Command-query separation (CQS) is a principle of imperative computer programming. It was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language.
It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer. More formally, methods should return a value only if they are referentially transparent and hence possess no side effects. It is noteworthy that rigid implementation of this specification makes tracking the number of times queries have been issued essentially impossible; it is clearly intended as a programming guideline rather than a rule for good coding, such as avoiding the use of a goto from a nested loop.
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Command-query separation is particularly well suited to a design by contract (DbC) methodology, in which the design of a program is expressed as assertions embedded in the source code, describing the state of the program at certain critical times. In DbC, assertions are considered design annotations – not program logic – and as such, their execution should not affect the program state. CQS is beneficial to DbC because any value-returning method (any query) can be called by any assertion without fear of modifying program state.
In theoretical terms, this establishes a measure of sanity, whereby one can reason about a program's state without simultaneously modifying that state. In practical terms, CQS allows all assertion checks to be bypassed in a working system to improve its performance without inadvertently modifying its behaviour. CQS may also prevent the occurrence of certain kinds of heisenbugs.
Even beyond the connection with design by contract, CQS is considered by its adherents to have a simplifying effect on a program, making its states (via queries) and state changes (via commands) more comprehensible in a manner reminiscent of how Edsger Dijkstra's admonition against gotos did the same for control flow.
CQS is well-suited to the object-oriented methodology, but can also be applied outside of object-oriented programming. There is nothing inherently object-oriented about the separation of side effects and return values, and so CQS can be profitably applied to any programming paradigm that requires reasoning about side effects.
CQS can make it more difficult to implement re-entrant and multi-threaded software correctly. This claim usually arises when a non-thread-safe pattern is used to implement the command query separation.
A simple example of a pattern that breaks CQS but is useful for multi-threaded software:
private int x; public int increment_and_return_x() { lock x; // by some mechanism x = x + 1; int x_copy = x; unlock x; // by some mechanism return x_copy; }
A common CQS pattern usable only in single threaded applications:
private int x; public int value() { return x; } void increment_x() { x = x + 1; }
Even in single-threaded programs, it is sometimes arguably significantly more convenient to have a method that is a combined query and command. Martin Fowler cites the pop() method of a stack as an example.[1]