Can one extend virtual interface without recompilation of client code?

Question

A library provides a class with virtual functions. Can this class be extended with new virtual functions without recompiling binaries dynamically linked to the library?

I beleive this is not possible in standard. Are there platforms allowing this?

Would that be easier if new functions are only added to the end of the class body?

Answer 1

The standard is not concerned with binary compatibility. It is concerned though with classes, and by "changing" the definition of a class from one translation unit to another you indeed invoke undefined behavior.

Most compilers do allow a number of changes without the need for recompilation, however the list is small... and for this one I would say that it might not be possible, depending on the a priori knowledge of the derived classes.

The problem I foresee lies with the optimization that compilers usually carry out on the virtual tables.

When you create a class with virtual functions, you get a virtual table that looks like so:

// B virtual table
0 - Offset to complete object
1 - RTTI
2 - func0
3 - func1
...

In order to gain some space, the derived class own virtual functions are usually "appended":

// D virtual table
Same as B
N+3 - func(N+1)
N+4 - func(N+2)

This way a D object only has one virtual pointer, than can be used as such even when the type is (statically) a B (through pointer or reference).

However, if you were to extend B without recompiling D, then it would just plain crash, since when calling the N+1 function of B you would instead call the N+1 function of D which would probably not even have the same arguments... oups!

It can be done, though, if you know than no derived class add any virtual function of its own.