The term 'command query separation' was coined by Bertrand Meyer in his book "Object Oriented Software Construction" - a book that is one of the most influential OO books during the early days of OO. (The first edition is the one that had the influence, the second edition is good but you'll need several months in a gym before you can lift it.)
The fundamental idea is that we should divide an object's methods into two sharply separated categories:
- Queries: Return a result and do not change the observable state of the system (are free of side effects).
- Commands: Change the state of a system but do not return a value.
Because the term 'command' is widely used in other contexts I prefer to refer to them as 'modifiers', you also see the term 'mutators'.
The really valuable idea in this principle is that it's extremely handy if you can clearly separate methods that change state from those that don't. This is because you can use queries in many situations with much more confidence, introducing them anywhere, changing their order. You have to be more careful with modifiers.
The notion in the principle is that the return type is the
give-away for the difference. It's a good convention because most of
the time it works well. Consider the java idiom for iterating through
a collection: the
next method both gives the next item in
the collection and advances the iterator. Personally I'd much prefer a
style that has separate
Meyer likes to use command-query separation absolutely, but there are exceptions. Popping a stack is a good example of a query that modifies state. Meyer correctly says that you can avoid having this method, but it is a useful idiom. So I prefer to follow this principle when I can, but I'm prepared to break it to get my pop.
It would be nice if the language itself would support this notion. I could imagine a language that would detect state changing methods, or at least allow the programmer to mark them. One reason that languages can't detect them automatically is that the rule about not changing state really only applies to the ObservableState of the system. Using programmer markings seems more reasonable but is rare. The only case I've really come across it is the const modifier in C++. Since I haven't used C++ for many years it's hard for me to assess how useful it is in practice. My sense is that good C++ers use const a lot and like it.