C++ Object slicing

Object slicing occurs when a derived class object is copied into a base class object by value, silently discarding derived-class members. The code compiles and runs without error, but loses data and polymorphic behavior.

Prevent slicing by passing polymorphic objects by reference or pointer, never by value.

This example shows how slicing loses derived class data when passing by value.

// compile: g++ -o slicing slicing.cpp
// run: ./slicing
// description: demonstrate object slicing when copying derived to base
 
#include <iostream>
 
class Animal {
public:
    virtual ~Animal() = default;
    virtual void speak() { std::cout << "Animal sound\n"; }
};
 
class Dog : public Animal {
public:
    int tricks = 5;
    void speak() override { std::cout << "Woof! (" << tricks << " tricks)\n"; }
};
 
void processAnimal(Animal a) {  // passed by value: slicing occurs!
    a.speak();
}
 
int main() {
    Dog dog;
    dog.tricks = 10;
 
    processAnimal(dog);  // Dog sliced to Animal; tricks lost, prints "Animal sound"
 
    return 0;
}