新的char实际上保证类型类型的对齐内存吗？ [英] Does new char actually guarantee aligned memory for a class type?

问题描述

通过 new char [sizeof（T）] 分配一个缓冲区，保证分配的内存与 T ，其中 T 的所有成员都有自然的，实现定义的对齐方式（也就是说，您没有使用 alignas 关键字修改它们的对齐方式）。

我已经看到这个保证在几个答案周围，但我不完全清楚标准如何到达这种保证。标准的5.3.4-10给出了基本要求：基本上 new char [] 必须与 max_align_t 对齐。

我缺少的是 alignof（T）的位总是一个有效的对齐方式最大值 max_align_t 。我的意思是，它似乎很明显，但必须得到一个结构的对齐最多 max_align_t ？甚至点3.11-3说，可能支持扩展对齐，因此编译器自己决定一个类是否是一个过度对齐的类型？

解决方案

我缺少的是 alignof（T）的位总是一个有效的对齐方式，最大值为 max_align_t 。我的意思是，它似乎很明显，但必须得到一个结构的对齐最多 max_align_t ？甚至点3.11-3说，可以支持扩展对齐，因此编译器可以自己决定一个类是否是一个过对齐的类型？

正如Mankarse所说，我可以得到的最好的报价是 [basic.align] / 3 ：

<具有扩展的对准要求的类型是过度对准类型。 [注意：
每个过对齐类型是或包含扩展对齐适用的类类型（可能通过非静态数据成员）。 -end note]

这似乎暗示扩展比对必须明确需要（然后传播），但不能

我宁愿更清楚地提及;对于编译器编写器来说意图是显而易见的，任何其他行为将是疯狂的，仍然...

Is allocating a buffer via new char[sizeof(T)] guaranteed to allocate memory which is properly aligned for the type T, where all members of T has their natural, implementation defined, alignment (that is, you have not used the alignas keyword to modify their alignment).

I have seen this guarantee made in a few answers around here but I'm not entirely clear how the standard arrives at this guarantee. 5.3.4-10 of the standard gives the basic requirement: essentially new char[] must be aligned to max_align_t.

What I'm missing is the bit which says alignof(T) will always be a valid alignment with a maximum value of max_align_t. I mean, it seems obvious, but must the resulting alignment of a structure be at most max_align_t? Even point 3.11-3 says extended alignments may be supported, so may the compiler decide on its own a class is an over-aligned type?

解决方案

What I'm missing is the bit which says alignof(T) will always be a valid alignment with a maximum value of max_align_t. I mean, it seems obvious, but must the resulting alignment of a structure be at most max_align_t ? Even point 3.11-3 says extended alignments may be supported, so may the compiler decide on its own a class is an over-aligned type ?

As noted by Mankarse, the best quote I could get is from [basic.align]/3:

A type having an extended alignment requirement is an over-aligned type. [ Note: every over-aligned type is or contains a class type to which extended alignment applies (possibly through a non-static data member). —end note ]

which seems to imply that extended alignment must be explicitly required (and then propagates) but cannot

I would have prefer a clearer mention; the intent is obvious for a compiler-writer, and any other behavior would be insane, still...

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