blocks|key|2670635|text|以下内容；|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2670636|enum+class+E+:+CL<E>::UndType;|code-block|syntax|javascript|2670637|在当前的一些实现(测试过的clang%2B%2B、g%2B%2B和MSVC)中不能被接受为有效的声明。在基于枚举的CL<E>::UndType中，它们不接受尚未完成的类型E。在测试的实现中给出的错误是，E在这一点上未声明。他们似乎将声明点放在enum-base的末尾，他们认为一旦完成就会声明它。|offset|length|style|CODE|2670638|在阅读说明书时；|2670639|§14.3.1/2模板类型参数|BOLD|2670640|2670641|注意:模板类型参数可能是不完整的类型(3.9)。-结束注释|blockquote|2670642|2670643|2670644|和|2670645|§7.2/6枚举声明|2670646|2670647|基础类型为固定类型的枚举从声明点(3.3.2)到紧跟在枚举基数(如果有)之后是不完整的类型，此时它将成为完整类型。|2670648|2670649|2670650|是否暗示它是可编译的；就像CRTP实现一样。|2670651|我确信这(即编译enum+class+E+:+CL<E>::UndType;失败)是意图，还是被认为是一个用例。从规范中，不透明的枚举声明被给予了一些“特殊”的处理w.r.t.它的基类型以及它必须是整型的要求。|2670652|假设，给定CWG#1482的分辨率，代码应该是可编译的。|2670653|2670654|至于目前的变通方法...|2670655|这个；|2670656|enum+class+E;+//+default+underlying+type+is+int|2670657|是最小声明。|2670658|不透明的声明可以是；|2670659|enum+class+E+:+int;+//+int+base|2670660|下面是一个完整的定义(包括枚举器)；|2670661|enum+class+E+:+int+{/*...*/};|2670662|或者，为了使用类模板，可以使用另一种类型(可能是void)。|2670663|enum+class+E+:+CL<void>::UndType;|2670664|entityMap|0|LINK|mutability|MUTABLE|url|http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1482^0|0|0|1D|E|25|1|2L|1|0|0|0|F|0|0|0|0|0|0|0|A|0|0|0|0|0|0|8|U|0|5|8|0|0|0|0|0|0|0|0|0|0|0|O|4|0|0^^$0|@$1|2|3|4|5|6|7|25|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|26|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|27|8|@$I|28|J|29|K|L]|$I|2A|J|2B|K|L]|$I|2C|J|2D|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|2E|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|2F|8|@$I|2G|J|2H|K|Q]]|9|@]|A|$]]|$1|R|3|-4|5|6|7|2I|8|@]|9|@]|A|$]]|$1|S|3|T|5|U|7|2J|8|@]|9|@]|A|$]]|$1|V|3|-4|5|6|7|2K|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|2L|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|2M|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|2N|8|@$I|2O|J|2P|K|Q]]|9|@]|A|$]]|$1|11|3|-4|5|6|7|2Q|8|@]|9|@]|A|$]]|$1|12|3|13|5|U|7|2R|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|2S|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|2T|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|2U|8|@]|9|@]|A|$]]|$1|18|3|19|5|6|7|2V|8|@$I|2W|J|2X|K|L]]|9|@]|A|$]]|$1|1A|3|1B|5|6|7|2Y|8|@]|9|@$I|2Z|J|30|1|31]]|A|$]]|$1|1C|3|-4|5|6|7|32|8|@]|9|@]|A|$]]|$1|1D|3|1E|5|6|7|33|8|@]|9|@]|A|$]]|$1|1F|3|1G|5|6|7|34|8|@]|9|@]|A|$]]|$1|1H|3|1I|5|D|7|35|8|@]|9|@]|A|$E|F]]|$1|1J|3|1K|5|6|7|36|8|@]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|37|8|@]|9|@]|A|$]]|$1|1N|3|1O|5|D|7|38|8|@]|9|@]|A|$E|F]]|$1|1P|3|1Q|5|6|7|39|8|@]|9|@]|A|$]]|$1|1R|3|1S|5|D|7|3A|8|@]|9|@]|A|$E|F]]|$1|1T|3|1U|5|6|7|3B|8|@$I|3C|J|3D|K|L]]|9|@]|A|$]]|$1|1V|3|1W|5|D|7|3E|8|@]|9|@]|A|$E|F]]|$1|1X|3|-4|5|6|7|3F|8|@]|9|@]|A|$]]]|1Y|$1Z|$5|20|21|22|A|$23|24]]]]

The following;

<pre><code>enum class E : CL&lt;E&gt;::UndType;
</code></pre>

Is not accepted as a valid declaration in some current implementations (tested clang++, g++ and MSVC). They do not accept the, as yet incomplete type <code>E</code>, in the enum-base <code>CL&lt;E&gt;::UndType</code>. The error given in the tested implementations is that <code>E</code> is undeclared at that point. They seem to place the point of declaration at the end of the enum-base, they consider it declared once it is complete.

When reading the specifications;

§14.3.1/2 Template type arguments

<blockquote>
 [ Note: A template type argument may be an incomplete type (3.9). — end note ]
</blockquote>

And 

§7.2/6 Enumeration declarations

<blockquote>
 An enumeration whose underlying type is fixed is an incomplete type from its point of declaration (3.3.2) to immediately after its enum-base (if any), at which point it becomes a complete type.
</blockquote>

Does hint at it being compilable; as is the case with CRTP implementations.

I'm note sure if this (i.e. the failure to compile <code>enum class E : CL&lt;E&gt;::UndType;</code>) is the intention or if it was considered as a use case. From the specification, the opaque enum declaration is given some "special" treatment w.r.t. its base type and the requirement that it must be an integral type.

Presumably, the code should be compilable given the resolution to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1482" rel="nofollow">CWG#1482</a>.

<hr>

As for current workarounds...

This;

<pre><code>enum class E; // default underlying type is int
</code></pre>

Is the minimum declaration.

The opaque declaration could be either;

<pre><code>enum class E : int; // int base
</code></pre>

The following would be a full definition (including enumerators);

<pre><code>enum class E : int {/*...*/};
</code></pre>

Or to use the class template, another type (possibly <code>void</code>) could be used.

<pre><code>enum class E : CL&lt;void&gt;::UndType;
</code></pre>

blocks|key|120470|text|现在，CWG2516已为此打开。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|120471|我认为这是标准中的一个错误，该标准禁止is_scoped_enum的可移植实现。|style|CODE|120472|entityMap|0|LINK|mutability|MUTABLE|url|https://wg21.link/cwg2516|1|https://en.cppreference.com/w/cpp/types/is_scoped_enum^0|3|7|0|0|J|E|J|E|1|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@$A|S|B|T|1|U]]|C|$]]|$1|D|3|E|5|6|7|V|8|@$A|W|B|X|F|G]]|9|@$A|Y|B|Z|1|10]]|C|$]]|$1|H|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]|P|$5|K|L|M|C|$N|Q]]]]

Now <a href="https://wg21.link/cwg2516" rel="nofollow noreferrer">CWG2516</a> is opened for this.
I believe it's a bug in the Standard which forbids a portable implementation of <a href="https://en.cppreference.com/w/cpp/types/is_scoped_enum" rel="nofollow noreferrer"><code>is_scoped_enum</code></a>.

What is the point of declaration of enumeration types? Is it immediately after the name of an enumeration? I saw Standard C++ 14 (n4296) §3.3.2/3:

<blockquote>
 The point of declaration for an enumeration is immediately after the
 identifier (if any) in either its enum-specifier (7.2) or its first
 opaque-enum-declaration (7.2), whichever comes first
</blockquote>

But when I try to reproduce it;

<pre><code>template &lt;class T&gt;
struct CL
{
 using UndType = int;
};

enum class E: CL&lt;E&gt;::UndType; //error: E is undefined
</code></pre>

I have got an error on all the compilers, although enum-base for enumeration <code>E</code> is placed after the identifier and must be visible.

Point of declaration for an enumeration

枚举类型的声明点是什么？它是否紧跟在枚举的名称之后？我看到了标准C++ 14 (n4296)§3.3.2/3：枚举的声明点紧跟在其枚举说明符(7.2)或其第一个不透明枚举声明(7.2)中的标识符(如果有)之后，以先出现的为准但是当我试图重现它的时候；template <class T>struct CL{ usi...

问枚举的声明点
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