blocks|key|836723|text|无论在何处使用，符号都是相同的对象和相同的内存分配：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|836724|>>+:hello.object_id
=>+331068
>>+a+=+:hello
=>+:hello
>>+a.object_id
=>+331068
>>+b+=+:hello
=>+:hello
>>+b.object_id
=>+331068
>>+a+=+"hello"
=>+"hello"
>>+a.object_id
=>+2149256980
>>+b+=+"hello"
=>+"hello"
>>+b.object_id
=>+2149235120
>>+b+=+"hell"+%2B+"o"|code-block|syntax|javascript|836725|两个字符串“相同”，因为它们包含相同的字符，它们可能不会引用相同的内存，这可能是低效的，如果您使用字符串进行散列。|836726|因此，符号对于减少内存开销非常有用。然而，它们是一个等待发生的内存泄漏，因为一旦创建，符号就不能被垃圾收集。创建成千上万的符号会分配内存，并且是不可恢复的。呀！|836727|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]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

A symbol is the same object and the same allocation of memory no matter where it is used:

<pre><code>&gt;&gt; :hello.object_id
=&gt; 331068
&gt;&gt; a = :hello
=&gt; :hello
&gt;&gt; a.object_id
=&gt; 331068
&gt;&gt; b = :hello
=&gt; :hello
&gt;&gt; b.object_id
=&gt; 331068
&gt;&gt; a = "hello"
=&gt; "hello"
&gt;&gt; a.object_id
=&gt; 2149256980
&gt;&gt; b = "hello"
=&gt; "hello"
&gt;&gt; b.object_id
=&gt; 2149235120
&gt;&gt; b = "hell" + "o"
</code></pre>

Two strings which are 'the same' in that they contain the same characters may not reference the same memory, which can be inefficient if you're using strings for, say, hashes.

So, symbols can be useful for reducing memory overhead. However - they are a memory leak waiting to happen, because symbols cannot be garbage collected once created. Creating thousands and thousands of symbols would allocate the memory and not be recoverable. Yikes!

blocks|key|836786|text|在没有根据某种白名单验证输入的情况下从用户输入创建符号可能特别糟糕(例如，对于RoR中的查询字符串参数)。如果在未经验证的情况下将用户输入转换为符号，则恶意用户可能会导致程序消耗大量内存，而这些内存永远不会被垃圾回收。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|836787|错误(无论用户输入如何，都会创建一个符号)：|836788|name+=+params[:name].to_sym|code-block|syntax|javascript|836789|好(只有在允许用户输入的情况下才会创建符号)：|836790|whitelist+=+['allowed_value',+'another_allowed_value']
raise+ArgumentError+unless+whitelist.include?(params[:name])
name+=+params[:name].to_sym|836791|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

It can be particularly bad to create symbols from user input without validating the input against some kind of a white-list (for example, for query string parameters in RoR). If user input is converted to symbols without validation, a malicious user can cause your program to consume large amounts of memory that will never be garbage collected. 

Bad (a symbol is created regardless of user input):

<pre><code>name = params[:name].to_sym
</code></pre>

Good (a symbol is only created if the user input is allowed):

<pre><code>whitelist = ['allowed_value', 'another_allowed_value']
raise ArgumentError unless whitelist.include?(params[:name])
name = params[:name].to_sym
</code></pre>

blocks|key|836847|text|如果您使用的是Ruby2.2.0或更高版本，那么动态创建大量符号通常是可以的，因为它们将根据链接到more+details+about+the+new+symbol+GC的Ruby+2.2.0-preview1+announcement被垃圾收集。但是，如果您将动态符号传递给某种将其转换为ID+(C源代码中使用的内部Ruby实现概念)的代码，那么在这种情况下，它将被固定并且永远不会被垃圾回收。我不确定这种情况有多常见。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|836848|您可以将符号视为某物的名称，将字符串(大致)视为一系列字符。在许多情况下，您可以使用符号或字符串，也可以混合使用这两者。符号是不可变的，这意味着它们在被创建后不能被改变。在实现符号的方式中，比较两个符号以查看它们是否相等是非常有效的，因此使用它们作为散列的键应该比使用字符串快一点。符号不像字符串那样有很多方法，比如start_with?，所以在调用这些方法之前，您必须使用to_s将符号转换为字符串。|style|CODE|836849|您可以在文档中阅读有关符号的更多信息：|836850|http://www.ruby-doc.org/core-2.1.3/Symbol.html|836851|entityMap|0|LINK|mutability|MUTABLE|url|https://bugs.ruby-lang.org/issues/9634|1|https://www.ruby-lang.org/en/news/2014/09/18/ruby-2.2.0-preview1-released/|2^0|1D|10|0|2E|W|1|0|4E|B|57|4|0|0|0|1A|2|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|6|7|13|8|@$A|14|B|15|F|G]|$A|16|B|17|F|G]]|9|@]|C|$]]|$1|H|3|I|5|6|7|18|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|19|8|@]|9|@$A|1A|B|1B|1|1C]]|C|$]]|$1|L|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|M|$N|$5|O|P|Q|C|$R|S]]|T|$5|O|P|Q|C|$R|U]]|V|$5|O|P|Q|C|$R|K]]]]

If you are using Ruby 2.2.0 or later, it should usually be OK to dynamically create a lot of symbols, because they will be garbage collected according to the <a href="https://www.ruby-lang.org/en/news/2014/09/18/ruby-2.2.0-preview1-released/" rel="nofollow">Ruby 2.2.0-preview1 announcement</a>, which has a link to <a href="https://bugs.ruby-lang.org/issues/9634" rel="nofollow">more details about the new symbol GC</a>. However, if you pass your dynamic symbols to some kind of code that converts it to an ID (an internal Ruby implementation concept used in the C source code), then in that case it will get pinned and never get garbage collected. I'm not sure how commonly that happens.

You can think of symbols as a name of something, and strings (roughly) as a sequence of characters. In many cases you could use either a symbol or a string, or you could use a mixture of the two. Symbols are immutable, which means they can't be changed after being created. The way symbols are implemented, it is very efficient to compare two symbols to see if they are equal, so using them as keys to hashes should be a little faster than using strings. Symbols don't have a lot the methods that strings do, such as <code>start_with?</code> so you would have to use <code>to_s</code> to convert the symbol into a string before calling those methods.

You can read more about symbols here in the documentation:

<a href="http://www.ruby-doc.org/core-2.1.3/Symbol.html" rel="nofollow">http://www.ruby-doc.org/core-2.1.3/Symbol.html</a>

blocks|key|836894|text|Starting+Ruby+2.2和更高版本的Symbols会自动进行垃圾回收，所以这应该不是问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|836895|entityMap|0|LINK|mutability|MUTABLE|url|https://www.ruby-lang.org/en/news/2014/09/18/ruby-2-2-0-preview1-released/|1|http://ruby-doc.org/core-2.3.1/Symbol.html^0|0|H|0|N|7|1|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]|$A|R|B|S|1|T]]|C|$]]|$1|D|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]]]

<a href="https://www.ruby-lang.org/en/news/2014/09/18/ruby-2-2-0-preview1-released/" rel="noreferrer">Starting Ruby 2.2</a> and above <a href="http://ruby-doc.org/core-2.3.1/Symbol.html" rel="noreferrer">Symbols</a> are automatically garbage collected and so this should not be an issue.

What is the function of symbol in ruby? what's difference between string and symbol?
Why is it not a good idea to dynamically create a lot of symbols?

Why is it not a good idea to dynamically create a lot of symbols in ruby (for versions before 2.2)?

符号在拼音中的作用是什么？字符串和符号有什么区别？为什么动态创建大量符号不是一个好主意？

问为什么在ruby中动态创建大量符号不是一个好主意(对于2.2之前的版本)？
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