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### 2377. `std::align` requirements overly strict

Section: 20.2.5 [ptr.align] Status: C++17 Submitter: Peter Dimov Opened: 2014-03-30 Last modified: 2017-07-30

Priority: 0

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Discussion:

`std::align` requires that its `alignment` argument shall be "a fundamental alignment value or an extended alignment value supported by the implementation in this context".

This requirement is overly strict. There are use cases that require a buffer aligned at values that are not tied to the C++ implementation, such as page size, cache line size, sector size. These come from the hardware or the OS and are generally not known until run time. The implementation of `std::align` does not depend on the requirement that `alignment` be a fundamental or an extended alignment value; any power of two would be handled the same way.

In addition, it is not possible for the user to even determine whether a value is "a fundamental alignment value or an extended alignment value supported by the implementation in this context". One would expect values coming from `alignof` to be fine, but I'm not sure whether even that is guaranteed in the presence of `alignas`.

Therefore, I propose that

> Requires:

> - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context

be changed to

> Requires:

> - `alignment` shall be a power of two

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Edit 20.2.5 [ptr.align] p2 as indicated:

   >     void* align(std::size_t alignment, std::size_t size,
   >       void*& ptr, std::size_t& space);

   > > -1- […]

   > > -2- Requires:

   > > - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context power of two