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Contents
# Instance Variable Assumption
## Introduction
Classes should not assume that instance variables are set or present outside of the current class definition.
Good:
```Ruby
class Foo
def initialize
@bar = :foo
end
def foo?
@bar == :foo
end
end
```
Good as well:
```Ruby
class Foo
def foo?
bar == :foo
end
def bar
@bar ||= :foo
end
end
```
Bad:
```Ruby
class Foo
def go_foo!
@bar = :foo
end
def foo?
@bar == :foo
end
end
```
## Example
Running Reek on:
```Ruby
class Dummy
def test
@ivar
end
end
```
would report:
```Bash
[1]:InstanceVariableAssumption: Dummy assumes too much for instance variable @ivar [https://github.com/troessner/reek/blob/master/docs/Instance-Variable-Assumption.md]
```
Note that this example would trigger this smell warning as well:
```Ruby
class Parent
def initialize(omg)
@omg = omg
end
end
class Child < Parent
def foo
@omg
end
end
```
The way to address the smell warning is that you should create an `attr_reader` to use `@omg` in the subclass and not access `@omg` directly like this:
```Ruby
class Parent
attr_reader :omg
def initialize(omg)
@omg = omg
end
end
class Child < Parent
def foo
omg
end
end
```
Directly accessing instance variables is considered a smell because it [breaks encapsulation](http://designisrefactoring.com/2015/03/29/organizing-data-self-encapsulation/) and makes it harder to reason about code.
If you don't want to expose those methods as public API just make them private like this:
```Ruby
class Parent
def initialize(omg)
@omg = omg
end
private
attr_reader :omg
end
class Child < Parent
def foo
omg
end
end
```
## Current Support in Reek
An instance variable must:
* be set in the constructor
* or be accessed through a method with lazy initialization / memoization.
If not, _Instance Variable Assumption_ will be reported.
## Configuration
_Instance Variable Assumption_ supports the [Basic Smell Options](Basic-Smell-Options.md).
Version data entries
18 entries across 18 versions & 1 rubygems