What happens if I delete an instance of a derived class through a pointer to its base class when the base class destructor is not marked virtual?

If the base destructor is not virtual, calling delete on a Base* pointing to a Derived instance invokes Undefined Behavior (UB)! In practice, the compiler will only execute the Base destructor, leaving any dynamically allocated members in the Derived class leaked in memory.

Always follow the Rule of Zero, or declare public virtual ~Base() = default; whenever a class contains at least one virtual method.