Difference with capturing the block, yield with an argument, and yield

block_yield.rb

class WithBlockVar
  attr_accessor :z
  def initialize(x, &blk)
    @x = x
    instance_eval(&blk) if block_given?
  end
end

# Using block variable
a = WithBlockVar.new("A") do
  self.z = "Z"
end
puts a.inspect
# => #<WithBlockVar:0x007fa27c169470 @x="A", @z="Z">


class WithYieldArg
  attr_accessor :z
  def initialize(x)
    @x = x
    yield self if block_given?
  end
end

# Using yield and block arg
b = WithYieldArg.new("B") do |b|
  b.z = "Z"
end
puts b.inspect
# => #<WithYieldArg:0x007fa27c169218 @x="B", @z="Z">


class WithYield
  attr_accessor :z
  def initialize(x)
    @x = x
    yield if block_given?
  end
end

# Using yield
c = WithYield.new("C") do
  self.z = "Z"
end
puts c.inspect
# => block_yield.rb:39:in `block in <main>': undefined method `z=' for main:Object (NoMethodError)
#         from block_yield.rb:21:in `initialize'
#         from block_yield.rb:38:in `new'
#         from block_yield.rb:38:in `<main>'