What is object slicing in C++ or Java?

Object slicing is used to describe the situation when you assign an object of a derived class to an instance of a base class. This causes a loss of methods and member variables for the derived class object. This is termed as information being sliced away. For example,

class Foo {
   int a;
};
   class Bar : public Foo {
   int b;
};

Since Bar extends Foo, it now has 2 member variables, a and b. So if you create a variable bar of type Bar and then create a variable of type Foo and assign bar, you'll lose the member variable b in the process. For example,

Bar bar;
Foo foo = bar;

In this case, the information in for about b is lost in a bar. This is known as member slicing.