what is local scope exactly?

问题:

To be honest, I still confused about the instance variable and local variable, not sure which should be used.

only one condition I know about local variable that can't be used is:

class MyClass
  def initialize
    local_var = 1
    @instance_var = 1
  end

  def show_local_var
   local_var
  end

  def show_instance_var
   @instance_var
  end
end

apparently, MyClass.new.show_instance_var works while MyClass.new_show_local_var not

the other thing about the two kind of variables is that the block seems share the same local scope, so the local variable can be referenced:

local_var = 1

3.times do
  puts local_var
end

There are all I know about the distinctions, is there any other available? please let me know

if there is any articles about this, that would be so helpful for me,

回答1:

A local variable is used "right here right now" and can't be accessed from anywhere else.

class MyClass
  def foo
    local_var = 2
    #I use this to do some sort of calculation
    @instance_var = local_var + 34
    local_var = 5 * @instance_var
    puts local_var
  end
  # From here, local_var is unaccessible.
end

Once you're out of scope (foo's end is passed) local_var is no more and can't be referred to.

The instance variable is available to the whole class at all times.

class MyClass
  def initialize
    @instance_var = 0
  end

  def foo
    local_var = 2
    #I use this to do some sort of calculation
    @instance_var = local_var + 34
  end

  def some_operation
    if @instance_var == 36
      @instance_var = 3
    else
      @instance_var = 1
    end
  end
end

So when you call m = MyClass.new and later on m.some_operation, it's working with the same @instance_var .

And while we're at it, there are also Class variables (defined @@class_var) that are accessible from any instance of the class.

I don't have an article in particular to provide you, but some googling about ruby variable scope and about each type of variable independently should provide you with all the information you need!

回答2:

The second example you describe is called a Closure. Paul puts it quite nicely there:

A closure is a block of code which meets three criteria:

• It can be passed around as a value and
• executed on demand by anyone who has that value, at which time
• it can refer to variables from the context in which it was created (i.e. it is closed with respect to variable access, in the mathematical sense of the word "closed").

For a nice, quick introduction about the available scopes in Ruby you may refer to the Ruby Programming wikibook.

There is

• Local Scope
• Global Scope
• Instance Scope
• Class Scope

The "Default Scope", as it is sometimes referred to when executing code with no scope modifiers surrounding it, as in

@iv = "Who do I belong to?"

is the scope of the 'main' object.

回答3:

Local scope is limited to the location in which the variable is declared, i.e. a function, or a for loop, but cannot be accessed from outside that location. However, if you nest other constructs within the function, for loop, etc. then the inner constructs can access the local variable. Instance variables are scoped to the instance of the class.

Article on ruby variable scope

Article on scope in general