class RuboCop::Cop::Naming::MemoizedInstanceVariableName

Checks for memoized methods whose instance variable name does not match the method name. Applies to both regular methods (defined with ‘def`) and dynamic methods (defined with `define_method` or `define_singleton_method`).

This cop can be configured with the EnforcedStyleForLeadingUnderscores directive. It can be configured to allow for memoized instance variables prefixed with an underscore. Prefixing ivars with an underscore is a convention that is used to implicitly indicate that an ivar should not be set or referenced outside of the memoization method.

@safety

This cop relies on the pattern `@instance_var ||= ...`,
but this is sometimes used for other purposes than memoization
so this cop is considered unsafe. Also, its autocorrection is unsafe
because it may conflict with instance variable names already in use.

@example EnforcedStyleForLeadingUnderscores: disallowed (default)

# bad
# Method foo is memoized using an instance variable that is
# not `@foo`. This can cause confusion and bugs.
def foo
  @something ||= calculate_expensive_thing
end

def foo
  return @something if defined?(@something)
  @something = calculate_expensive_thing
end

# good
def _foo
  @foo ||= calculate_expensive_thing
end

# good
def foo
  @foo ||= calculate_expensive_thing
end

# good
def foo
  @foo ||= begin
    calculate_expensive_thing
  end
end

# good
def foo
  helper_variable = something_we_need_to_calculate_foo
  @foo ||= calculate_expensive_thing(helper_variable)
end

# good
define_method(:foo) do
  @foo ||= calculate_expensive_thing
end

# good
define_method(:foo) do
  return @foo if defined?(@foo)
  @foo = calculate_expensive_thing
end

@example EnforcedStyleForLeadingUnderscores: required

# bad
def foo
  @something ||= calculate_expensive_thing
end

# bad
def foo
  @foo ||= calculate_expensive_thing
end

def foo
  return @foo if defined?(@foo)
  @foo = calculate_expensive_thing
end

# good
def foo
  @_foo ||= calculate_expensive_thing
end

# good
def _foo
  @_foo ||= calculate_expensive_thing
end

def foo
  return @_foo if defined?(@_foo)
  @_foo = calculate_expensive_thing
end

# good
define_method(:foo) do
  @_foo ||= calculate_expensive_thing
end

# good
define_method(:foo) do
  return @_foo if defined?(@_foo)
  @_foo = calculate_expensive_thing
end

@example EnforcedStyleForLeadingUnderscores :optional

# bad
def foo
  @something ||= calculate_expensive_thing
end

# good
def foo
  @foo ||= calculate_expensive_thing
end

# good
def foo
  @_foo ||= calculate_expensive_thing
end

# good
def _foo
  @_foo ||= calculate_expensive_thing
end

# good
def foo
  return @_foo if defined?(@_foo)
  @_foo = calculate_expensive_thing
end

# good
define_method(:foo) do
  @foo ||= calculate_expensive_thing
end

# good
define_method(:foo) do
  @_foo ||= calculate_expensive_thing
end