


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:
- Aligns the data member to meet its alignment requirement (a multiple of its size).
- Updates the common alignment requirement as the least common multiple of the current requirement and the data member's requirement.
- 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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