How Does C Compiler Determine the Size of a Class Based on Data Members and Alignment?

Determining the Size of a C Class

C classes occupy memory in a manner determined by specific rules during compilation. The size of a class is calculated based on the data members it contains and their respective alignments.

When the compiler analyzes a class, it evaluates each data member in sequence:

1. Aligns the data member to meet its alignment requirement (a multiple of its size).
2. Updates the common alignment requirement as the least common multiple of the current requirement and the data member's requirement.
3. Increases the class size by the data member's size.

After processing all data members, the compiler adjusts the class size to be a multiple of the common alignment requirement.

In the example provided:

TestClass1 and TestClass2

Both have similar data members (two char arrays of size 8), resulting in a size of 16 bytes.

TestClass3

It contains the same data members as TestClass1 and TestClass2, but the __m128i data member requires 16-byte alignment. This alignment overrides the alignment of the char arrays, resulting in a class size of 48 bytes (16 bytes for alignment, then 16 bytes for __m128i, then 16 bytes for more alignment).

TestClass4

This class has the same data members as TestClass3, but with a different order. However, the alignment rules remain the same, leading to a class size of 32 bytes (8 bytes for the first char array, 16 bytes for alignment, 16 bytes for __m128i, then 8 bytes for the second char array).

These rules ensure efficient memory allocation and alignment for optimized data access and performance.

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