Object alignment requirement

cppreference Object # alignment requirement

Every object type has the property called alignment requirement, which is an integer value (of type std::size_t, always a power of 2) representing the number of bytes between successive addresses at which objects of this type can be allocated.

since C++11

The alignment requirement of a type can be queried with alignof or std::alignment_of. The pointer alignment function std::align can be used to obtain a suitably-aligned pointer within some buffer, and std::aligned_storage can be used to obtain suitably-aligned storage.

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Each object type imposes its alignment requirement on every object of that type; stricter alignment (with larger alignment requirement) can be requested using alignas (since C++11).

In order to satisfy alignment requirements of all non-static members of a class, padding may be inserted after some of its members.

stackoverflow Order between STDCPP_DEFAULT_NEW_ALIGNMENT and alignof(std::max_align_t)

With GCC and Clang on x86-64/Linux alignof(std::max_align_t) and __STDCPP_DEFAULT_NEW_ALIGNMENT__ are both equal to 16.

With MSVC on x86-64/Windows alignof(std::max_align_t) is 8 and __STDCPP_DEFAULT_NEW_ALIGNMENT__ is 16.

The standard defines the two terms corresponding to these quantities in [basic.align]/3:

An extended alignment is represented by an alignment greater than alignof(std::max_­align_­t). [...] A type having an extended alignment requirement is an over-aligned type. [...] A new-extended alignment is represented by an alignment greater than _­_­STDCPP_­DEFAULT_­NEW_­ALIGNMENT_­_­.

A

1、std::max_align_t: The alignment of the biggest scalar type

2、__STDCPP_DEFAULT_NEW_ALIGNMENT__: The alignment of allocated memory

NOTE:

C++17引入