Tuesday, April 14, 2009

slicing - [[c++ primer]]

Update -- c# has no slicing because the only composite type to be passed by cloning is struct but structs is never subclassed. Java is even cleaner. C# class/array/Delegate instances are like java objects -- always "pbref" rather than "pbclone" i.e. pass-by-value.

Slicing is the #1 pitfall in c++ inheritance, subsumption, pass-by-value (pbclone -- see other posts). It's common and tricky.

class Base{};
class Derived : public Base {};
Base b;
Derived d;

Q: when do we see slicing of a big object into a small object?
A: assigning qq(b = d). The LHS base object has a smaller real estate than the RHS subclass object
A: copying qq[Base b = d] or qq[Base b(d)]
A: {special case} when pbclone passing a child object into a parent parameter, essentially copying
A: a container with nonref type arg qq[ myMultiset.insert(d) ]
A: in all these cases, the LHS object is a parent object -- sufficient memory to contain parent's fields -- no room for child's toys:).

Q: When can we avoid slicing by sticking to just one object? Note copier/assignment always involve 2 objects -- always slicing.
A: pbref using ref or ptr, bypassing field-by-field cloning. However, you can't avoid slicing completely. If i have an object of type D, to be passed into a field [2] of type B, I get slicing of some sort always, since i can't access the child portion of the D object, unless i dynamic_cast.

[2] or stackVar incl params

onion -- slice off the outer layer.

See [[c++ primer]] P578

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