Dynamic method calling in Ruby

问题:

As far as I am aware there are three ways to dynamically call a method in Ruby:

Method 1:

s = SomeObject.new
method = s.method(:dynamic_method)
method.call

Method 2:

s = SomeObject.new
s.send(:dynamic_method)

Method 3:

s = SomeObject.new
eval "s.dynamic_method"

By benchmarking them I have established that Method 1 is by far the fastest, Method 2 is slower, and Method 3 is by far the slowest.

I have also found that .call and .send both allow calling private methods, while eval does not.

So my question is: is there any reason to ever use .send or eval? Why would you not always just use the fastest method? What other differences do these methods of calling dynamic methods have?

回答1:

is there any reason to ever use send?

call needs a method object, send doesn't:

class Foo
  def method_missing(name)
    "#{name} called"
  end
end

Foo.new.send(:bar)         #=> "bar called"
Foo.new.method(:bar).call  #=> undefined method `bar' for class `Foo' (NameError)

is there any reason to ever use eval?

eval evaluates arbitrary expressions, it's not just for calling a method.

---

Regarding benchmarks, send seems to be faster than method + call:

require 'benchmark'

class Foo
  def bar; end
end

Benchmark.bm(4) do |b|
  b.report("send") { 1_000_000.times { Foo.new.send(:bar) } }
  b.report("call") { 1_000_000.times { Foo.new.method(:bar).call } }
end

Result:

           user     system      total        real
send   0.210000   0.000000   0.210000 (  0.215181)
call   0.740000   0.000000   0.740000 (  0.739262)

回答2:

Think of it this way:

Method 1 (method.call): Single run-time

If you run Ruby once on your program straight through, you control the entire system and you can hold onto a "pointer to your method" via the "method.call" approach. All you are doing is holding on to a handle to "live code" that you can run whenever you want. This is basically as fast as calling the method directly from within the object (but it is not as fast as using object.send - see benchmarks in other answers).

Method 2 (object.send): Persist name of method to database

But what if you want to store the name of the method you want to call in a database and in a future application you want to call that method name by looking it up in the database? Then you would use the second approach, which causes ruby to call an arbitrary method name using your second "s.send(:dynamic_method)" approach.

Method 3 (eval): Self-modifying method code

What if you want to write/modify/persist code to a database in a way that will run the method as brand new code? You might periodically modify the code written to the database and want it to run as new code each time. In this (very unusual case) you would want to use your third approach, which allows you to write your method code out as a string, load it back in at some later date, and run it in its entirety.

For what it's worth, generally it is regarded in the Ruby world as bad form to use Eval (method 3) except in very, very esoteric and rare cases. So you should really stick with methods 1 and 2 for almost all problems you encounter.

回答3:

I updated the benchmark from @Stefan to check if there are some speed improvements when saving reference to method. But again – send is much faster than call

require 'benchmark'

class Foo
  def bar; end
end

foo = Foo.new
foo_bar = foo.method(:bar)

Benchmark.bm(4) do |b|
  b.report("send") { 1_000_000.times { foo.send(:bar) } }
  b.report("call") { 1_000_000.times { foo_bar.call } }
end

These are the results:

           user     system      total        real
send   0.080000   0.000000   0.080000 (  0.088685)
call   0.110000   0.000000   0.110000 (  0.108249)

So send seems to be the one to take.

回答4:

Here is all possible method calls:

require 'benchmark/ips'

class FooBar
  def name; end
end

el = FooBar.new

Benchmark.ips do |x|
  x.report('plain') { el.name }
  x.report('eval') { eval('el.name') }
  x.report('method call') { el.method(:name).call }
  x.report('send sym') { el.send(:name) }
  x.report('send str') { el.send('name') }
  x.compare!
end

And results are:

Warming up --------------------------------------
               plain   236.448k i/100ms
                eval    20.743k i/100ms
         method call   131.408k i/100ms
            send sym   205.491k i/100ms
            send str   168.137k i/100ms
Calculating -------------------------------------
               plain      9.150M (± 6.5%) i/s -     45.634M in   5.009566s
                eval    232.303k (± 5.4%) i/s -      1.162M in   5.015430s
         method call      2.602M (± 4.5%) i/s -     13.009M in   5.010535s
            send sym      6.729M (± 8.6%) i/s -     33.495M in   5.016481s
            send str      4.027M (± 5.7%) i/s -     20.176M in   5.027409s

Comparison:
               plain:  9149514.0 i/s
            send sym:  6729490.1 i/s - 1.36x  slower
            send str:  4026672.4 i/s - 2.27x  slower
         method call:  2601777.5 i/s - 3.52x  slower
                eval:   232302.6 i/s - 39.39x  slower

It's expected that plain call is the fastest, no any additional allocations, symbol lookups, just lookup and evaluation of method.

As for send via symbol, it is faster than via string as its much more easer to allocate memory for symbol. Once it's defined it's stored for long term in memory and there no reallocations.

The same reason can be said about method(:name) (1) it's requires to allocate memory for Proc object (2) we are calling the method in class which leads for additional method lookup which takes time too.

eval is runs interpreter so it's the heaviest.

回答5:

The whole point of send and eval is that you can change the command dynamically. If the method you want to execute is fixed, then you can hard-wire that method without using send or eval.

receiver.fixed_method(argument)

But when you want to invoke a method that varies or you do not know in advance, then you cannot write that directly. Hence the use of send or eval.

receiver.send(method_that_changes_dynamically, argument)
eval "#{code_to_evaluate_that_changes_more_dramatically}"

Additional use of send is that, as you noticed, you can call a method with explicit receiver using send.