# Struct constructor

**URL:** <https://www.ruby-forum.com/t/struct-constructor/120292>\
**Category:** Ruby\
**Created:** [November 15, 2007, 4:01pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292 "2007-11-15T16:01:04Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 4:01pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/1 "2007-11-15T16:01:04Z")

</div>

It isn’t possible to define a class method [] (to make a constructor)  
for a struct is it?

If you have:

```
 Customer = Struct.new( "Customer", :name, :address, :zip )
 joe = Customer.new( "Joe S.", "123 Maple, Anytown NC", 12345 )

```

but you want it to be:

```
 joe = ["Joe S.", "123 Maple, Anytown NC", 12345]

```

Ruby will just make joe an array, right? No way around this that I  
can see, is there?

---

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 4:11pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/2 "2007-11-15T16:11:48Z")

</div>

Alle giovedì 15 novembre 2007, Wayne M. ha scritto:

> ```
> joe = ["Joe S.", "123 Maple, Anytown NC", 12345]
> 
> ```
> 
> Ruby will just make joe an array, right? No way around this that I  
> can see, is there?

You can’t instruct ruby to create a class different from Array for the  
[x,  
y, …] construct, but you can define a [] method for a class, and use  
it to  
create a new istance of the class:

class A

def initialize x, y  
@x = x  
@y = y  
end

def A.[](x, y)  
new x, y  
end

end

a = A[1, 2]

Since your Customer class was created using Struct.new, it already  
provides  
this feature (at least, my trials show this, although I couldn’t find  
documentation about this). So, you can write:

joe = Customer[“Joe S.”, “123 Maple, Anytown NC”, 12345]

I hope this helps

Stefano

---

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 4:53pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/3 "2007-11-15T16:53:58Z")

</div>

2007/11/15, Wayne M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> ```
> joe = ["Joe S.", "123 Maple, Anytown NC", 12345]
> 
> ```
> 
> Ruby will just make joe an array, right?

Of course you cannot change the semantics of the array constructor.

> No way around this that I  
> can see, is there?

Of course there is, this is Ruby. 🙂

irb(main):001:0\> Test = Struct.new :foo, :bar do  
irb(main):002:1\* class \<\<self  
irb(main):003:2\> alias :initialize :[]  
irb(main):004:2\> end  
irb(main):005:1\> end  
=\> Test  
irb(main):006:0\> Test[1,2]  
=\> #  
irb(main):007:0\>

Kind regards

robert

---

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 5:11pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/4 "2007-11-15T17:11:33Z")

</div>

On Fri, 16 Nov 2007 01:05:44 +0900, Wayne M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> So how is [] defined in Ruby? Just curious. It must be a special-case.

It’s just a singleton method on the class object; “[]” is a valid method  
name.

joe = Customer.send(:[], “Joe S.”, “123 Maple, Anytown NC”, 12345)

You can define a #[] method on anything.

-mental

---

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 5:05pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/5 "2007-11-15T17:05:47Z")

</div>

Stefano C. wrote:

> this feature (at least, my trials show this, although I couldn’t find  
> documentation about this). So, you can write:
> 
> joe = Customer[“Joe S.”, “123 Maple, Anytown NC”, 12345]

I also missed the docs for this since I was looking in the old pick-axe  
book included with Ruby dists. It’s actually documented in the second  
edition book on page 627. Thanks.

So how is [] defined in Ruby? Just curious. It must be a special-case.

---

<div class="post-metadata">

**Author:** ![wemagor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/8edcca/32.png) [@wemagor](https://www.ruby-forum.com/u/wemagor)\
**Post date:** [November 15, 2007, 5:23pm UTC](https://www.ruby-forum.com/t/struct-constructor/120292/6 "2007-11-15T17:23:50Z")

</div>

Wayne M. wrote:

> So how is [] defined in Ruby? Just curious.

obj[stuff] is syntactig sugar for obj. and  
obj[stuff]=otherstuff is  
syntactic sugar for obj.[]=(stuff,otherstuff). Stuff can be multiple  
values  
seperated by commas, otherstuff can’t.

> It must be a special-case.

Not more so than \<, \>, \<=, \>=, \<\<, \>\>, ~, ^, &, +, +@, -, -@, \*, \*\* and  
so on.  
Also all methods ending with a equals sign (where you can write obj.bla  
= blo  
instead of obj.bla=(blo)).

HTH,  
Sebastian