XSPEC12 uses C++ streams for I/O. Instead of performing I/O through C++ std::cin and std::out, however, it defines new streams tcin, tcout that read and write data through objects called XSstreams derived from their C++ equivalents.

In C++, stream objects read from and write to internal streambuf objects that fill or drain a character buffer. The XSstreams re-implement this buffer to point to different read/write operations. Specifically these operations can optionally be:

• Reading and writing through a tcl channel (Tcl_Channel artefact)
• Creating and reading from a widget, for example implemented with a tk script.

The abstract interface written for XSPEC12 is not dependent on the choice of tcl as the scripting language - one can in principle use any language that has a C language Application Programming Interface (API) that exposes the low-level read/write capabilities to the user (although of course we have only implemented tcl).

Logging user input and program output is implemented by having the low-level output implementation write to two channels, one of them being a text file. Degree of verbosity to either or both channels can be controlled by a chatter parameter set by the calling function.

Implication: The object of this somewhat more sophisticated I/O architecture is to provide the ability for XSPEC12 to implement either a command-line interface (as, we expect, many experienced users will continue to prefer) or a GUI interface with a console prompt. Switching between the two is achieved by setting a single boolean flag. Because the command driver functions are separated from the Problem Domain code (the Model layer) we can also, with the same Model layer code, set up a different set of commands at startup that implement a widget based interface. This allows the GUI, to provide whatever operations the widget set supports, without therefore tying the widgets to acting simply as a "better looking command prompt."

Although the planned GUI has not yet been developed, the concept of user interface switching described above has already been successfully demonstrated.