C++ Class packing / member alignment

Question

I know structure packing is a common thing in C++ programming (at least on low memory systems). But what about classes. I know it works because I tried it

#include <iostream>

#pragma pack(push, 1)
class Test_Packed {
    uint8_t t;
    uint32_t test;
};
#pragma pack(pop)

class Test_Unpacked {
    uint8_t t;
    uint32_t test;
};

int main() {
    std::cout<<sizeof(Test_Packed) << " / " << sizeof(Test_Unpacked)<<std::endl;
    return 0;
}

Which correctly outputs "5 / 8".

Can I assume this to be the case on all conforming Compilers, or is this implementation defined?

I know that adding virtual members (and thus needing a vtable) will add additional data in the front. What can be other reasons for this to fail?

Can this cause any problems except for poor performance on some platforms?

This question itself is a great demo of proper #pragma pack usage and syntax. Just what I was looking for on how to use #pragma pack! — Gabriel Staples
Keep in mind, in C++, a struct is just a class where all members default to public if not otherwise specified, and a class is just a class/struct where all members default to private unless otherwise specified, so it seems to me that more narrowly packing just the desired members within a class or struct, rather than the entire class or struct as you have done, would affect a class or struct identically. — Gabriel Staples
See also: stackoverflow.com/questions/3318410/pragma-pack-effect — Gabriel Staples

Ðаn Ðаn · Accepted Answer · 2017-05-31T14:23:54

The documentation for #pragma states:

Implementation defined behavior is controlled by #pragma directive.

So the effect, if any, of #pragma pack(push, 1) and #pragma pack(pop) is completely "implementation defined."

C++ Class packing / member alignment

1 Answers