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如何创建char / std :: byte的std :: vector，其中第一个字节与16个字节对齐，但是没有填充？

[英]How to create std::vector of char/std::byte where first byte is aligned to 16 byes, but there is no padding?

原文 2019-01-25 12:09:35 6 1 c++/ alignment/ alignas

There is an existing question that requires C++03 and has no answer, so I will open a new one. 有一个问题需要C ++ 03并且没有答案，所以我将打开一个新问题。

Problem I have is that I want to have std::vector of std::byte , but so that .data() (first element of data array) is 16 byte aligned. 我遇到的问题是我想要std::vector的std::byte ，但是.data() （数据数组的第一个元素）是16字节对齐的。

alignas on wrapped char does not help because I do not want to have alignment gaps in the array. 包装char上的alignas没有帮助，因为我不想在数组中有对齐间隙。 In other words I want to keey alignment of 1 for elements, but I want alignment of 16 for the array. 换句话说，我想对元素进行1对齐，但我希望对齐数组16。

Ideally I would like to avoid using a custom allocator. 理想情况下，我想避免使用自定义分配器。 If there is any TBB or boost vector that does what I want that would be also great. 如果有任何TBB或增强矢量可以做我想要的那样也会很棒。

1 个解决方案

Aligning the data in a vector ain't provided by default. 默认情况下不提供对齐矢量中的数据。 Not even for aligned classes. 甚至没有对齐的课程。

The best way of doing alignment is with the aligned_allocator of boost. 做对齐的最佳方法是使用boost的aligned_allocator 。

Unfortunately, it doesn't prevent padding, it even overallocates to adapt the pointer on the alignment. 不幸的是，它不会阻止填充，它甚至可以根据对齐方式调整指针。 From C++17, it can used aligned new (see std::aligned_val_t overloads). 从C ++ 17开始，它可以使用对齐的new （参见std::aligned_val_t重载）。 However, all implementations I've seen actually use the same trick. 但是，我见过的所有实现实际上都使用相同的技巧。

An alternative is allocating a whole page at once, and do your own memory management with a custom allocator. 另一种方法是一次分配整个页面，并使用自定义分配器进行自己的内存管理。 You can do it, though it might take a lot of time to do correctly. 您可以这样做，但可能需要花费大量时间才能正确完成。