Ruby class override without inheritance

Question

I've just made this experiment:

class A < Hash
  def foo
    'foo'
  end
end

class A < Hash
  def bar 
    'bar'
  end
end

So far I get the result I expected, the second declaration extends the first one. However I was surprised of this:

class A
  def call
    puts foo
    puts bar
  end
end

The code above works, but only if I declare it later. Otherwise I get:

TypeError: superclass mismatch for class A

Can I assume that in Ruby, it is safe skipping the superclass specification without side effects after making sure that the "original-first" declaration was parsed?

In you last attempt, it tries to make an Object class, as its superclass.. So the error you got.. and it is expected.. — Arup Rakshit
– Arup Rakshit, Commented Jan 31, 2014 at 15:18

Малъ Скрылевъ · Accepted Answer · 2014-01-31 15:28:04Z

3

You are able to declare inheritance only on the first occurince of the class definition, so below variants will work:

When you've defined the same class inheritance:
```
class A < Hash
end

class A < Hash
end
```
When you've used default inheritance in the second case, that is treated as undefined inheritance:
```
class A < Hash
end

class A
end
```
When you've used default inheritance in both cases, the default inheritance is of Object class:
```
class A
end

class A
end
```

And below will not:

When you've used default inheritance in the first case, and next you tried to redefine it explicitly:
```
class A
end

class A < Hash
end

TypeError: superclass mismatch for class A
```
When you've used specified inheritance (in example String) in the first case, and next you tried to redefine it explicitly (in example with Hash):
```
class A < String
end

class A < Hash
end

TypeError: superclass mismatch for class A
```

edited Jan 31, 2014 at 15:28

answered Jan 31, 2014 at 15:20

Малъ Скрылевъ

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Comments

Community · Accepted Answer · 2017-05-23 10:31:41Z

1

@Малъ Скрылевъ explained this case a better way, so I wouldn't attempt that. But I would show you another way to do this.

One way to avoid the error is in your situation :

Instead of writing

class A
  def call
    puts foo
    puts bar
  end
end

Write it as below using Module#class_eval:

Evaluates the string or block in the context of mod, except that when a block is given, constant/class variable lookup is not affected. This can be used to add methods to a class. module_eval returns the result of evaluating its argument.

A.class_eval do
  def _call
    puts foo
    puts bar
  end
end

edited May 23, 2017 at 10:31

CommunityBot

11 silver badge

answered Jan 31, 2014 at 15:23

Arup Rakshit

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