::cerr << "Octave interpreter initialization failed!"
          //                 << std::endl;
          //       exit (1);
          //     }
          //
          // You may skip this step if you don't need to do anything
          // between reading the startup files and telling the interpreter
          // that you are ready to execute commands.

          // Tell the interpreter that we're ready to execute commands:

          int status = interpreter.execute ();

          if (status != 0)
            {
              std::cerr << "creating embedded Octave interpreter failed!"
                        << std::endl;
              return status;
            }

          octave_idx_type n = 2;
          octave_value_list in;

          for (octave_idx_type i = 0; i < n; i++)
            in(i) = octave_value (5 * (i + 2));

          octave_value_list out = octave::feval ("gcd", in, 1);

          if (out.length () > 0)
            std::cout << "GCD of ["
                      << in(0).int_value ()
                      << ", "
                      << in(1).int_value ()
                      << "] is " << out(0).int_value ()
                      << std::endl;
          else
            std::cout << "invalid\n";
        }
      catch (const octave::exit_exception& ex)
        {
          std::cerr << "Octave interpreter exited with status = "
                    << ex.exit_status () << std::endl;
        }
      catch (const octave::execution_exception&)
        {
          std::cerr << "error encountered in Octave evaluator!" << std::endl;
        }

      return 0;
    }

which, as before, is compiled and run as a standalone application with

    $ mkoctfile --link-stand-alone embedded.cc -o embedded
    $ ./embedded
    GCD of [10, 15] is 5
    $

It is worth re-iterating that, if only built-in functions are to be called from a C++ standalone program then it does not need to initialize the interpreter. The general rule is that for a built-in function named `function_name` in the interpreter, there will be a C++ function named `Ffunction_name` (note the prepended capital `F`) accessible in the C++ API. The declarations for all built-in functions are collected in the header file `builtin-defun-decls.h`. This feature should be used with care as the list of built-in functions can change. No guarantees can be made that a function that is currently a built-in won’t be implemented as a `.m` file or as a dynamically linked function in the future. An example of how to call built-in functions from C++ can be seen in the code

    #include <iostream>
    #include <octave/oct.h>
    #include <octave/builtin-defun-decls.h>

    int
    main ()
    {
      int n = 2;
      Matrix a_matrix = Matrix (n, n);

      for (octave_idx_type i = 0; i < n; i++)
        for (octave_idx_type j = 0; j < n; j++)
          a_matrix(i,j) = (i + 1) * 10 + (j + 1);

      std::cout << "This is a matrix:" << std::endl
                << a_matrix            << std::endl;

      octave_value_list in;
      in(0) = a_matrix;

      octave_value_list out = octave::Fnorm (in, 1);
      double norm_of_the_matrix = out(0).double_value ();

      std::cout << "This is the norm of the matrix:" << std::endl
                << norm_of_the_matrix                << std::endl;

      return 0;
    }

which is compiled and run as a standalone application with

    $ mkoctfile --link-stand-alone standalonebuiltin.cc -o standalonebuiltin
    $ ./standalonebuiltin
    This is a matrix:
     11 12
     21 22

    This is the norm of the matrix:
    34.4952
    $