Question

我在学习Ruby时正在研究一个简单的Pi Generator，但我在RubyMine 6.3.3上一直得到NoMethodError，所以我决定尽可能简单地创建一个新项目和新类，并且我 STILL < / strong>获取NoMethodError。有什么原因吗？

class Methods def hello (player) print "Hello, " << player end hello ("Annie") end

我得到的错误是：

C:/Users/Annie the Eagle/Documents/Coding/Ruby/Learning Environment/methods.rb:5:in `<class:Methods>': undefined method `hello' for Methods:Class (NoMethodError)

Answer 1

您已定义了实例方法，并尝试将其称为类的方法。因此，您需要使方法hello成为类方法，而不是类Methods的实例方法。

class Methods def self.hello(player) print "Hello, " << player end hello("Annie") end

或者，如果您想将其定义为实例方法，请按以下方式调用它：

class Methods def hello(player) print "Hello, " << player end end Methods.new.hello("Annie")

Answer 2

您尝试将实例方法作为类方法调用。

这里有一些代码说明了ruby中两者之间的区别：

class Person

  # This is a class method - note it's prefixed by self
  # (which in this context refers to the Person class)
  def self.species
    puts 'Human'
    # Note: species is OK as a class method because it's the same 
    # for all instances of the person class - ie, 'Bob', 'Mary', 
    # 'Peggy-Sue', and whoever else, are ALL Human.
  end

  # The methods below aren't prefixed with self., and are
  # therefore instance methods

  # This is the construct, called automatically when
  # a new object is created
  def initialize(name)
    # @name is an instance variable
    @name = name
  end

  def say_hello
    puts "Hello from #{@name}"
  end

end

现在尝试一下，调用方法......

# Call a class method...
# We're not referring to any one 'instance' of Person,
Person.species #=> 'Human'

# Create an instance
bob = Person.new('Bob')

# Call a method on the 'Bob' instance
bob.say_hello #=> 'Hello from Bob'

# Call a method on the Person class, going through the bob instance
bob.class.species #=> 'Human'

# Try to call the class method directly on the instance
bob.species #=> NoMethodError

# Try to call the instance method on the class
# (this is the error you are getting)
Person.say_hello #=> NoMethodError

Answer 3

您已经创建了一个实例方法，但是您正在调用一个类方法。要调用hello("Annie")，您必须创建一个方法实例。例如：

class Methods
  def self.hello(player)
      print "Hello, " << player
  end
end

my_method = Methods.new
my_method.hello("Annie")

这会输出Hello, Annie

Answer 4

通过使用 def method_name args 定义方法，您定义的实例方法将包含在该类的每个对象中，但不包含在类本身中。

另一方面，通过 def self.method_name args ，您将获得一个直接在类中的类方法，而无需从中实例化对象。

所以如果你有这个：

Class Test
   def self.bar
   end

   def foo
   end

end

您可以这样执行实例方法：

a = Test.new
a.foo

至于班级应该是：

Test.foo

Ruby：NoMethodError，但为什么呢？

4 个答案: