Many applications in use today are driven by a
command line interface rather than a graphical interface. Most of
these applications are robust, popular and proven in general usage.
The convenience and ease of use that a graphical interface provides
might make these applications available to a wider range of users.
In this work, we describe a technique that exploits ostreams and
subtyping to integrate a graphical interface into a command driven
application. The technique that we propose does not create a new
process to construct the communication linkage nor does it require
a sentinel to terminate the linkage.
We use the technique to integrate a graphical interface into legacy
code, a grader program, that maintains a list of student grades.
The grader program is coded in C++, exploits the Command and
Envelope-Letter patterns and uses the standard library containers,
algorithms and iterators to implement much of its functionality.