sizeof(): where is the problem with calculation?

Question

First of all, on my system the following hold: sizeof(char) == 1 and sizeof(char*) == 4. So simply, when we calculate the total size of the class below:

class SampleClass { char c; char* c_ptr; };

we could say that sizeof(SampleClass) = 5. HOWEVER, when we compile the code, we easily see that sizeof(SampleClass) = 8.

So the question is "where is the problem with calculation?" :S

Language: C++ Compiler: gcc 4.4.0 OS: Tinycore

Answer 1

Compilers usually add padding to structures to align them on word boundaries (because accessing word-aligned locations requires fewer memory accesses and hence is faster).

So even though the char takes only 1 byte, c_ptr is shifted to the next 4-byte boundary, hence the result of 8 bytes.

Answer 2

This is caused by padding.
The compiler is adding padding:

• to make access to members as fast as possible
• also to make arrays of the object pack so that access to elements effecient.

So objects that have a size of 1 can be aligned to 1 byte boundaries and still be easy/efficient to read. While objects of size of 4 need to be aligned on 4 byte boundaries (as appropriate to your compiler (technically you can align to 1 byte boundaries but this means you usually need multiple instructions to extract and combine and thus it is more efficient to write to 4 byte boundaries)).

Thus for optimum alignment of structures it is best to order the members by size (largest first) This will give you the optimum packing strategy in most normal situations.

This will not stop your object being eight bytes though.
As the compiler is also taking into account that your class may be used in arrays. Thus each element in the array needs to be aligned so that the largest member of each element is aligned appropriately.