The derived object that arrived as a base

#include <iostream>

struct Base {
    virtual void who() { std::cout << "base\n"; }
    virtual ~Base() = default;
};
struct Derived : Base {
    void who() override { std::cout << "derived\n"; }
};

void show(Base b) { b.who(); }

int main() {
    Derived d;
    show(d);
}

show takes Base by value, so the Derived is copied into a Base — sliced. The derived part is gone before who() runs, and virtual dispatch on the sliced copy calls the base version.

Observed output: base.

Fix: take const Base&.

Lesson: pass polymorphic objects by reference, never by value.

Facts verified 2026-11-04. The code on this page was compiled and run on the box with GCC 16.2.0 at -std=c++23 -Wall -Wextra; the outputs quoted are the observed outputs. MSVC was not tested.

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