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The system containing the policy-based design is a military message-handling system (MHS) developed by a company. Among its capabilities is a message interface that allows data in messages stored in the MHS to be exchanged with practically any external system via a programmable command-line-based interface. For example, an incoming message containing the position of enemy forces may be sent through the message interface and used to update a geographic information system. Or logistics information in a command-and-control system's database may be retrieved through the message interface and used to generate a message in the MHS. The policy-based design idea actually works in real life, and that it is not that difficult to use. The MHS is available in UNIX and Windows versions and none of the compilers have had any problems digesting the code. The policy-based design is a specialized tool with a rather limited applicability. Its purpose is to structure a set of related classes that vary along a number of independent dimensions, and such design problems do not occur frequently. This is no fault of policy-based design as such. Any design pattern has a limited field of applicability. The only drawback of the design is that it is difficult to explain a policy-based solution to people not familiar with the concept.

This article presents a technique that uses the fill power of C++ to avoid excessive coupling, lack of type safety and monolithic message-handling functions when transmitting data using generic message passing facilities. It describes a common feature of the object-oriented techniques where all message classes are derived from a common base class. It also acknowledges the functionality of the Dynamic Double Dispatch technique in which the message processing function is selected using overload resolution at compile time.