Size of a class

[pepribal]

I've defined a class MyClass; it has no member functions, just data. Data is: 11 double's and 3 unsigned short's.

That should amount a total of 94 bytes (11*8 + 3*2). However, if I print a sizeof(MyClass) to the console, it comes out with 96 bytes... Why?

And it becomes more puzzling: the same code in windows using MSYS and MinGW, comes out with a result of 104 bytes!!

I've checked, and in both platforms, double's are 8 bytes and unsigned short's are 2 bytes... So what are these extra bytes? How can I make sure the class will be the same size on both platforms (I intend to do a multiplatform app).

Thanks.

[ppesci]

Size depends on how compiler work and what libraries you use. The extra bytes is for wrap your data in a class I think. If you want the same size, maybe you get better (but not guaranteed) results, if you use a struct to hold your data.

[markkuk]

Originally Posted by pepribal

I've checked, and in both platforms, double's are 8 bytes and unsigned short's are 2 bytes... So what are these extra bytes?

The compiler may add extra bytes to align the data members for optimal access speed.

Originally Posted by pepribal

How can I make sure the class will be the same size on both platforms

You can't do that in a portable way. Write functions that pack/unpack data into byte arrays with known byte ordering for binary data storage and exchange between different platforms. Alternatively, use text-based data exchange formats.

[pepribal]

OK, thanks a lot, guys.