Editing Eiffel (programming language) (section)

===Features: commands and queries===
The primary characteristic of a class is that it defines a set of features: as a class represents a set of run-time objects, or "instances", a feature is an operation on these objects. There are two kinds of features: queries and commands. A query provides information about an instance. A command modifies an instance.

The command-query distinction is important to the Eiffel method. In particular:

* [[Uniform-Access Principle]]: from the point of view of a software client making a call to a class feature, whether a query is an attribute (field value) or a function (computed value) should not make any difference. For example, <code>a_vehicle.speed</code> could be an attribute accessed on the object <code>a_vehicle</code>, or it could be computed by a function that divides distance by time. The notation is the same in both cases, so that it is easy to change the class's implementation without affecting client software.
* [[Command-Query Separation|Command-Query Separation Principle]]: Queries must not modify the instance. This is not a language rule but a methodological principle. So in good Eiffel style, one does not find "get" functions that change something and return a result; instead there are commands (procedures) to change objects, and queries to obtain information about the object, resulting from preceding changes.