Can I force a C++ class to compile using the minimum amount of space?

Question

I have a class that I will literally instantiate billions of. I did my best to put as little data as possible in it, and I got it down to 10 bytes when compiled for an x64 architecture: an unsigned short int and a size_t. When I mouse over sizeof(myclass) (in VS), however, it reports 16 bytes. If I comment out either of the fields, the size of the class is what you'd expect, 10 bytes and 2 bytes respectively.

Is there a way to instruct the compiler to use only 10 bytes for my class?

Thanks! Throw `#pragma pack(2)` in an answer, and I'll mark it as the best answer. — dx_over_dt, Oct 08 '17 at 02:29
@RemyLebeau why `pack(1)` when I know my object is an even number of bytes? — dx_over_dt, Oct 08 '17 at 02:47
@dfoverdx if someday you decide to add a 1-byte `char` or `bool` member, `pack(2)` won't produce the smallest size possible, there will be 1 byte of padding. If you want the smallest size possible, use the smallest alignment possible. — Remy Lebeau, Oct 08 '17 at 03:28

score 0 · Accepted Answer · answered Oct 08 '17 at 04:15

#pragma pack can make your structures smaller. On the other hand, it can make retrieving the member data slower, since the alignment of the bytes is not guaranteed. As a result the compiler will need extra instructions to fetch the data from different memory locations. For more info, this link has an excellent post: #pragma pack effect

Can I force a C++ class to compile using the minimum amount of space?

1 Answers1