Rspec doubles - normal double, instance double, class doublea

Test doubles are any object that are suppose to stand in and represent real objects during testing. Rspec offers three types of doubles - ordinary doubles(just double), instance doubles, and class doubles.

All doubles are strict by default. What that means is that if any un-allowed or unexpected methods are invoked, the test will fail. For example:

x = double()
a.bar // error because `bar` was not allowed.

However, you can make any double loose by appending `as_null_object`.

Now lets look at differences.

Ordinary doubles

x = double()

These doubles are super barebones. You can allow any messages on these methods.

Instance doubles

x = instance_double('ClassName')

These doubles are aware of the instance methods of class 'ClassName' - you can only allow messages that are defined.

Class doubles

x = class_double('ClassName')

These doubles are aware of the class / module methods of any class or module named 'ClassName'. Just like instance doubles, only defined messages are allowable.

In short, instance and class doubles go a step farther than verifying the state of an object (was 'X' called?). They also verify behavior (is 'X' a thing that this object does?).

When do you use one over the other?

I see ordinary doubles as good for creating dummy objects during a test. For example, if you need to fulfill a parameter requirement for a method where you know the object isn't being used. However, you generally want to use instance and class doubles for the stricter check.