I declare a struct: struct ABC { int a; int b; char c; in d; } I use VC++ compiler. In main, I call function sizeof(ABC) and it returns 16. Why? Because I think sizeof(ABC) = sizeof(int) + sizeof(int) + sizeof(char) + sizeof(int) = 4+4+1+4 = 13. What happened?
It returns 16 because of the compiler using the packaging format. I would try to explain. When you have data type which holds the maximum chunk of byte it allocates the other smaller byte as a multiple of the highest possible bytes taken for optimization of the memory and nowadays memory is quite cheap. Try putting a float and or a double and see how the things change.
Thank alot. I will try. I sent a message to you about uml tool you are using. Please see it in the picture: http://www.go4expert.com/images/articles/designpattern/Visitor.png I am learning Design Pattern. Thanks.
With your answer, 3 remaining bytes will be used for what? With 16 bytes, you can tell me the position of struct's members. Example: a from byte 0->byte 3, ... Please continue to explain for me clearly. Why the compiler use this calculation way? Thanks.
You can choose to pack the struct so that it uses the number of bytes you think it should use, rather than aligning members on certain boundaries. In doing so you will cause a failure on systems that do not allow access on every byte boundary (Bus Fault). If you are not intimately familiar with your implementation, you will do well to accept the compiler's safer arrangement, despite the loss of a couple bytes of memory.
I can't understand clearly. I am reading about option /Zp in Visual C++ Compiler. Maybe I will give myseft some examples to understand about this. Thanks.
