Aligned Memory Allocation in C++

This post contains the summary of the note about aligned memory allocation in C++.

Names Note
alignof Built-in operator to get the native byte alignment of input type.
std::align Regard it as an operation to finding the starting memory location to place the target scalar, struct/class data onto the target memory buffer prior allocated before the call. Suitable to be used in custom allocator implementation working with external allocated buffer.
std::alignment_of Provide the constant value for alignment requirement for input type. Underlying it uses alignof operator to find such value. If type is array type, it returns the element type. If it’s reference type, it returns the type which pointer refers to.
std::aligned_storage Regard it as an aligned data structure to hold any type. Underlying it uses union with two field members, one for backend data, and another for a trick to align the whole union. It bases on array of unsigned char thus users have to use reinterpret_cast to cast data to proper type to work with. Suitable to be used as building block of custom allocator implementation, thus no need to manually handle and define backend data to hold aligned bytes.
std::aligned_alloc C++17 Dynamically aligned allocate memory space with input of alignment size, and size to allocate. It came from C thus need to include <cstdlib>. Alternatively, it’s possible to implement this ourselves see mem.h/mem.c.


Names Note
alignas A specifier to align stack definition. This is equivalent to GCC’s feature of __attribute__(aligned) but it’s defined in C++11 standard instead. It seems to have maximum alignment value of 128.

Example Codes

Topic URLs
std::align Multiple types aligned allocator on stack - Align.cpp, Single type with improvement aligned allocator on stack - Align2.cpp
std::alignment_of AlignmentOf.cpp
std::aligned_storage AlignedStorage.cpp
std::aligned_alloc AlignedAlloc.cpp


April, 15, 2020

First published on Oct, 18, 2019

Written by Wasin Thonkaew
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