[[Property:title|CA004 - Command-Query Separation]]
[[Property:link_title|CA004]]
[[Property:weight|0]]
[[Property:uuid|7d58b7d3-e7e5-d3ec-83c8-84fca33bf638]]
__NOTOC__
=Description=
A function should never change the state of an object. A function containing a procedure call, an assignment to an attribute, or creating an attribute is a strong indication that this principle is violated. This rule applies exactly in these three cases.

There are rather exceptional but sometimes useful class designs in which the externally visible state of an object (i. e. the values of exported queries) does not change even though the function contains a rule-violating instruction.

:{| class="doctable"
|-
| '''Scope'''
| Class
|-
| '''Status'''
| Enabled
|-
| '''Severity'''
| Warning
|-
| '''Applicability'''
| All
|-
| '''Score'''
|  60
|}


=Example of violation=
<eiffel>
height: INTEGER
		-- Height in pixel of Current.

width: INTEGER
		-- Width in pixel of Current.
	do
		height := height + 10
		Result := 100 - height
	end</eiffel>

=Recommendation=
Ensures that no query changes the state of the current object.

In the example, one could replace the routine `width` by an attribute and calling `update_width` before querying `width` where `update_width` would be:
<eiffel>update_width
		-- Update `height' and `width' with ....
	do
		height := height + 10
		width := 100 - height
	end</eiffel>

Or you can remove the line `height := height + 10` from the body of `width`.


