# Named Constructor Idiom in Ruby?

**URL:** <https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410>\
**Category:** Ruby\
**Created:** [August 18, 2007, 3:37pm UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410 "2007-08-18T15:37:40Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 18, 2007, 3:37pm UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/1 "2007-08-18T15:37:40Z")

</div>

Hi all,

I’m almost certain there’s a good way to do this, but I seem to have  
lost hold of the thought of how – is there a straight-forward/neat way  
to do the named constructor idiom in Ruby?

In C++ terms, a class like this implements it:

class A {  
public:  
static A something() {  
A a;  
// do something special to a  
return a;  
}

```
static A otherthing() {
    A a;
    // some other special thing to a
    return a;
}

```

protected:  
A();  
};

The result is that we can’t construct A directly, only by one of the two  
‘named’ constructors. Is there any way to achieve a similar thing using  
protection in Ruby?

Thanks,

Arlen.

---

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 18, 2007, 3:44pm UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/2 "2007-08-18T15:44:40Z")

</div>

Alle sabato 18 agosto 2007, Arlen Christian Mart C. ha scritto:

> ```
> static A something() {
> 
> ```
> 
> Arlen.  
> You need to make the new class method of class A private. Then, you  
> implement  
> the class methods you want to make public to create the instances of A:

class A

private\_class\_method :new

def self.special\_method\_1  
a = A.new  
#do something special  
a  
end

def self.special\_method\_2  
a = A.new  
#do something else special  
a  
end

end

a1 = A.special\_method1  
a2 = A.special\_method2

I hope this helps

Stefano

---

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 18, 2007, 3:40pm UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/3 "2007-08-18T15:40:49Z")

</div>

Arlen Christian Mart C. wrote:

> Hi all,
> 
> I’m almost certain there’s a good way to do this, but I seem to have  
> lost hold of the thought of how – is there a straight-forward/neat way  
> to do the named constructor idiom in Ruby?
> 
> In C++ terms, a class like this implements it:
> 
> class A {  
> public:  
> static A something() {  
> A a;  
> // do something special to a  
> return a;  
> }
> 
> ```
> static A otherthing() {
> A a;
> // some other special thing to a
> return a;
> }
> 
> ```
> 
> protected:  
> A();  
> };
> 
> The result is that we can’t construct A directly, only by one of the two  
> ‘named’ constructors. Is there any way to achieve a similar thing using  
> protection in Ruby?
> 
> Thanks,
> 
> Arlen.

I think you want something like this:

# somewhat contrieved example

class Clock  
class \<\<self  
private :new  
def from\_string(string)  
new(\*string.match(SomeRegexp).captures)  
end  
def from\_seconds(int)  
new(int)  
end  
end  
end

HTH

regards  
Stefan

---

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 18, 2007, 3:56pm UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/4 "2007-08-18T15:56:41Z")

</div>

Exactly what I needed, both Stefan and Stefano! Thanks very much. 🙂

Cheers,  
Arlen.

---

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 20, 2007, 11:28am UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/5 "2007-08-20T11:28:14Z")

</div>

2007/8/18, Stefano C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> > public:  
> > }
> 
> ```
> a = A.new
> 
> ```
> 
> end
> 
> a1 = A.special\_method1  
> a2 = A.special\_method2
> 
> I hope this helps

A small glitch:

irb(main):020:0\> a1 = A.special\_method\_1  
NoMethodError: private method `new' called for A:Class from (irb):6:in `special\_method\_1’  
from (irb):20  
from :0

You need to remove the “A.” when calling “new” after making it  
private. It’s not needed anyway.

irb(main):021:0\> def A.special\_method\_1  
irb(main):022:1\> a=new  
irb(main):023:1\> a  
irb(main):024:1\> end  
=\> nil  
irb(main):025:0\> a1 = A.special\_method\_1  
=\> #\<A:0x7ff63428\>

Kind regards

robert

---

<div class="post-metadata">

**Author:** ![unnali](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/97f17d/32.png) [@unnali](https://www.ruby-forum.com/u/unnali)\
**Post date:** [August 20, 2007, 11:56am UTC](https://www.ruby-forum.com/t/named-constructor-idiom-in-ruby/112410/6 "2007-08-20T11:56:45Z")

</div>

Arlen Christian Mart C. wrote:

> lost hold of the thought of how – is there a straight-forward/neat way  
> to do the named constructor idiom in Ruby?

You have some good answers, but here is yet another way that is quite  
interesting:

class A  
class \<\< self; private :new; end

def self.something  
a = self.allocate  
# set up a somehow  
a  
end

def self.otherthing  
a = self.allocate  
# set up a somehow  
a  
end  
end

allocate is like new but without calling initialize. I believe that new  
is basically defined as: (1) allocate, (2) initialize.

The only benefit of this over the other solutions is that it avoids  
calling new altogether. I have found this useful in a case where the new  
was very generalized and therefore slow.

best,  
Dan