blocks|key|70435|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|70436|为什么我的代码片段链接？这里到底发生了什么？|blockquote|70437|70438|70439|好吧，这没什么可联系的。因为必须考虑显式实例化的影响。来自n3337：|70440|70441|+temp.explicit+(强调我的)|offset|length|style|BOLD|70442|70443|10+除了内联函数和类模板专门化之外，显式实例化声明还具有抑制它们所引用的实体的隐式实例化的效果。[注意:其意图是，作为显式实例化声明的主题的内联函数在使用odr+(basic.def.odr)时仍将被隐式实例化，以便可以考虑将主体用于内联，但不会在翻译单元中生成内联函数的行外副本。-结束语]|70444|70445|因此，类模板专门化X<int>的隐式实例化不会被抑制。它也是一个聚合，所以它的初始化发生在内联中，我们没有得到任何链接。但是，如果它有任何成员，这些成员将在8段中被取消|CODE|70446|70447|命名类模板专门化的显式实例化也是其每个成员(不包括从基类继承的成员)的相同类型(声明或定义)的显式实例化，其先前未在包含显式实例化的转换单元中显式专门化，除非如下所述。|70448|70449|70450|因此，如果你有一个类似于下面这样的聚合：|70451|template+<typename>
struct+X+{
++++X();
};

template+<typename+T>
X<T>::X()+{}+++++

extern+template+struct+X<int>;

int+main()
{
++++X<int>{};
}|code-block|syntax|javascript|70452|正如您所料，这将失败，因为它的ODR使用的构造函数的定义从未实例化。声明是实例化的，因为如上所述，封闭的专门化是实例化的。但是在显式实例化声明的抑制作用下，我们永远得不到任何定义。|70453|entityMap|0|LINK|mutability|MUTABLE|url|https://timsong-cpp.github.io/cppwp/n3337/temp.explicit#10|1|https://timsong-cpp.github.io/cppwp/n3337/temp.explicit#8^0|0|0|0|0|0|0|1|D|0|0|3|D|0|2|0|0|0|9|6|26|1|1|0|0|0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|1J|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|1K|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|1L|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|1M|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|1N|7|@]|8|@]|9|$]]|$1|H|3|-4|4|5|6|1O|7|@]|8|@]|9|$]]|$1|I|3|J|4|C|6|1P|7|@$K|1Q|L|1R|M|N]]|8|@]|9|$]]|$1|O|3|-4|4|5|6|1S|7|@]|8|@]|9|$]]|$1|P|3|Q|4|5|6|1T|7|@$K|1U|L|1V|M|N]]|8|@$K|1W|L|1X|1|1Y]]|9|$]]|$1|R|3|-4|4|5|6|1Z|7|@]|8|@]|9|$]]|$1|S|3|T|4|5|6|20|7|@$K|21|L|22|M|U]]|8|@$K|23|L|24|1|25]]|9|$]]|$1|V|3|-4|4|5|6|26|7|@]|8|@]|9|$]]|$1|W|3|X|4|C|6|27|7|@]|8|@]|9|$]]|$1|Y|3|-4|4|5|6|28|7|@]|8|@]|9|$]]|$1|Z|3|-4|4|5|6|29|7|@]|8|@]|9|$]]|$1|10|3|11|4|5|6|2A|7|@]|8|@]|9|$]]|$1|12|3|13|4|14|6|2B|7|@]|8|@]|9|$15|16]]|$1|17|3|18|4|5|6|2C|7|@]|8|@]|9|$]]|$1|19|3|-4|4|5|6|2D|7|@]|8|@]|9|$]]]|1A|$1B|$4|1C|1D|1E|9|$1F|1G]]|1H|$4|1C|1D|1E|9|$1F|1I]]]]

<blockquote>
 Why does my code snippet link? What is actually happening here?
</blockquote>

Well, there's nothing to link. For one has to consider the effects of the explicit instantiation. From n3337:

<blockquote>
 [temp.explicit] (emphasis mine)
 
 <a href="https://timsong-cpp.github.io/cppwp/n3337/temp.explicit#10" rel="nofollow noreferrer">10</a> Except for inline functions and class template
 specializations, explicit instantiation declarations have the effect
 of suppressing the implicit instantiation of the entity to which they
 refer. [ Note: The intent is that an inline function that is the
 subject of an explicit instantiation declaration will still be
 implicitly instantiated when odr-used ([basic.def.odr]) so that the
 body can be considered for inlining, but that no out-of-line copy of
 the inline function would be generated in the translation unit. — end
 note ]
</blockquote>

So the implicit instantiation of the class template specialization <code>X&lt;int&gt;</code>, is not suppressed. It's also an aggregate, so its initialization occurs inline, and we get nothing to link against. However, if it had any members, those would be suppressed under paragraph <a href="https://timsong-cpp.github.io/cppwp/n3337/temp.explicit#8" rel="nofollow noreferrer">8</a>:

<blockquote>
 An explicit instantiation that names a class template specialization
 is also an explicit instantiation of the same kind (declaration or
 definition) of each of its members (not including members inherited
 from base classes) that has not been previously explicitly specialized
 in the translation unit containing the explicit instantiation, except
 as described below.
</blockquote>

So if you had instead of an aggregate something akin to this:

<pre><code>template &lt;typename&gt;
struct X {
 X();
};

template &lt;typename T&gt;
X&lt;T&gt;::X() {} 

extern template struct X&lt;int&gt;;

int main()
{
 X&lt;int&gt;{};
}
</code></pre>

That would fail as you expect, since it ODR uses a constructor whose definition is never instantiated. The declaration is instantiated, because the enclosing specialization is instantiated, as mentioned above. But we never get any definition, under the suppressing effect of the explicit instantiation declaration.

blocks|key|2786835|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2786836|这是否意味着我发布的代码片段是格式错误的NDR？|blockquote|2786837|2786838|2786839|是的，就像你引用的temp.explink/13中的句子一样。“一个实体”就是这个意思。如果显式实例化声明在其他方面没有规范效果，这并不重要。|2786840|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|J|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|K|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|L|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|M|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|N|7|@]|8|@]|9|$]]|$1|H|3|-4|4|5|6|O|7|@]|8|@]|9|$]]]|I|$]]

<blockquote>
 Does this mean that the code snippet I posted is ill-formed, NDR?
</blockquote>

Yes, by the exact sentence from [temp.explicit]/13 that you quoted. "An entity" means just that. It does not matter if an explicit instantiation declaration otherwise have no normative effect.

Consider the following code snippet:

<pre><code>template &lt;typename&gt;
struct X { };

extern template struct X&lt;int&gt;;

int main()
{
 X&lt;int&gt;{};
}
</code></pre>

It compiles and links: <a href="https://gcc.godbolt.org/z/Na5kZx" rel="noreferrer">live example on godbolt.org</a>. I would expect it not to link due to the <code>extern template</code> declaration.

My understanding is that <code>extern template</code> means: "please don't instantiate this particular template specialization in this TU, it will be provided by some other TU and you can link against it".

The examples/descriptions. I've seen on isocpp and cppreference seem to validate my mental model. E.g.

<blockquote>
 From <a href="https://en.cppreference.com/w/cpp/language/class_template" rel="noreferrer">https://en.cppreference.com/w/cpp/language/class_template</a>:
 
 <blockquote>
 An explicit instantiation declaration (an extern template) skips implicit instantiation step: the code that would otherwise cause an implicit instantiation instead uses the explicit instantiation definition provided elsewhere (resulting in link errors if no such instantiation exists). This can be used to reduce compilation times by explicitly declaring a template instantiation in all but one of the source files using it, and explicitly defining it in the remaining file. 
 </blockquote>
</blockquote>

Why does my code snippet link? What is actually happening here?

<hr>

EDIT - found this in the latest Standard draft:

<blockquote>
 <a href="http://eel.is/c++draft/temp.explicit#:instantiation,explicit" rel="noreferrer">[temp.explicit]</a>
 
 <blockquote>
 If an entity is the subject of both an explicit instantiation declaration and an explicit instantiation definition in the same translation unit, the definition shall follow the declaration. An entity that is the subject of an explicit instantiation declaration and that is also used in a way that would otherwise cause an implicit instantiation in the translation unit shall be the subject of an explicit instantiation definition somewhere in the program; otherwise the program is ill-formed, no diagnostic required.
 </blockquote>
</blockquote>

Does this mean that the code snippet I posted is ill-formed, NDR?

Using `extern template` to prevent implicit instantiation of a template class

考虑下面的代码片段：template <typename>struct X { };extern template struct X<int>;int main(){ X<int>{};}它编译并链接：。由于extern template声明，我希望它不会链接。我的理解是，extern template的意思是：...

问使用` `extern template`防止模板类的隐式实例化
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