Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
Bertrand Meyer is an academic, author, and consultant in the field of computer languages. He created the Eiffel programming language.-Education and academic career:...
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
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 effect
Side effect (computer science)
In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world...
s. 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
Goto
goto is a statement found in many computer programming languages. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call normally returns control...
from a nested loop.
Connection with design by contract
Command-query separation is particularly well suited to a design by contract
Design by contract
Design by contract , also known as programming by contract and design-by-contract programming, is an approach to designing computer software...
(DbC) methodology, in which the design of a program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
is expressed as assertions embedded in the source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
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.
Broader impact on software engineering
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
Edsger Dijkstra
Edsger Wybe Dijkstra ; ) was a Dutch computer scientist. He received the 1972 Turing Award for fundamental contributions to developing programming languages, and was the Schlumberger Centennial Chair of Computer Sciences at The University of Texas at Austin from 1984 until 2000.Shortly before his...
goto is a statement found in many computer programming languages. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call normally returns control...
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
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.
Drawbacks
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
Martin Fowler
-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
