Question

I use C++ for embedded programming.

Supposing I have to implement a strictly-defined (i.e. byte-by-byte) class type, can I add a constructor and some other non-virtual methods to it without objects of that type changing at the byte level? That is, can I assume that no additional data will be added to it?

I assume that RTTI is turned off.

I would like to be sure wheteher C++ standard defines this.

Answer 1

Yes, if you only add constructors and/or non-virtual methods you will not change the size or the layout of the class, because the original class and the new class will be layout compatible (9.2 Class members [class.mem] #17), but only if they are standard-layout classes.

The standard-layout classes are defines as:

9 Classes [class]

A standard-layout class is a class that:

— has no non-static data members of type non-standard-layout class (or
array of such types) or reference,

— has no virtual functions (10.3) and no virtual base classes (10.1),

— has the same access control (Clause 11) for all non-static data
members,

— has no non-standard-layout base classes,

— either has no non-static data members in the most derived class and
at most one base class with non-static data members, or has no base
classes with non-static data members, and

— has no base classes of the same type as the first non-static data
member.