FAQ 27.07 What is a capability query?

A capability query is an inspector member function (see FAQ 14.07) that allows users to determine whether an object supports some other member function. Capability queries invite inflexibility.

The benefit of capability queries is that they allow a class designer to avoid thinking about how users will use the objects, instead forcing the user code to explicitly test the classes and objects to see what capabilities they support.

The problem with capability queries is that they allow a class designer to avoid thinking about how users will use the objects, instead forcing the user code to explicitly test the classes and objects to see what capabilities they support.

Capability queries export complexity from the server to the users, from the few to the many. User code often needs explicit control flow to select operations based on the results of a capability query user code uses code to find code (see FAQ 27.04). This impacts existing user code when new derived classes are added.

Capability queries are not normally recommended.