# What's the suggested raise Exception idiom?

**URL:** <https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624>\
**Category:** Ruby\
**Created:** [March 23, 2010, 6:25pm UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624 "2010-03-23T18:25:50Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![ate50eggs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/fbc32d/32.png) [@ate50eggs](https://www.ruby-forum.com/u/ate50eggs)\
**Post date:** [March 23, 2010, 6:25pm UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624/1 "2010-03-23T18:25:50Z")

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What’s the distinction between:  
raise ArgumentError, “the argument was invalid”  
and  
raise ArgumentError.new(“the argument was invalid”)

How does this apply in the case of exceptions with custom initializers  
for attributes? For example:  
class StatusCodeError \< StandardError  
attr\_reader :status\_code  
def initialize(status\_code)  
@status\_code = status\_code  
end  
end

I’ve been reading a lot of code, books and blogs, but haven’t gotten  
as clear of an understanding on why people choose one way over  
another, so I’m looking for additional opinions and commentary.

Thanks

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**Author:** ![ate50eggs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/fbc32d/32.png) [@ate50eggs](https://www.ruby-forum.com/u/ate50eggs)\
**Post date:** [March 23, 2010, 10:52pm UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624/2 "2010-03-23T22:52:46Z")

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On 03/23/2010 06:22 PM, Nathan B. wrote:

> ```
> @status_code = status_code
> end
> 
> ```
> 
> end
> 
> I’ve been reading a lot of code, books and blogs, but haven’t gotten  
> as clear of an understanding on why people choose one way over  
> another, so I’m looking for additional opinions and commentary.

I believe raise a,b,c,d… is merely syntactig sugar for raise  
a.new(b,c,d…) with a bit of added functionality (e.g. if a is not an  
exception class there is an error, if a is a String a StandardError is  
raised).

To answer your question, normally there is no reason to use a variant  
with “new” - rather save yourself a bit of typing and use the shorter  
form. My 0.02EUR.

Kind regards

robert

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**Author:** ![ate50eggs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/fbc32d/32.png) [@ate50eggs](https://www.ruby-forum.com/u/ate50eggs)\
**Post date:** [March 24, 2010, 4:10am UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624/3 "2010-03-24T04:10:35Z")

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On Mar 23, 2010, at 2:45 PM, Robert K. wrote:

> > ```
> > def initialize(status_code)
> > 
> > ```
> 
> exception class there is an error, if a is a String a StandardError is  
> raised).

Not exactly so. According to the passage from RI below, “raise” with an  
exception class as a first paremeter accepts up to 2 additional  
parameters – the first one will be used as an argument to new when the  
exception object is instantiated, and the second, if any, must be an  
array representing callback information.

When I need an exception class with more than one argument, I use  
something like the following:

class DimensionError \< RuntimeError  
attr\_reader :width, :hight  
def initialize(\*args)  
@width, @hight = \*args  
super “Wrong dimension #{width}x#{hight}”  
end  
end

begin  
raise DimensionError, [5, 3]  
rescue DimensionError =\> error  
error.wigth  
error.length  
end

= OR =

class DimensionError \< RuntimeError  
attr\_reader :width, :hight  
def initialize(options = {})  
@width = options[:width]  
@hight = options[:hight]  
super “Wrong dimension #{width}x#{hight}”  
end  
end

begin  
raise DimensionError, :width =\> 5, :hight =\> 3  
rescue DimensionError =\> error  
error.wigth  
error.length  
end

## ----------------------------------------------------------- Kernel#raise raise raise(string) raise(exception [, string [, array]]) fail fail(string) fail(exception [, string [, array]])

```
 With no arguments, raises the exception in $! or raises a
 RuntimeError if $! is nil. With a single String argument, raises a
 RuntimeError with the string as a message. Otherwise, the first
 parameter should be the name of an Exception class (or an object
 that returns an Exception object when sent an exception message).
 The optional second parameter sets the message associated with the
 exception, and the third parameter is an array of callback
 information. Exceptions are caught by the rescue clause of
 begin...end blocks.

    raise "Failed to create socket"
    raise ArgumentError, "No parameters", caller
```

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**Author:** ![ate50eggs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/fbc32d/32.png) [@ate50eggs](https://www.ruby-forum.com/u/ate50eggs)\
**Post date:** [March 24, 2010, 9:32am UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624/4 "2010-03-24T09:32:28Z")

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2010/3/24 Gennady B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> On Mar 23, 2010, at 2:45 PM, Robert K. wrote:

> > I believe raise a,b,c,d… is merely syntactig sugar for raise  
> > a.new(b,c,d…) with a bit of added functionality (e.g. if a is not an  
> > exception class there is an error, if a is a String a StandardError is  
> > raised).
> 
> Not exactly so. According to the passage from RI below, “raise” with an exception class as a first paremeter accepts up to 2 additional parameters – the first one will be used as an argument to new when the exception object is instantiated, and the second, if any, must be an array representing callback information.

Right you are. Thanks for the education! Apparently I never had the  
need for this. 🙂

> begin  
> raise DimensionError, [5, 3]  
> rescue DimensionError =\> error  
> error.wigth  
> error.length  
> end

That does not seem to work as you expect on 1.9:

irb(main):038:0\> class E1 \< Exception  
irb(main):039:1\> def initialize(\*a) p a end  
irb(main):040:1\> end  
=\> nil  
irb(main):041:0\> begin; raise E1,[1,2]; rescue Exception =\> e; p e; end  
[[1, 2]]  
#\<E1: E1\>  
=\> #\<E1: E1\>  
irb(main):042:0\> class E1 \< Exception  
irb(main):043:1\> def initialize(\*a) x,y=\*a; p x,y end  
irb(main):044:1\> end  
=\> nil  
irb(main):045:0\> begin; raise E1,[1,2]; rescue Exception =\> e; p e; end  
[1, 2]  
nil  
#\<E1: E1\>  
=\> #\<E1: E1\>  
irb(main):046:0\> RUBY\_VERSION  
=\> “1.9.1”

You would have to do

irb(main):047:0\> class E1 \< Exception  
irb(main):048:1\> def initialize(a) x,y=\*a; p x,y end  
irb(main):049:1\> end  
=\> nil  
irb(main):050:0\> begin; raise E1,[1,2]; rescue Exception =\> e; p e; end  
1  
2  
#\<E1: E1\>  
=\> #\<E1: E1\>  
irb(main):051:0\>

Note the missing splash operator in initialize’s argument list.

Kind regards

robert

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**Author:** ![ate50eggs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/fbc32d/32.png) [@ate50eggs](https://www.ruby-forum.com/u/ate50eggs)\
**Post date:** [March 24, 2010, 4:40pm UTC](https://www.ruby-forum.com/t/whats-the-suggested-raise-exception-idiom/185624/5 "2010-03-24T16:40:23Z")

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On Mar 24, 2010, at 1:31 AM, Robert K. wrote:

> > end
> 
> irb(main):044:1\> end
> 
> Note the missing splash operator in initialize’s argument list.

Well, I am still on 1.8.6, missed the issue on 1.9.x ;-). Thanks for  
pointing this out. Besides, an exception’s initialize() accepts only one  
parameter anyway, so your no-splash version should work for 1.8.x as  
well. I started using Hash-based initialize() recently, and it seems to  
work same way both in 1.8 and 1.9.

Gennady.