blocks|key|2044262|text|也许你可以用defined来做呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2044263|例如：|2044264|if+defined?(MyClassName)+==+'constant'+&&+MyClassName.class+==+Class++
+++puts+"its+a+class"+
end|code-block|syntax|javascript|2044265|注意:类检查是必填项，例如：|2044266|Hello+=+1+
puts+defined?(Hello)+==+'constant'+#+returns+true|2044267|要回答原来的问题：|2044268|puts+"enter+the+name+of+the+Class+to+see+if+it+exists"
nameofclass=gets.chomp
eval("defined?(#{nameofclass})+==+'constant'+and+#{nameofclass}.class+==+Class")|2044269|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

perhaps you can do it with defined? 

eg:

<pre><code>if defined?(MyClassName) == 'constant' &amp;&amp; MyClassName.class == Class 
 puts "its a class" 
end
</code></pre>

Note: the Class check is required, for example:

<pre><code>Hello = 1 
puts defined?(Hello) == 'constant' # returns true
</code></pre>

To answer the original question: 

<pre><code>puts "enter the name of the Class to see if it exists"
nameofclass=gets.chomp
eval("defined?(#{nameofclass}) == 'constant' and #{nameofclass}.class == Class")
</code></pre>

blocks|key|1830358|text|类名是常量。可以使用defined?方法查看是否定义了常量。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1830359|defined?(String)++++#+=>+"constant"
defined?(Undefined)+#+=>+nil|code-block|syntax|javascript|1830360|如果你感兴趣，你可以阅读更多关于defined?的工作原理。|1830361|entityMap|0|LINK|mutability|MUTABLE|url|http://redhanded.hobix.com/inspect/methodCheckDefined.html^0|A|8|0|0|G|8|G|8|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]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|M|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

Class names are constants. You can use the <code>defined?</code> method to see if a constant has been defined.

<pre><code>defined?(String) # =&gt; "constant"
defined?(Undefined) # =&gt; nil
</code></pre>

You can read more about how <a href="http://redhanded.hobix.com/inspect/methodCheckDefined.html" rel="noreferrer"><code>defined?</code></a> works if you're interested.

blocks|key|2044304|text|您可以使用Module.const_get来获取字符串引用的常量。它将返回常量(通常类由常量引用)。然后，您可以检查该常量是否为类。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2044305|我会按照这样的思路做一些事情：|2044306|def+class_exists?(class_name)
++klass+=+Module.const_get(class_name)
++return+klass.is_a?(Class)
rescue+NameError
++return+false
end|code-block|syntax|javascript|2044307|此外，如果可能，我在接受用户输入时总是避免使用eval；我怀疑这是否会用于任何严肃的应用程序，但值得注意其中的安全风险。|2044308|entityMap^0|5|G|0|0|0|N|4|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

You can use <code>Module.const_get</code> to get the constant referred to by the string. It will return the constant (generally classes are referenced by constants). You can then check to see if the constant is a class.

I would do something along these lines:

<pre><code>def class_exists?(class_name)
 klass = Module.const_get(class_name)
 return klass.is_a?(Class)
rescue NameError
 return false
end
</code></pre>

Also, if possible I would always avoid using <code>eval</code> when accepting user input; I doubt this is going to be used for any serious application, but worth being aware of the security risks.

blocks|key|1830379|text|defined?(DatabaseCleaner)+#+=>+nil
require+'database_cleaner'
defined?(DatabaseCleaner)+#+=>+constant|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1830380|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>defined?(DatabaseCleaner) # =&gt; nil
require 'database_cleaner'
defined?(DatabaseCleaner) # =&gt; constant
</code></pre>

blocks|key|1830386|text|如果您通过调用Module.const_get来查找某个范围内的常量，则可以避免从Module#const_defined?("SomeClass")中抢救NameError。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1830387|调用它的常见作用域是Object，例如：Object.const_defined?("User")。|1830388|参见："Module“。|1830389|entityMap|0|LINK|mutability|MUTABLE|url|http://ruby-doc.org/core/classes/Module.html#M001691^0|7|G|15|Y|0|K|T|0|4|6|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|Z|8|@]|D|@$9|10|A|11|1|12]]|E|$]]|$1|J|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

You can avoid having to rescue the NameError from <code>Module.const_get</code> if you are looking the constant within a certain scope by calling <code>Module#const_defined?("SomeClass")</code>. 

A common scope to call this would be Object, eg: <code>Object.const_defined?("User")</code>.

See: "<a href="http://ruby-doc.org/core/classes/Module.html#M001691" rel="noreferrer">Module</a>".

blocks|key|1830406|text|我用它来查看是否在运行时加载了一个类：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1830407|def+class_exists?(class_name)
++ObjectSpace.each_object(Class)+{%7Cc%7C+return+true+if+c.to_s+==+class_name+}
++false
end|code-block|syntax|javascript|1830408|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|$]]

I used this to see if a class was loaded at runtime:

<pre><code>def class_exists?(class_name)
 ObjectSpace.each_object(Class) {|c| return true if c.to_s == class_name }
 false
end
</code></pre>

blocks|key|1830413|text|下面是一个更简洁的版本：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1830414|def+class_exists?(class_name)
++eval("defined?(#{class_name})+&&+#{class_name}.is_a?(Class)")+==+true
end

class_name+=+"Blorp"
class_exists?(class_name)
=>+false

class_name+=+"String"
class_exists?(class_name)
=>+true|code-block|syntax|javascript|1830415|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|$]]

Here's a more succinct version:

<pre><code>def class_exists?(class_name)
 eval("defined?(#{class_name}) &amp;&amp; #{class_name}.is_a?(Class)") == true
end

class_name = "Blorp"
class_exists?(class_name)
=&gt; false

class_name = "String"
class_exists?(class_name)
=&gt; true
</code></pre>

blocks|key|2044379|text|我有时会做一些事情来解决这个问题。可以将以下方法添加到String类中，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2044380|class+String
++++def+to_class
++++++++my_const+=+Kernel.const_get(self)
++++++++my_const.is_a?(Class)+?+my_const+:+nil
++++rescue+NameError+
++++++++nil
++++end

++++def+is_a_defined_class?
++++++++true+if+self.to_class
++++rescue+NameError
++++++++false
++++end
end|code-block|syntax|javascript|2044381|然后：|2044382|'String'.to_class
=>+String
'unicorn'.to_class
=>+nil
'puppy'.is_a_defined_class?
=>+false
'Fixnum'.is_a_defined_class?
=>+true|2044383|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Here's something I sometimes do to tackle this very issue. You can add the following methods to the String class like so:

<pre><code>class String
 def to_class
 my_const = Kernel.const_get(self)
 my_const.is_a?(Class) ? my_const : nil
 rescue NameError 
 nil
 end

 def is_a_defined_class?
 true if self.to_class
 rescue NameError
 false
 end
end
</code></pre>

Then:

<pre><code>'String'.to_class
=&gt; String
'unicorn'.to_class
=&gt; nil
'puppy'.is_a_defined_class?
=&gt; false
'Fixnum'.is_a_defined_class?
=&gt; true
</code></pre>

blocks|key|1830443|text|如果类没有被加载，我假设你会采取一些措施。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1830444|如果您想要一个文件，为什么不直接检查require的输出|offset|length|style|CODE|1830445|require+'already/loaded'++
=>+false|code-block|syntax|javascript|1830446|entityMap^0|0|I|7|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|S|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I assume you'll take some action if the class is not loaded.

If you mean to require a file, why not just check the output of <code>require</code>?

<pre><code>require 'already/loaded' 
=&gt; false
</code></pre>

blocks|key|2044435|text|只有一行代码，我会这样写：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2044436|!!Module.const_get(nameofclass)+rescue+false|code-block|syntax|javascript|2044437|仅当给定的nameofclass属于已定义的类时才返回true。|offset|length|style|CODE|2044438|entityMap^0|0|0|5|B|R|4|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]|$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

In just one line, I would write:

<pre><code>!!Module.const_get(nameofclass) rescue false
</code></pre>

that will return <code>true</code>only if the given <code>nameofclass</code> belongs to a defined class.

blocks|key|1830616|text|Kernel.const_defined?("Fixnum")+#+=>+true|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1830617|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>Kernel.const_defined?("Fixnum") # =&gt; true
</code></pre>

blocks|key|1830638|text|如果你想打包一些东西，finishing_moves+gem会添加一个class_exists?方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1830639|class_exists?+:Symbol
#+=>+true
class_exists?+:Rails
#+=>+true+in+a+Rails+app
class_exists?+:NonexistentClass
#+=>+false|code-block|syntax|javascript|1830640|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/forgecrafted/finishing_moves|1|https://github.com/forgecrafted/finishing_moves/wiki/Kernel#kernelclass_exists^0|B|F|Z|D|B|F|0|Z|D|1|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]|$9|12|A|13|1|14]]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]|S|$5|N|O|P|E|$Q|T]]]]

If you want something packaged, the <a href="https://github.com/forgecrafted/finishing_moves" rel="nofollow noreferrer"><code>finishing_moves</code></a> gem adds a <a href="https://github.com/forgecrafted/finishing_moves/wiki/Kernel#kernelclass_exists" rel="nofollow noreferrer"><code>class_exists?</code></a> method.

<pre><code>class_exists? :Symbol
# =&gt; true
class_exists? :Rails
# =&gt; true in a Rails app
class_exists? :NonexistentClass
# =&gt; false
</code></pre>

blocks|key|1830661|text|上面的答案对我来说都不起作用，可能是因为我的代码存在于子模块的作用域中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1830662|我决定使用Fred+Wilmore's+reply+to+"How+do+I+check+if+a+class+is+defined?"中的代码在我的模块中创建一个class_exists?方法，并最终停止了咒骂。|offset|length|style|CODE|1830663|def+class_exists?(name)
+++name.constantize.is_a?(Class)+rescue+false+#+rubocop:disable+Style/RescueModifier
end|code-block|syntax|javascript|1830664|对于好奇的人来说，完整的代码：|1830665|module+Some
++module+Thing
++++def+self.build(object)
++++++name+=+"Some::Thing::#{object.class.name}"
++++++class_exists?(name)+?+name.constantize.new(object)+:+Base.new(object)
++++end

++++def+self.class_exists?(name)
++++++name.constantize.is_a?(Class)+rescue+false+#+rubocop:disable+Style/RescueModifier
++++end

++++private_class_method+:class_exists?
++end
end|1830666|我将其用作工厂，根据作为参数传递的对象的类来构建对象：|1830667|Some::Thing.build(something)
=>+#+A+Some::Thing::Base+object
Some::Thing.build(something_else)
=>+#+Another+object,+which+inherits+from+Some::Thing::Base|1830668|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/25453213/766301^0|0|2A|D|5|1R|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@$D|14|E|15|F|G]]|9|@$D|16|E|17|1|18]]|A|$]]|$1|H|3|I|5|J|7|19|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|1A|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|1B|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|1C|8|@]|9|@]|A|$]]|$1|S|3|T|5|J|7|1D|8|@]|9|@]|A|$K|L]]|$1|U|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]]]

None of the answers above worked for me, maybe because my code lives in the scope of a submodule.

I settled on creating a <code>class_exists?</code> method in my module, using code found in <a href="https://stackoverflow.com/a/25453213/766301">Fred Wilmore's reply to "How do I check if a class is defined?"</a> and finally stopped cursing.

<pre><code>def class_exists?(name)
 name.constantize.is_a?(Class) rescue false # rubocop:disable Style/RescueModifier
end
</code></pre>

Full code, for the curious:

<pre><code>module Some
 module Thing
 def self.build(object)
 name = "Some::Thing::#{object.class.name}"
 class_exists?(name) ? name.constantize.new(object) : Base.new(object)
 end

 def self.class_exists?(name)
 name.constantize.is_a?(Class) rescue false # rubocop:disable Style/RescueModifier
 end

 private_class_method :class_exists?
 end
end
</code></pre>

I use it as a factory which builds objects depending on the class of the object passed as argument:

<pre><code>Some::Thing.build(something)
=&gt; # A Some::Thing::Base object
Some::Thing.build(something_else)
=&gt; # Another object, which inherits from Some::Thing::Base
</code></pre>

blocks|key|2044677|text|如果它是一个简单的独立类，例如|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2044678|class+Foo
end|code-block|syntax|javascript|2044679|然后我们就可以使用Object.const_defined?("Foo")了。而且，如果您的类位于任何模块中，例如|offset|length|style|CODE|2044680|module+Bar
++class+Foo
++end
end|2044681|然后我们就可以使用Module.const_get("Bar::Foo")了。|2044682|请注意，如果Module.const_get('Bar::Foo')找不到这个类，那么它将引发异常。而Object.const_defined?('Foo')将返回true或false。|2044683|entityMap^0|0|0|9|S|0|0|9|S|0|6|S|1F|S|2A|4|2F|5|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@$I|14|J|15|K|L]|$I|16|J|17|K|L]|$I|18|J|19|K|L]|$I|1A|J|1B|K|L]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|T|$]]

If it is a simple standalone class e.g.
<pre class="lang-rb prettyprint-override"><code>class Foo
end
</code></pre>
then we can use <code>Object.const_defined?(&quot;Foo&quot;)</code>. And, if your class is inside any module e.g.
<pre class="lang-rb prettyprint-override"><code>module Bar
 class Foo
 end
end
</code></pre>
then we can use <code>Module.const_get(&quot;Bar::Foo&quot;)</code>.
Please note that if <code>Module.const_get('Bar::Foo')</code> doesn't find the class then it will raise exception. While, <code>Object.const_defined?('Foo')</code> will return <code>true</code> or <code>false</code>.

How do I check if a class already exists in Ruby?

My code is:

<pre><code>puts "enter the name of the Class to see if it exists" 
nameofclass=gets.chomp 
eval (" #{nameofclass}...... Not sure what to write here")
</code></pre>

I was thinking of using:

<pre><code>eval "#{nameofclass}ancestors. ....."
</code></pre>

How to check if a class already exists in Ruby

Kernel

如何检查类在Ruby中是否已经存在？我的代码是：puts "enter the name of the Class to see if it exists"   nameofclass=gets.chomp  eval (" #{nameofclass}......  Not sure what to write he...

问如何检查Ruby中是否已存在类
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