AMDG Domagoj Saric wrote:
why is boost::optional laid out so that the bool comes before the object's placeholder/storage?
this takes up to 7 extra bytes of storage with default alignement
These two structs should both take up 16 bytes. You don't save anything by reordering the members. struct s1 { bool initialized; double value; }; struct s2 { double value; bool initialized; };
and, more importantly, it forces pointer offset adjustment/calculation when accessing a boost::optional<> through a pointer (and trying to access the actual contained object)...
Is the difference in performance actually measurable in any way?
it also makes life harder for those of us keen to hackery ;) because it makes the address of an optional different from the address of the actual object within the optional...
Um, you shouldn't rely on this anyway. The standard only makes guarantees about the addresses of sub-objects for POD types.
ps. it would be 'cool' if all of the functionality from boost::optional<> not related to the 'is initialized' bool (conditional destruction, etc...) could be extracted into a separate class (usefull for cases where you have to delay/in-place construct an object but are certain that you will construct it by the point where it needs to be used or destroyed...so you do not need the 'conditional' overhead)...
In Christ, Steven Watanabe