What does slicing mean in C++?

Question

It is mentioned in C++ FAQ site -- "larger derived class objects get sliced when passed by value as a base class object", what does slicing mean? Any sample to demonstrate?

http://www.parashift.com/c++-faq-lite/value-vs-ref-semantics.html#faq-31.8

I am using VSTS 2008 + native C++ as my development environment.

Answer 1

Quoting this lecture:

Slicing

Suppose that class D is derived from class C. We can think of D as class C with some extra data and methods. In terms of data, D has all the data that C has, and possible more. In terms of methods, D cannot hide any methods of C, and may have additional methods. In terms of existing methods of C, the only thing that D can do is to override them with its own versions.

If x is an object of class D, then we can slice x with respect to C, by throwing away all of the extensions that made x a D, and keeping only the C part. The result of the slicing is always an object of class C.

slicing http://webdocs.cs.ualberta.ca/~hoover/Courses/201/201-New-Notes/lectures/slides/slice/slide1.gif

Design Principle: Slicing an object with respect to a parent class C should still produce a well-formed object of class C.

Usage Warning: Even though D is-a C, you must be careful. If you have a argument type that is a C and you supply a D it will be sliced if you are doing call by value, pointer, or reference. See the example below.

Note on virtual functions. Their signatures are used to identify which one to execute.

Watch out for the sliced = operator, it can make the lhs inconsistent. Also, the operator= is never virtual, it wouldn't make sense. For example, suppose classes A, B are both subclasses of class C. Just because an A is a C, and a B is a C, it doesn't mean you can assign a B object to an A object. Without run-time type information you cannot make a safe assignment.

Answer 2

Slicing means that the derived class information is lost. The base class parameter forces an object of the base class to created which share the same base-class state as the derived class. E.g:

struct B { int x; };

struct D : B { double y; };

void f(B b) {}

int main() {
  D d;
  f(d);
}

In the function f you will not be able to access D::y.

Edit: Thanks to everyone for editing. Please make sure that the edit adds value. Note that

• For structs the default inheritance is public.
• If you fix the code, make sure you update the body too