类模板Example< int> ;;语句对于C ++ 11意味着什么？ [英] What does `class template Example<int>;` statement mean with C++11?
问题描述
在cplusplus.com上，我曾被引用显式模板实例化 ，给出以下示例：
I've been referred to "Explicit Template Instantiation" at cplusplus.com, which gives the following example:
template <typename T> class Example
{
public:
Example( T test )
{
_data = test;
}
void setTest(T test)
{
_data = T;
}
private:
T _data;
};
class template Example<int>;
class template Example<float>;
class template Example<double>;
除了对我来说似乎遗漏的错误（尝试将类型分配给成员）变量- _data = T 而不是我假设的应该是 _data = test -我不明白
Apart from what looks like an omission error to me where a type is attempted to be assigned to a member variable -- _data = T instead of what I assume should be _data = test -- what I don't understand is what do the last 3 lines declare or instruct the compiler to do, exactly?
我知道最后的三行是什么，或者确切地指示编译器做什么？实例化和专业化。我对后两者的理解可能确实有一些漏洞，但是我通常使用例如 template class Example< int> ;; 表单，而不是摘要中显示的表单。
I know what templates are, have built programs with them, and know in general about their instantiation and specialization. I do probably have some holes in the understanding of the latter two, but I typically instruct an explicit template instantiation using e.g. template class Example<int>; form and not the one shown in the snippet.
我试图使用 g ++ -std = c ++ 11 -pedantic 编译代码段，它编译得很好并且没有警告（我纠正了 _date = T 上面的错误）。
I've tried to compile the snippet using g++ -std=c++11 -pedantic and it compiles just fine and without warnings (I corrected the _date = T error above first).
这是在我评论有关问题的答案，但我仍不确定代码段中的后3行是模板特化还是实例化。
This came after I commented on an answer to a related question and I am still unsure whether either of the last 3 lines in the snippet is a template specialization or instantiation.
我有还尝试在类之后允许模板的语法） ：//www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf rel = nofollow noreferrer>由ISO发布的C ++ 11草案，但是空手而归。
I have also tried to locate the relevant grammar production rule (one allowing template after class) in the C++11 draft published by ISO but came empty handed.
推荐答案
我们可以从以下Godbolt中看到示例，根据clang和MSVC格式不正确，请查看显式实例化部分 [temp.explicit] 我看不到gcc接受它的任何理由。
We can see from the following godbolt example this is ill-formed according to clang and MSVC and looking at the draft standard section on Explicit instantiation section [temp.explicit] I don't see any justification for gcc to accepts it.
我认为给定该主题的文章可能的意思是：
I believe what the article "possibly" meant given the topic was:
template class Example<int>;
template class Example<float>;
template class Example<double>;
在C ++ 11之前的版本中，似乎允许这种语法，请参见缺陷报告1707：精化类型说明符中的模板没有嵌套名称说明符（强调我的）：
It looks like pre C++11 this grammar was allowed, see defect report 1707: template in elaborated-type-specifier without nested-name-specifier (emphasis mine):
精心制作的类型语法10.1.7.3 [dcl.type.elab]中的说明符
部分读取
The grammar for elaborated-type-specifier in 10.1.7.3 [dcl.type.elab] reads, in part,
elaborated-type-specifier:
class-key nested-name-specifieropt templateopt simple-template-id
这允许使用不带
嵌套名称说明符的模板关键字，例如结构模板S。 b与template关键字的其他用法不一致。将生产一分为二可能更好
，并且只允许在嵌套名称说明后面加上关键字
This allows use of the template keyword without a nested-name-specifier, e.g., struct template S. This is inconsistent with other uses of the template keyword. It might be better to split the production in two and only allow the keyword following a nested-name-specifier,
...。
所以这更有意义带有此评论，表示 -ansi 会引起警告。
So this makes a little more sense with this comment that -ansi causes a warning.
另一个 answerer提交了两个错误报告。
cppreference 具有很好的显式实例化这样的问题显式实例化-什么时候使用？解释了更多细节，为什么这很有用。
cppreference has a good dicssuion of Explicit instantiation and this SO question Explicit instantiation - when is it used? explains more details why this is useful.
另外请注意，我们可以看到此元发布：L墨水已更改为cppreference.com ，表明该网站的信息不正确，并且一般而言，社区更喜欢 cppreference 作为可靠的C ++参考。
Also note, we can see this Meta post: Links being changed to cppreference.com that the site has been known to have incorrect information and in general the community prefers cppreference as a solid C++ reference.
这篇关于类模板Example< int> ;;语句对于C ++ 11意味着什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
