Passing derived class by reference to function taking base class as parameter
c++, polymorphism
Solution
This is known as slicing.
When passing `A` by value, a local copy is made, by copying just the `A` part of the caller's object. Since this has both a static and dynamic type of `A`, that's the override of `print` that's chosen.
Passing by reference, the function gets a reference to the caller's object with dynamic type `B`, so that override is chosen.
Using abstract base classes can prevent confusing behaviour like this - they can't be directly instantiated, and so can't be passed by value.
Problem
Consider the following code: ``` class A { public: virtual ~A() { } virtual void print() const { std::cout << "In A" << std::endl; } }; class B : public A { public: B() { } virtual ~B() { } virtual void print() const { std::cout << "In B" << std::endl; } }; void doSomething(A* a) { a->print(); } void doSomething(const A a) { a.print(); } int main() { A* a = new B(); doSomething(a); doSomething(B()); return 0; } ``` Why does this output: ``` In B In A ``` But when you change `doSomething` to `doSomething(const A& a)` it outputs: ``` In B In B ```