# Best way to dynamically create new instance methods

**URL:** <https://www.ruby-forum.com/t/best-way-to-dynamically-create-new-instance-methods/79047>\
**Category:** Ruby\
**Created:** [December 11, 2006, 9:28pm UTC](https://www.ruby-forum.com/t/best-way-to-dynamically-create-new-instance-methods/79047 "2006-12-11T21:28:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dandante](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9fc29f/32.png) [@dandante](https://www.ruby-forum.com/u/dandante)\
**Post date:** [December 11, 2006, 9:28pm UTC](https://www.ruby-forum.com/t/best-way-to-dynamically-create-new-instance-methods/79047/1 "2006-12-11T21:28:09Z")

</div>

Hi, I’ve been sifting through module\_eval, instance\_eval, define\_method,  
and the like and I can’t seem to accomplish what I am trying to do,  
which is something like this:

class Foo  
def initialize  
list\_of\_method\_names =  
get\_list\_of\_method\_names\_from\_some\_external\_function  
for method\_name in list\_of\_method\_names\_do  
create\_method(method\_name)  
end  
end

def create\_method(method\_name)  
# some magic here as yet unknown  
end

end

The list retrieval is in initialize() because the list is constantly  
changing so I always want the most recent one when I initialize my  
class.

What I want create\_method() to do (simplified here) is, given the  
argument “baz”, add a method to the current instance of Foo that would  
look like this if it were a traditional method (that is, not dynamically  
created):

def baz(\*args)  
args[0]  
end

That seems pretty simple but I have run into all sorts of problems with  
various ways of doing it. Rather than recount all those ways and why  
they didn’t work, I instead present the minimal example of the problem I  
am trying to solve and hope that someone can solve it.

Thanks in advance.  
Dan

---

<div class="post-metadata">

**Author:** ![dandante](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9fc29f/32.png) [@dandante](https://www.ruby-forum.com/u/dandante)\
**Post date:** [December 11, 2006, 10:25pm UTC](https://www.ruby-forum.com/t/best-way-to-dynamically-create-new-instance-methods/79047/2 "2006-12-11T22:25:57Z")

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Dan T. wrote:

> That seems pretty simple but I have run into all sorts of problems with  
> various ways of doing it. Rather than recount all those ways and why  
> they didn’t work, I instead present the minimal example of the problem I  
> am trying to solve and hope that someone can solve it.

Rather than try to sort out a way to create methods on the fly, perhaps  
it  
would be better if you were to explain what you are trying to do …  
what  
problem does this approach solve?

There are any number of ways to produce results for given input data,  
and  
creating new methods for a class at runtime is not necessarily the most  
efficient way to accomplish your goal.

What I am saying is you are now struggling to solve, not the original  
problem your program is meant to solve, but the problems created by your  
solution.

One possible alternative is a hash with keys consisting of identifying  
labels, and values consisting of lambdas that can produce various kinds  
of  
results. The lambdas could easily stand in for your custom methods, and  
this approach might end up being easier to implement.

Example:

* * *

#!/usr/bin/ruby -w

hash = {  
“+” =\> lambda { |x,y| x + y },  
“-” =\> lambda { |x,y| x - y },  
“\*” =\> lambda { |x,y| x \* y },  
“/” =\> lambda { |x,y| x / y },  
}

puts hash["/"].call(1.0,3.0)

* * *

Output:

0.333333333333333

Depending on what you are trying to do, this might be easier to use and  
might be faster as well, especially if you take your development time  
into  
account.

---

<div class="post-metadata">

**Author:** ![dandante](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9fc29f/32.png) [@dandante](https://www.ruby-forum.com/u/dandante)\
**Post date:** [December 11, 2006, 11:16pm UTC](https://www.ruby-forum.com/t/best-way-to-dynamically-create-new-instance-methods/79047/3 "2006-12-11T23:16:17Z")

</div>

On Dec 11, 2006, at 2:28 PM, Dan T. wrote:

> What I want create\_method() to do (simplified here) is, given the  
> argument “baz”, add a method to the current instance of Foo that would  
> look like this if it were a traditional method (that is, not  
> dynamically  
> created):
> 
> def baz(\*args)  
> args[0]  
> end

> > class Dynamic  
> > def initialize(name)  
> > create\_method(name)  
> > end  
> > private  
> > def create\_method(name)  
> > class \<\< self; self end.send(:define\_method, name) { |\*args|  
> > args.first }
> 
> > end  
> > end  
> > =\> nil
> 
> > foo = Dynamic.new(:foo)=\> #Dynamic:0x71dbd4  
> > bar = Dynamic.new(:bar)  
> > =\> #Dynamic:0x71b3fc
> 
> > foo.foo(“Hi”)  
> > =\> “Hi”
> 
> > foo.bar(“Hi”)  
> > NoMethodError: undefined method `bar’ for #Dynamic:0x71dbd4  
> > from (irb):33
> 
> > bar.bar(“Hi”)  
> > =\> “Hi”

Hope that helps.

James Edward G. II