blocks|key|748623|text|通过委托构造函数，您可以执行如下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|748624|template<typename+T>
struct+Test+{
++++template+<std::size_t...Is,+typename...+B>
++++Test(std::index_sequence<Is...>,+B&&...+vv)+:
++++++++b{std::get<sizeof...(Is)+-+1+-+Is>(std::tie(vv...))...}
++++{}

public:
++++template<typename...+B>
++++explicit+Test(B...+vv)+:+Test(std::index_sequence_for<B...>{},+vv...)+{}
private:
++++std::byte+b[sizeof(T)];
};|code-block|syntax|javascript|748625|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|$]]

With delegating constructor, you may do something like:

<pre><code>template&lt;typename T&gt;
struct Test {
 template &lt;std::size_t...Is, typename... B&gt;
 Test(std::index_sequence&lt;Is...&gt;, B&amp;&amp;... vv) :
 b{std::get&lt;sizeof...(Is) - 1 - Is&gt;(std::tie(vv...))...}
 {}

public:
 template&lt;typename... B&gt;
 explicit Test(B... vv) : Test(std::index_sequence_for&lt;B...&gt;{}, vv...) {}
private:
 std::byte b[sizeof(T)];
};
</code></pre>

blocks|key|2384749|text|您可以利用index_sequence按反向顺序提取项：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2384750|#include+<array>
#include+<iostream>
#include+<utility>
#include+<cstddef>

template<::std::size_t...+VIndexes>+constexpr+auto
Make_ReversedArrayOfBytes_Impl
(
++++::std::index_sequence<VIndexes...>
,+++::std::array<::std::byte,+sizeof...(VIndexes)>+bytes
)
{
++++return
++++(
++++++++::std::array<::std::byte,+sizeof...(VIndexes)>
++++++++{
++++++++++++bytes[sizeof...(VIndexes)+-+1+-+VIndexes]...
++++++++}
++++);
}

template<typename...+TItems>+constexpr+auto
Make_ReversedArrayOfBytes(TItems...+bytes)
{
++++return
++++(
++++++++Make_ReversedArrayOfBytes_Impl
++++++++(
++++++++++++::std::make_index_sequence<sizeof...(TItems)>()
++++++++,+++::std::array<::std::byte,+sizeof...(TItems)>
++++++++++++{
++++++++++++++++static_cast<::std::byte>(bytes)...
++++++++++++}
++++++++)
++++);
}

int+main()
{
++++constexpr+const+auto+items{Make_ReversedArrayOfBytes(0,+1,+2,+3,+4)};
++++for(auto+const+&+item:+items)
++++{
++++++++::std::cout+<<+static_cast<::std::size_t>(item)+<<+::std::endl;
++++}
++++return+0;
}|code-block|syntax|javascript|2384751|在联机编译器中运行|2384752|entityMap|0|LINK|mutability|MUTABLE|url|https://wandbox.org/permlink/uKzbwTYCdahU1I82^0|5|E|0|0|0|9|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|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

You utilize <code>index_sequence</code> to extract items in reverse order:

<pre><code>#include &lt;array&gt;
#include &lt;iostream&gt;
#include &lt;utility&gt;
#include &lt;cstddef&gt;

template&lt;::std::size_t... VIndexes&gt; constexpr auto
Make_ReversedArrayOfBytes_Impl
(
 ::std::index_sequence&lt;VIndexes...&gt;
, ::std::array&lt;::std::byte, sizeof...(VIndexes)&gt; bytes
)
{
 return
 (
 ::std::array&lt;::std::byte, sizeof...(VIndexes)&gt;
 {
 bytes[sizeof...(VIndexes) - 1 - VIndexes]...
 }
 );
}

template&lt;typename... TItems&gt; constexpr auto
Make_ReversedArrayOfBytes(TItems... bytes)
{
 return
 (
 Make_ReversedArrayOfBytes_Impl
 (
 ::std::make_index_sequence&lt;sizeof...(TItems)&gt;()
 , ::std::array&lt;::std::byte, sizeof...(TItems)&gt;
 {
 static_cast&lt;::std::byte&gt;(bytes)...
 }
 )
 );
}

int main()
{
 constexpr const auto items{Make_ReversedArrayOfBytes(0, 1, 2, 3, 4)};
 for(auto const &amp; item: items)
 {
 ::std::cout &lt;&lt; static_cast&lt;::std::size_t&gt;(item) &lt;&lt; ::std::endl;
 }
 return 0;
}
</code></pre>

<a href="https://wandbox.org/permlink/uKzbwTYCdahU1I82" rel="nofollow noreferrer">Run in online compiler</a>

blocks|key|378053|text|如果b是一个非静态数据成员，以下更紧凑的版本在优化(*)后可能会变成等效的版本：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|378054|template<typename+T>
struct+Test+{
public:
++++template<typename...+B>
++++explicit+Test(B...+vv){+auto+rb+=+std::rbegin(b);+((*rb%2B%2B+=+vv),...);+}
private:
++++char+b[sizeof(T)];
};|code-block|syntax|javascript|378055|(*)除非，Jarod42的解决方案结束时，已经颠倒的B在数据部分时，议员.|378056|编辑:看起来树干嘎吱声和gcc都很聪明，把倒置的B放在数据部分，不管是在Jarod42的代码中，还是在我的！|378057|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|$]]

given that b is a non static data member, the following more compact version may turn out equivalent once optimized(*):

<pre><code>template&lt;typename T&gt;
struct Test {
public:
 template&lt;typename... B&gt;
 explicit Test(B... vv){ auto rb = std::rbegin(b); ((*rb++ = vv),...); }
private:
 char b[sizeof(T)];
};
</code></pre>

(*) unless, Jarod42's solution ends up with the already reversed B's in data section when constexpr...

EDIT: it looks like both trunk clang and gcc are so smart to place the reversed B's in the data section both in Jarod42's code and mine !

In the following class template I want to initalize the member-array <code>b</code> with the values of the parameter-pack <code>vv</code> in reverse-oder!

<pre><code>template&lt;typename T&gt;
struct Test {
public:
 template&lt;typename... B&gt;
 explicit Test(B... vv) : b{vv...,} // how to initialize member b with pack expansion v... in reverse order
 {
 }
private:
 std::byte b[sizeof(T)];
};
</code></pre>

I can't imagine how to reverse the parameter-pack expansion in a form, that it is as well usable in an initializer list.

Compile-time reversal of parameter-pack expansion

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在下面的类模板中，我希望使用反向程序中参数-pack b的值来初始化成员数组vv！template<typename T>struct Test {public: template<typename... B> explicit Test(B... vv) : b{vv...,} // how to ini...

问参数包展开的编译时反转
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