如何在ruby中从外部访问类变量？

Question

我正在尝试从类外部的方法访问类变量。

这是我的班级：

class Book

  @@bookCount = 0
  @@allBooks = []

  def self.allBooks
    @@allBooks
  end

  def self.bookCount
    @@bookCount
  end

  attr_accessor :name,:author,:date,:genre,:rating

  def initialize(name, author, date, genre, rating)
      @name = name
      @author = author
      @date = date
      @genre = genre
      @rating = rating
      @@bookCount += 1
      @@allBooks << self
  end

end

这是尝试访问类变量@@ bookCount的方法

def seeBookShelf
  if @@bookCount == 0
    puts "Your bookshelf is empty."
  else
    puts "You have " + @bookCount + " books in your bookshelf:"
    puts allBooks
  end
end

当我尝试执行该方法时，我得到了这个：

undefined local variable or method `bookCount' for main:Object (NameError)

如何从外部访问bookCount？

Answer 1

使用class_variable_get访问类外的类变量：

class Foo
  @@a = 1
end

Foo.class_variable_get(:@@a)
=> 1

Answer 2

对于大多数情况，类实例变量优先于类变量。 当与继承一起使用时，后者容易出现各种奇怪的行为。

考虑：

class Book
  @book_count = 0
  @all_books = []

  class << self
    attr_reader :book_count
    attr_reader :all_books
  end

  # further code omitted.

end

使用此代码Book.book_count和Book.all_books获取预期数据。

Answer 3

您可以使用class_eval到具体的类别范围内评估的代码块：

class Book
  @@bookCount = 1
end

Book.class_eval '@@bookCount'
# => 1

而且只是为了好玩......你可以用class_eval实际上做各种各样的class_eval例如在类中定义一个没有猴子修补的新方法：

Book.class_eval { @@bookCount = 5 }
Book.class_eval '@@bookCount'
# => 5

Book.class_eval do
  def self.hey_look_a_new_method
    return "wow"
  end
end

Book.hey_look_a_new_method
# => "wow"

Answer 4

您必须指定变量的类：

def seeBookShelf
  if Book.bookCount == 0
    puts "Your bookshelf is empty."
  else
    puts "You have " + Book.bookCount + " books in your bookshelf:"
    puts Book.allBooks
  end
end

Answer 5

你需要一个getter来访问类变量，试试这个代码。 有关说明，请参阅http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/ 。 你也最好使用字符串插值，否则你会得到一个Type错误，也就是更多的Rubyesque。

class Book
  @@bookCount = 0

  def self.bookCount
    @@bookCount
  end
end

def seeBookShelf
  if Book.bookCount == 0
    puts "Your bookshelf is empty."
  else
    puts "You have #{Book.bookCount} books in your bookshelf:"
  end
end

seeBookShelf # Your bookshelf is empty.