Wrong sizeof of structures ????

[tonnot]

struct AA {
    float a;
    float b;
    unsigned char metadata;
};

The size of this are 4+4+1 , isn't it ?

AA *aa = new AA;
sizeof *aa = 12 ?????

I have read something like

your compiler may have a "#pragma option" feature that you can use to
wrap your structure definition, to enforce the byte alignment you want
You have to refer to your documentation for details.

[Disch]

Memory that is aligned on a 4-byte boundary may be faster to access, so as an optimization the compiler pads the structure size (and possibly members within the structure) to be on 4-byte boundaries.

You can turn that optimization off my changing your compiler settings or with #pragmas, but there's not really any reason to. It's not like losing 3 bytes is significant.