在那里我可以在C ++ 11使用alignas（） [英] where can I use alignas() in C++11

问题描述

在为了规范我的code，使之更便于携带，我换成

In an effort to standardize my code and make it more portable, I replaced

#ifdef __GNUC__
typedef __attribute__((aligned(16))) float aligned_block[4];
#else
typedef __declspec(align(16)) float aligned_block[4];
#endif

与

typedef float alignas(16) aligned_block[4];

在C ++ 11。然而，GNU（4.8）不喜欢这一点，但抱怨

in C++11. However, gnu (4.8) doesn't like that but complains

test.cc:3:9: warning: attribute ignored [-Wattributes]
  typedef float alignas(16) aligned_block[4];
                ^
test.cc:3:9: note: an attribute that appertains to a type-specifier is ignored

而铿锵3.2创建无警告（甚至 -Weverything -Wno-C ++ 98-compat的-pedantic ）。 所以我想我的code以上是否是正确的，更普遍，其中 alignas（），不能放置。

whereas clang 3.2 creates no warning (even with -Weverything -Wno-c++98-compat -pedantic). So I wonder whether my code above is correct and, more generally, where alignas() can and cannot be placed.

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与标准相关的文章是7.6.2，特别是7.6.2.1

The relevant article from the standard is 7.6.2, in particular 7.6.2.1

这是对准说明符可以被应用到一个变量或一类数据成员，但它不应被施加到一个位字段，一个功能参数，的catch子句（15.3）的形式参数，或变量使用寄存器存储说明符进行声明。对准说明符也可被应用到类或枚举类型的声明。一个对齐说明符以省略号是一个扩展包（14.5.3）。

An alignment-specifier may be applied to a variable or to a class data member, but it shall not be applied to a bit-field, a function parameter, the formal parameter of a catch clause (15.3), or a variable declared with the register storage class specifier. An alignment-specifier may also be applied to the declaration of a class or enumeration type. An alignment-specifier with an ellipsis is a pack expansion (14.5.3).

为已经挖出了由红十三。不过，我不是专家，足以知道这是什么意思了我上面的测试。

as already dug out by Red XIII. However, I'm not expert enough to know what this means for my test above.

如果说铛接受我的属性的事实意味着什么，那或许值得一提的是，尝试使用，而不是的typedef <一使用指令时， / code>，铛也抱怨。此外，相对于这一问题早期版本的说法，GCC不只是发出警告，但确实是忽略了我的愿望一致。

If the fact that clang accepts my attribute means anything, it's perhaps worth mentioning that when trying to use a using directive instead of a typedef, clang also complains. Also, contrary to a statement in an earlier version of this question, gcc does not only warn, but indeed ignores my wish for alignment.

推荐答案

我觉得你刚才放置的 alignas 在错误的位置。如果将它直接的在的标识符，GCC和Clang的是快乐和应用的对齐方式：

I think you just placed the alignas in the wrong position. If you move it directly after the identifier, both GCC and Clang are happy and apply the alignment:

typedef float aligned_block alignas(16) [4];
typedef float aligned_block [4] alignas(16);

这也是如此，如果你使用使用，其中的差别也变得更加明显。这里有两个版本是的不是的海湾合作​​委员会（警告，对准忽略）受理：

this is also true if you use using, where the difference also becomes more apparent. Here are two versions that are not accepted by GCC (warning, alignment ignored):

using aligned_block = float alignas(16)[4];
using aligned_block = float[4] alignas(16);

和这里的接受之一：

using aligned_block alignas(16) = float[4];

我认为，海湾合作委员会适用

I think that GCC applies

² A 的typedef名的，也可以通过的别名声明引入的。在标识符的继使用关键字变成的的typedef名的和在可选的属性说明-SEQ 的继的代码的 appertains到的的typedef名的。它，就好像它是由`的typedef符引入相同的语义。 [...]

7.1.3 The typedef specifier [dcl.typedef]

² A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. It has the same semantics as if it were introduced by the `typedef specifier. [...]

（重点煤矿）

以上是相当明确的使用为的typedef 有s $ P $通过几个段落垫规则包括在§8.3/ 1，在这里你找到结束：

The above is quite clear for using, the rules for typedef are spread through several paragraphs, including at the end of §8.3/1, where you find:

¹ [...]可选的属性 - 符-SEQ 的下面一个的声明符-ID 的 appertains该声明的实体。

8.3 Meaning of declarators

¹ [...] The optional attribute-specifier-seq following a declarator-id appertains to the entity that is declared.

（再次，重点煤矿）

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