blocks|key|192974|text|你的想法是对的，但你说它的方式是错的。它应该是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|192975|raise+MyCustomError.new(an_object,+"A+message")|code-block|syntax|javascript|192976|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Your idea is right, but the way you call it is wrong. It should be

<pre><code>raise MyCustomError.new(an_object, "A message")
</code></pre>

blocks|key|3024649|text|raise已经设置了消息，所以您不必将其传递给构造函数：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3024650|class+MyCustomError+<+StandardError
++attr_reader+:object

++def+initialize(object)
++++@object+=+object
++end
end

begin
++raise+MyCustomError.new("an+object"),+"a+message"
rescue+MyCustomError+=>+e
++puts+e.message+#+=>+"a+message"
++puts+e.object+#+=>+"an+object"
end|code-block|syntax|javascript|3024651|我已经用rescue+MyCustomError替换了rescue+Exception，参见Why+is+it+a+bad+style+to+rescue+Exception+=>+e+in+Ruby?。|3024652|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/10048173/why-is-it-bad-style-to-rescue-exception-e-in-ruby^0|0|5|0|0|4|K|R|G|1Z|L|1A|1J|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]]|E|$]]|$1|M|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

<code>raise</code> already sets the message so you don't have to pass it to the constructor:

<pre><code>class MyCustomError &lt; StandardError
 attr_reader :object

 def initialize(object)
 @object = object
 end
end

begin
 raise MyCustomError.new("an object"), "a message"
rescue MyCustomError =&gt; e
 puts e.message # =&gt; "a message"
 puts e.object # =&gt; "an object"
end
</code></pre>

I've replaced <code>rescue Exception</code> with <code>rescue MyCustomError</code>, see <a href="https://stackoverflow.com/questions/10048173/why-is-it-bad-style-to-rescue-exception-e-in-ruby">Why is it a bad style to `rescue Exception =&gt; e` in Ruby?</a>.

blocks|key|3024657|text|我想做一些类似的事情。我希望将一个对象传递给#new，并根据传递的对象的一些处理来设置消息。以下是有效的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3024658|class+FooError+<+StandardError
++attr_accessor+:message+#+this+is+critical!
++def+initialize(stuff)
++++@message+=+stuff.reverse
++end
end

begin
++raise+FooError.new("!dlroW+olleH")
rescue+FooError+=>+e
++puts+e.message+#=>+Hello+World!
end|code-block|syntax|javascript|3024659|请注意，如果您不声明attr_accessor+:message，那么它将无法工作。为了解决OP的问题，您还可以将消息作为附加参数传递，并存储您喜欢的任何内容。关键部分似乎是重写#message。|offset|length|style|CODE|3024660|entityMap^0|0|0|A|M|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I wanted to do something similar. I wanted to pass an object to #new and have the message set based on some processing of the passed object. The following works.

<pre><code>class FooError &lt; StandardError
 attr_accessor :message # this is critical!
 def initialize(stuff)
 @message = stuff.reverse
 end
end

begin
 raise FooError.new("!dlroW olleH")
rescue FooError =&gt; e
 puts e.message #=&gt; Hello World!
end
</code></pre>

Note that if you don't declare <code>attr_accessor :message</code> then it will not work. Addressing the OP's issue, you could also pass the message as an additional argument and store anything you like. The crucial part appears to be overriding #message.

blocks|key|190849|text|鉴于所有其他错误都继承自Exception的ruby核心文档对#message的描述|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|190850|190851|返回调用exception.to_s的结果。通常会返回异常的消息或名称。通过提供to_str方法，异常同意在需要字符串的地方使用。|blockquote|190852|190853|190854|190855|我会选择重新定义to_s/to_str或初始化器。下面是一个示例，我们希望以一种人类可读的方式知道外部服务何时做不到某件事。|190856|注意:下面的第二个策略使用了rails的字符串方法，比如demodualize，这可能有点复杂，因此在异常中这样做可能是不明智的。如果需要，您还可以向方法签名添加更多参数。|190857|覆盖#to_s策略而不是#to_str，它的工作方式不同|BOLD|190858|module+ExternalService

++class+FailedCRUDError+<+::StandardError
++++def+to_s
++++++'failed+to+crud+with+external+service'
++++end
++end

++class+FailedToCreateError+<+FailedCRUDError;+end
++class+FailedToReadError+<+FailedCRUDError;+end
++class+FailedToUpdateError+<+FailedCRUDError;+end
++class+FailedToDeleteError+<+FailedCRUDError;+end
end|code-block|syntax|javascript|190859|控制台输出|190860|begin;+raise+ExternalService::FailedToCreateError;+rescue+=>+e;+e.message;+end
#+=>+"failed+to+crud+with+external+service"

begin;+raise+ExternalService::FailedToCreateError,+'custom+message';+rescue+=>+e;+e.message;+end
#+=>+"failed+to+crud+with+external+service"

begin;+raise+ExternalService::FailedToCreateError.new('custom+message');+rescue+=>+e;+e.message;+end
#+=>+"failed+to+crud+with+external+service"

raise+ExternalService::FailedToCreateError
#+ExternalService::FailedToCreateError:+failed+to+crud+with+external+service|190861|覆盖#initialize+Strategy|190862|这是我在rails中使用过的最接近实现的策略。如上所述，它使用demodualize、underscore和humanize+ActiveSupport方法。但这可以很容易地删除，就像在之前的策略中一样。|190863|module+ExternalService
++class+FailedCRUDError+<+::StandardError
++++def+initialize(service_model=nil)
++++++super("#{self.class.name.demodulize.underscore.humanize}+using+#{service_model.class}")
++++end
++end

++class+FailedToCreateError+<+FailedCRUDError;+end
++class+FailedToReadError+<+FailedCRUDError;+end
++class+FailedToUpdateError+<+FailedCRUDError;+end
++class+FailedToDeleteError+<+FailedCRUDError;+end
end|190864|190865|begin;+raise+ExternalService::FailedToCreateError;+rescue+=>+e;+e.message;+end
#+=>+"Failed+to+create+error+using+NilClass"

begin;+raise+ExternalService::FailedToCreateError,+Object.new;+rescue+=>+e;+e.message;+end
#+=>+"Failed+to+create+error+using+Object"

begin;+raise+ExternalService::FailedToCreateError.new(Object.new);+rescue+=>+e;+e.message;+end
#+=>+"Failed+to+create+error+using+Object"

raise+ExternalService::FailedCRUDError
#+ExternalService::FailedCRUDError:+Failed+crud+error+using+NilClass

raise+ExternalService::FailedCRUDError.new(Object.new)
#+RuntimeError:+ExternalService::FailedCRUDError+using+Object|190866|演示工具|190867|这是一个演示，展示了上述实现的救援和消息传递。引发异常的类是Cloudinary的假API。只需将上述策略之一转储到您的rails控制台，然后执行以下操作。|190868|require+'rails'+#+only+needed+for+second+strategy+

module+ExternalService
++class+FailedCRUDError+<+::StandardError
++++def+initialize(service_model=nil)
++++++@service_model+=+service_model
++++++super("#{self.class.name.demodulize.underscore.humanize}+using+#{@service_model.class}")
++++end
++end

++class+FailedToCreateError+<+FailedCRUDError;+end
++class+FailedToReadError+<+FailedCRUDError;+end
++class+FailedToUpdateError+<+FailedCRUDError;+end
++class+FailedToDeleteError+<+FailedCRUDError;+end
end

#+Stub+service+representing+3rd+party+cloud+storage
class+Cloudinary

++def+initialize(*error_args)
++++@error_args+=+error_args.flatten
++end

++def+create_read_update_or_delete
++++begin
++++++try_and_fail
++++rescue+ExternalService::FailedCRUDError+=>+e
++++++e.message
++++end
++end

++private+def+try_and_fail
++++raise+*@error_args
++end
end

errors_map+=+[
++#+Without+an+arg
++ExternalService::FailedCRUDError,
++ExternalService::FailedToCreateError,
++ExternalService::FailedToReadError,
++ExternalService::FailedToUpdateError,
++ExternalService::FailedToDeleteError,
++#+Instantiated+without+an+arg
++ExternalService::FailedCRUDError.new,
++ExternalService::FailedToCreateError.new,
++ExternalService::FailedToReadError.new,
++ExternalService::FailedToUpdateError.new,
++ExternalService::FailedToDeleteError.new,
++#+With+an+arg
++[ExternalService::FailedCRUDError,+Object.new],
++[ExternalService::FailedToCreateError,+Object.new],
++[ExternalService::FailedToReadError,+Object.new],
++[ExternalService::FailedToUpdateError,+Object.new],
++[ExternalService::FailedToDeleteError,+Object.new],
++#+Instantiated+with+an+arg
++ExternalService::FailedCRUDError.new(Object.new),
++ExternalService::FailedToCreateError.new(Object.new),
++ExternalService::FailedToReadError.new(Object.new),
++ExternalService::FailedToUpdateError.new(Object.new),
++ExternalService::FailedToDeleteError.new(Object.new),
].inject({})+do+%7Cerrors,+args%7C
++begin+
++++errors.merge!(+args+=>+Cloudinary.new(args).create_read_update_or_delete)
++rescue+=>+e
++++binding.pry
++end
end

if+defined?(pp)+%7C%7C+require('pp')
++pp+errors_map
else
++errors_map.each{+%7Cset%7C+puts+set.inspect+}
end|190869|entityMap^0|C|9|V|8|0|0|0|0|0|0|8|4|D|6|0|S|B|0|0|9|0|0|0|0|0|M|0|V|B|17|A|1I|8|1R|D|0|0|0|0|0|4|0|0|0^^$0|@$1|2|3|4|5|6|7|1J|8|@$9|1K|A|1L|B|C]|$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|1O|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|1P|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|1Q|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1S|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|1T|8|@$9|1U|A|1V|B|C]|$9|1W|A|1X|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|1Y|8|@$9|1Z|A|20|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|21|8|@$9|22|A|23|B|S]]|D|@]|E|$]]|$1|T|3|U|5|V|7|24|8|@]|D|@]|E|$W|X]]|$1|Y|3|Z|5|6|7|25|8|@]|D|@]|E|$]]|$1|10|3|11|5|V|7|26|8|@]|D|@]|E|$W|X]]|$1|12|3|13|5|6|7|27|8|@$9|28|A|29|B|S]]|D|@]|E|$]]|$1|14|3|15|5|6|7|2A|8|@$9|2B|A|2C|B|C]|$9|2D|A|2E|B|C]|$9|2F|A|2G|B|C]|$9|2H|A|2I|B|C]]|D|@]|E|$]]|$1|16|3|17|5|V|7|2J|8|@]|D|@]|E|$W|X]]|$1|18|3|Z|5|6|7|2K|8|@]|D|@]|E|$]]|$1|19|3|1A|5|V|7|2L|8|@]|D|@]|E|$W|X]]|$1|1B|3|1C|5|6|7|2M|8|@$9|2N|A|2O|B|S]]|D|@]|E|$]]|$1|1D|3|1E|5|6|7|2P|8|@]|D|@]|E|$]]|$1|1F|3|1G|5|V|7|2Q|8|@]|D|@]|E|$W|X]]|$1|1H|3|-4|5|6|7|2R|8|@]|D|@]|E|$]]]|1I|$]]

Given what the ruby core documentation of <code>Exception</code>, from which all other errors inherit, states about <code>#message</code>

<blockquote>
 Returns the result of invoking exception.to_s. Normally this returns
 the exception’s message or name. By supplying a to_str method,
 exceptions are agreeing to be used where Strings are expected.
</blockquote>

<a href="http://ruby-doc.org/core-1.9.3/Exception.html#method-i-message" rel="noreferrer">http://ruby-doc.org/core-1.9.3/Exception.html#method-i-message</a>

I would opt for redefining <code>to_s</code>/<code>to_str</code> or the initializer. Here is an example where we want to know, in a mostly human readable way, when an external service has failed to do something. 

NOTE: The second strategy below uses the rails pretty string methods, such as <code>demodualize</code>, which may be a little complicated and therefore potentially unwise to do in an exception. You could also add more arguments to the method signature, should you need.

Overriding #to_s Strategy not #to_str, it works differently

<pre><code>module ExternalService

 class FailedCRUDError &lt; ::StandardError
 def to_s
 'failed to crud with external service'
 end
 end

 class FailedToCreateError &lt; FailedCRUDError; end
 class FailedToReadError &lt; FailedCRUDError; end
 class FailedToUpdateError &lt; FailedCRUDError; end
 class FailedToDeleteError &lt; FailedCRUDError; end
end
</code></pre>

Console Output 

<pre><code>begin; raise ExternalService::FailedToCreateError; rescue =&gt; e; e.message; end
# =&gt; "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError, 'custom message'; rescue =&gt; e; e.message; end
# =&gt; "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError.new('custom message'); rescue =&gt; e; e.message; end
# =&gt; "failed to crud with external service"

raise ExternalService::FailedToCreateError
# ExternalService::FailedToCreateError: failed to crud with external service
</code></pre>

Overriding #initialize Strategy

This is the strategy closest to implementations I've used in rails. As noted above, it uses the <code>demodualize</code>, <code>underscore</code>, and <code>humanize</code> <code>ActiveSupport</code> methods. But this could be easily removed, as in the previous strategy.

<pre><code>module ExternalService
 class FailedCRUDError &lt; ::StandardError
 def initialize(service_model=nil)
 super("#{self.class.name.demodulize.underscore.humanize} using #{service_model.class}")
 end
 end

 class FailedToCreateError &lt; FailedCRUDError; end
 class FailedToReadError &lt; FailedCRUDError; end
 class FailedToUpdateError &lt; FailedCRUDError; end
 class FailedToDeleteError &lt; FailedCRUDError; end
end
</code></pre>

Console Output 

<pre><code>begin; raise ExternalService::FailedToCreateError; rescue =&gt; e; e.message; end
# =&gt; "Failed to create error using NilClass"

begin; raise ExternalService::FailedToCreateError, Object.new; rescue =&gt; e; e.message; end
# =&gt; "Failed to create error using Object"

begin; raise ExternalService::FailedToCreateError.new(Object.new); rescue =&gt; e; e.message; end
# =&gt; "Failed to create error using Object"

raise ExternalService::FailedCRUDError
# ExternalService::FailedCRUDError: Failed crud error using NilClass

raise ExternalService::FailedCRUDError.new(Object.new)
# RuntimeError: ExternalService::FailedCRUDError using Object
</code></pre>

Demo Tool

This is a demo to show rescuing and messaging of the above implementation. The class raising the exceptions is a fake API to Cloudinary. Just dump one of the above strategies into your rails console, followed by this.

<pre><code>require 'rails' # only needed for second strategy 

module ExternalService
 class FailedCRUDError &lt; ::StandardError
 def initialize(service_model=nil)
 @service_model = service_model
 super("#{self.class.name.demodulize.underscore.humanize} using #{@service_model.class}")
 end
 end

 class FailedToCreateError &lt; FailedCRUDError; end
 class FailedToReadError &lt; FailedCRUDError; end
 class FailedToUpdateError &lt; FailedCRUDError; end
 class FailedToDeleteError &lt; FailedCRUDError; end
end

# Stub service representing 3rd party cloud storage
class Cloudinary

 def initialize(*error_args)
 @error_args = error_args.flatten
 end

 def create_read_update_or_delete
 begin
 try_and_fail
 rescue ExternalService::FailedCRUDError =&gt; e
 e.message
 end
 end

 private def try_and_fail
 raise *@error_args
 end
end

errors_map = [
 # Without an arg
 ExternalService::FailedCRUDError,
 ExternalService::FailedToCreateError,
 ExternalService::FailedToReadError,
 ExternalService::FailedToUpdateError,
 ExternalService::FailedToDeleteError,
 # Instantiated without an arg
 ExternalService::FailedCRUDError.new,
 ExternalService::FailedToCreateError.new,
 ExternalService::FailedToReadError.new,
 ExternalService::FailedToUpdateError.new,
 ExternalService::FailedToDeleteError.new,
 # With an arg
 [ExternalService::FailedCRUDError, Object.new],
 [ExternalService::FailedToCreateError, Object.new],
 [ExternalService::FailedToReadError, Object.new],
 [ExternalService::FailedToUpdateError, Object.new],
 [ExternalService::FailedToDeleteError, Object.new],
 # Instantiated with an arg
 ExternalService::FailedCRUDError.new(Object.new),
 ExternalService::FailedToCreateError.new(Object.new),
 ExternalService::FailedToReadError.new(Object.new),
 ExternalService::FailedToUpdateError.new(Object.new),
 ExternalService::FailedToDeleteError.new(Object.new),
].inject({}) do |errors, args|
 begin 
 errors.merge!( args =&gt; Cloudinary.new(args).create_read_update_or_delete)
 rescue =&gt; e
 binding.pry
 end
end

if defined?(pp) || require('pp')
 pp errors_map
else
 errors_map.each{ |set| puts set.inspect }
end
</code></pre>

I can't seem to find much information about custom exception classes.

What I do know

You can declare your custom error class and let it inherit from <code>StandardError</code>, so it can be <code>rescue</code>d:

<pre><code>class MyCustomError &lt; StandardError
end
</code></pre>

This allows you to raise it using:

<pre><code>raise MyCustomError, "A message"
</code></pre>

and later, get that message when rescuing

<pre><code>rescue MyCustomError =&gt; e
 puts e.message # =&gt; "A message"
</code></pre>

What I don't know

I want to give my exception some custom fields, but I want to inherit the <code>message</code> attribute from the parent class. I found out reading <a href="http://www.ruby-forum.com/topic/207448" rel="noreferrer">on this topic</a> that <code>@message</code> is not an instance variable of the exception class, so I'm worried that my inheritance won't work.

Can anyone give me more details to this? How would I implement a custom error class with an <code>object</code> attribute? Is the following correct:

<pre><code>class MyCustomError &lt; StandardError
 attr_reader :object
 def initialize(message, object)
 super(message)
 @object = object
 end
end
</code></pre>

And then:

<pre><code>raise MyCustomError.new(anObject), "A message"
</code></pre>

to get:

<pre><code>rescue MyCustomError =&gt; e
 puts e.message # =&gt; "A message"
 puts e.object # =&gt; anObject
</code></pre>

will it work, and if it does, is this the correct way of doing things?

Ruby custom error classes: inheritance of the message attribute

我似乎找不到太多关于自定义异常类的信息。我所知道的您可以声明您的自定义error类，并让它从StandardError继承，因此它可以是rescued：class MyCustomError < StandardErrorend这使您可以使用以下命令来引发它：raise MyCustomError, "A messag...

问Ruby自定义错误类:消息属性的继承
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