Creating an instance from a variable

[Peter Hickman]

I have a class like this:

class Builder
def Builder.create( klass, data )
return klass.new(data)
end
end

I want to call it as:

x = Builder.create( "Fred", data )

and get a object of type Fred.

I'm sure you can see what I am trying to do but for the life of me I
can't get the syntax correct.

 

[Matt Mower]

Hi Peter,

I have a class like this:

class Builder
def Builder.create( klass, data )
return klass.new(data)
end
end

how about:

Class Builder
def Builder.create( klass_sym, data )
klass = Class.const_get( klass_sym )
klass.new( data )
end
end

x = Builder.create( :Fred, data )

Regards,

Matt

 

[Luc Heinrich]

return klass.new(data)

return Kernel.const_get(klass).new(data)

Note that const_get will throw an exception if the passed class name is
unknown, so be prepared to deal with that.

 

[Peter Hickman]

Thanks for that it was just what I needed.

Thank you

 

[Thomas Leitner]

On Mon, 17 Jan 2005 19:54:47 +0900

| I have a class like this:
|
| class Builder
| def Builder.create( klass, data )
| return klass.new(data)
| end
| end

use (I have not tested this):

class Builder
def Builder.create( klass, data )
return Object.const_get(klass).new(data)
end
end

*hth*,
Thomas

 

[CT]

I'm not sure if this is what you want, but I think Kernel.const_get
can help you.
See changes below:

class Builder
def Builder.create( klass, data )

return Kernel.const_get(klass).new(data) #can pass in a string

end
end

You might wanna do some error-catching too.

HTH!
CT

 

[Peter Hickman]

return Kernel.const_get(klass).new(data)

Note that const_get will throw an exception if the passed class name is
unknown, so be prepared to deal with that.

Actually, trying to catch the error seems to be harder than it should
be. For example:

class Builder
def Builder.create( klass_sym, data )
begin
return Kernel.const_get( klass_sym ).new( data )
rescue
raise "There was an error"
end
end
end

Should report "There was an error" when const_get fails. However this is
untrapped.

xx.rb:26:in `const_get': uninitialized constant Bernie at Kernel (NameError)
from xx.rb:26:in `create'
from xx.rb:43

I've tried splitting this up into individual steps:

x = Kernel.const_get( klass_sym )
x.new( data )

but this makes no difference.

 

[trans. (T. Onoma)]

Or try Ruby Facets:

require 'facet/object/constant'

class Builder
def Builder.create( klass, *data )
constant( klass ).new( *data )
end
end

T.

P.S. I debate with myself that the method might be better place in Kerenl, so
it might move there in future.


| >> return klass.new(data)
| >
| >return Kernel.const_get(klass).new(data)
| >
| >Note that const_get will throw an exception if the passed class name is
| >unknown, so be prepared to deal with that.
|
| Actually, trying to catch the error seems to be harder than it should
| be. For example:
|
| class Builder
| def Builder.create( klass_sym, data )
| begin
| return Kernel.const_get( klass_sym ).new( data )
| rescue
| raise "There was an error"
| end
| end
| end
|
| Should report "There was an error" when const_get fails. However this is
| untrapped.
|
| xx.rb:26:in `const_get': uninitialized constant Bernie at Kernel
| (NameError) from xx.rb:26:in `create'
| from xx.rb:43
|
| I've tried splitting this up into individual steps:
|
| x = Kernel.const_get( klass_sym )
| x.new( data )
|
| but this makes no difference.

 

[Robert Klemme]

Actually, trying to catch the error seems to be harder than it should
be. For example:

class Builder
def Builder.create( klass_sym, data )
begin
return Kernel.const_get( klass_sym ).new( data )
rescue
raise "There was an error"
end
end
end

Should report "There was an error" when const_get fails. However this is
untrapped.

xx.rb:26:in `const_get': uninitialized constant Bernie at Kernel (NameError)
from xx.rb:26:in `create'
from xx.rb:43

I've tried splitting this up into individual steps:

x = Kernel.const_get( klass_sym )
x.new( data )

but this makes no difference.

You need to catch NameError.

class Builder
def Builder.create( klass_sym, data )
begin
return Kernel.const_get( klass_sym ).new( data )
rescue NameError => e
raise "There was an error: #{e}"
end
end
end


robert

 

[Peter Hickman]

You need to catch NameError.

class Builder
def Builder.create( klass_sym, data )
begin
return Kernel.const_get( klass_sym ).new( data )
rescue NameError => e
raise "There was an error: #{e}"
end
end
end


robert

Thanks for that, I thought that an open rescue would have caught it.
Live and learn.

Thanks

 

[Florian Gross]

I have a class like this:

class Builder
def Builder.create( klass, data )
return klass.new(data)
end
end

I want to call it as:

x = Builder.create( "Fred", data )

Why don't just do x = Builder.create(Fred, data)?

 

[Bertram Scharpf]

Hi,

Am Montag, 17. Jan 2005, 19:54:47 +0900 schrieb Peter Hickman:

I have a class like this:

class Builder
def Builder.create( klass, data )
return klass.new(data)
end
end

I want to call it as:

x = Builder.create( "Fred", data )

I just call

class Fred
def initialize data
...
end
end
x = Builder.create Fred, data

or even

r = Builder.create Regexp, '.*'

Bertram

 

[Robert Klemme]

Why don't just do x = Builder.create(Fred, data)?

Why then not "Fred.new data"?


Regards

robert

 

[Bertram Scharpf]

Hi Robert,

Am Dienstag, 18. Jan 2005, 01:06:06 +0900 schrieb Robert Klemme:

Why then not "Fred.new data"?

I'm porting my Make replacement Bake to Ruby and I do something
like

class Real ; ...
class Phony ; ...

b = Builder.create target, name, prereqs, cmd

where `target' is either `Real' or `Phony'.

Bertram

 

[Archit Baweja]

Hi

If you want to use strings, you could also do this (slight change)

class Builder
def Builder.create(klass_name, data)
Class.const_get(klass_name.intern).new(data)
end
end

To note is the use of "intern" method which converts a string to a
symbol

HTH
Archit

 

[Robert Klemme]

Hi Robert,

Am Dienstag, 18. Jan 2005, 01:06:06 +0900 schrieb Robert Klemme:

I'm porting my Make replacement Bake to Ruby and I do something
like

class Real ; ...
class Phony ; ...

b = Builder.create target, name, prereqs, cmd

where `target' is either `Real' or `Phony'.

??? Why can't you do

b = target.new name, prereqs, cmd

I mean, a class *is* a factory for its instances. If you don't have the
requirement that the actual class of created instances might be different or
want to keep that flexibility I don't see any added value in introducing a
method for this.

Regards

robert

 

[Bertram Scharpf]

Hi Robert,

Am Dienstag, 18. Jan 2005, 17:01:12 +0900 schrieb Robert Klemme:

??? Why can't you do

b = target.new name, prereqs, cmd

I mean, a class *is* a factory for its instances. If you don't have the
requirement that the actual class of created instances might be different
or want to keep that flexibility I don't see any added value in introducing
a method for this.

Hm, seems to be a long time since I designed this. I
remember having thought very long and thoroughly about it.

I think I didn't tell the whole truth above. My code
actually looks like

Rule.new Real, 'prog', 'prog.o', 'gcc ...'
Rule.new Phony, 'all', 'prog'
Suffix.new Real, '.o', '.c', 'gcc -c ...'

Multiple kinds of rules can instantiate multiple types of
targets. So, it is not possible to derive one from the
other.

Further, the suffix rule has to determine the target name
before it holds enough information to instantiate the target
itself.

(The whole program will be available soon.)

Bertram

 

[Robert Klemme]

Hi Robert,

Am Dienstag, 18. Jan 2005, 17:01:12 +0900 schrieb Robert Klemme:

Hm, seems to be a long time since I designed this. I
remember having thought very long and thoroughly about it.


I think I didn't tell the whole truth above. My code
actually looks like

Rule.new Real, 'prog', 'prog.o', 'gcc ...'
Rule.new Phony, 'all', 'prog'
Suffix.new Real, '.o', '.c', 'gcc -c ...'

Multiple kinds of rules can instantiate multiple types of
targets. So, it is not possible to derive one from the
other.

Further, the suffix rule has to determine the target name
before it holds enough information to instantiate the target
itself.

Ok, I see.

Kind regards

robert