How is the Size of a C Class Determined, and What Role Does Alignment Play?

Understanding Class Size Determination in C

Determining the size of a C class is a crucial aspect of memory management and object alignment.

Rules for POD Classes

For Plain Old Data (POD) classes, the compiler follows specific rules to calculate the size:

1. Each member has a size (s) and alignment requirement (a).
2. The compiler initializes the class size (S) to zero and alignment (A) to one.

3. For each member, consider its alignment:

   • If S is not divisible by a, adjust S to make it a multiple of a.
   • Update A to the least common multiple (LCM) of A and a.
   • Allocate s bytes for the member.

4. After processing all members, ensure the class alignment is satisfied:

   • If S is not divisible by A, adjust S to make it a multiple of A.

Alignment Considerations

The alignment requirement ensures that data members are placed at addresses that align with their respective sizes. This improves performance on some hardware architectures by allowing for efficient access and manipulation of data.

• char buf[8] has a size of 8 bytes and alignment of 1.
• __m128i vect has a size of 16 bytes and alignment of 16.

Example: TestClass3

Consider TestClass3:

<code class="c++">class TestClass3 {
  char buf[8];
  __m128i vect;
  char buf2[8];
};</code>

Applying the rules:

• Initial S is 0 and A is 1.
• buf[8] requires 8 bytes and alignment 1. Adjust S to 8 and maintain A.
• vect requires 16 bytes and alignment 16. Increase S to 16, set A to LCM(A, 16) = 16, and allocate 16 bytes.
• buf2[8] requires 8 bytes and alignment 1. Increase S to 24 and maintain A.
• Final class alignment is not satisfied, so increase S to 32 (multiple of A = 16).

Therefore, TestClass3 is 32 bytes in size.

Conclusion

By understanding these rules and alignment requirements, developers can optimize memory usage and enhance the performance of their C applications. This knowledge is essential for designing efficient data structures and managing memory effectively.

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