Le 14/08/2012 13:11, Warp a écrit :
>> IIRC, C++ does not duplicates the class function members (not even
>> > pointers to them) in each instance. Only the actual class (as 1 pointer)
>> > is stored within each instance. The virtual function is just an index.
>> > The first virtual/inheritance cost a pointer in the class. The next are
>> > free.(in fact, with RTTI, even without inheritance, you might already
>> > have that pointer: so no additional cost)
> But if your class were eg. the size of one pointer, then adding any virtual
> functions would double its size.


Tetracapilli... if you compile with RTTI (default for most, including
gnu), an instance of an empty class is already the size of a pointer: to
the class description.

Adding any virtual function in such case would not increase the size of
the instance.

Also, remember that even if the root class is size of a pointer, its
children can only get the same size or larger. If every layer adds a
pointer, the second layer is already the size of two: one proper, one
from parent field (and a third for RTTI)

Removing RTTI <==> no dynamic_cast<>,... might break some code, yet it's
possible (for tight embedded system, including the Embedded C++ spec,
which are just PITA and was made for very low memory system).
In Embedded C++, there is no multiple inheritance, no C++ cast (neither
down or up), no mutable attribut, no namespace, no exception, no
templates and no stdio.
Usage of EC++ is low, but that's might already be too much. It's kind of
C on very fascist mode.

Post a reply to this message