Understanding Class Size Determination in C
Determining the size of a C class at compile-time requires an understanding of data alignment principles. Each member within a class has a size and alignment requirement.
Size and Alignment Calculation Process
During compilation, the compiler initializes a running size (S) to zero and an alignment requirement (A) to one (byte). For each member in the class:
- The alignment requirement (a) is checked. If S is not a multiple of a, S is adjusted to the nearest multiple. This determines the offset of the member.
- The least common multiple (LCM) of A and a is calculated, and A is updated to LCM.
- Space (s) for the member is added to S.
After processing all members, the final size of the class is determined. It is the value of S adjusted to be a multiple of the alignment requirement of the entire class (A).
Alignment Considerations
- Arrays: Size is the number of elements multiplied by element size, and the alignment requirement is the same as an element.
- Structures: Size and alignment requirements are determined recursively.
- Unions: Size is the size of the largest member, rounded up to the LCM of all members' alignments.
Example
The provided code demonstrates this process:
<code class="cpp">#include <xmmintrin.h> class TestClass1 { __m128i vect; }; // Size: 16 bytes class TestClass2 { char buf[8]; char buf2[8]; }; // Size: 16 bytes class TestClass3 { char buf[8]; __m128i vect; char buf2[8]; }; // Size: 48 bytes class TestClass4 { char buf[8]; char buf2[8]; __m128i vect; }; // Size: 32 bytes</code>
TestClass3, despite having the same members as TestClass1 and TestClass2, is larger (48 bytes) due to the alignment requirement of __m128i, which forces a 16-byte boundary. TestClass4, with a different data member ordering, has a different alignment and size (32 bytes).
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