How does memory layout differ between Structures of Arrays (SoA) and Arrays of Structures (AoS) in C/C ?

Memory Layout of Structures

In C/C , when defining a structure, members are not necessarily allocated contiguously in memory. This is a result of struct padding. For instance, consider the following test structure:

<code class="c">struct test
{
   double height;
   int    age;
   char   gender;
}</code>

While A.height, A.age, and A.gender may appear in order logically, they may not occupy adjacent memory locations. This is because the compiler may insert padding between members to align them on specific boundaries, optimizing for faster memory access.

Layout of Structures with Arrays

The layout of a structure of arrays (SoA) differs from that of an array of structures (AoS).

SoA:

<code class="text">-----------------------------------------------------------------------------------
| double | double | double | *pad* | int | int | int | *pad* | char | char | char |
-----------------------------------------------------------------------------------</code>

In SoA, members are grouped by type, with padding between arrays. This can enhance efficiency for operations that involve accessing elements of a specific type across multiple instances.

AoS:

<code class="text">-----------------------------------------------------------------------------------
| double | int | char | *pad* | double | int | char | *pad* | double | int | char |
-----------------------------------------------------------------------------------</code>

In AoS, structures are stored contiguously, with padding within each structure. This layout may be more straightforward for human readability but may result in reduced performance for accessing elements across multiple instances of the same member.

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