Why do we need a virtual destructor in C++?

If you delete a derived object through a base pointer and the base destructor is not virtual, only the base destructor runs. This is undefined behavior, and the resources of the derived class leak.

#include <iostream>
#include <memory>

struct Base {
    virtual ~Base() { std::cout << "~Base\n"; }
};

struct Derived : Base {
    ~Derived() override { std::cout << "~Derived\n"; }
};

int main() {
    std::unique_ptr<Base> p = std::make_unique<Derived>();
}   // both destructors run, because ~Base is virtual

Rule: a class which is used as a base class for polymorphism must have a virtual destructor (or a protected non virtual one). See Polymorphism.

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