C++ Brain Teasers: Exercise Your Mind/

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Discussion: The Phantom Spaceship

Discussion: The Phantom Spaceship

Execute the code to understand the output and gain insights into object construction and virtual function behavior.

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Run the code

Now, it’s time to execute the code and observe the output.

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#include <iostream>
#include <string>
struct GameObject
{
    GameObject() { std::cout << "Created a " << getType() << "\n"; }
    void render() const { std::cout << "Rendered a " << getType() << "\n"; }
    virtual std::string getType() const { return "GameObject"; }
};
class Spaceship : public GameObject
{
    std::string getType() const override { return "Spaceship"; }
};
void display(const GameObject &gameObject) { gameObject.render(); }
int main()
{
    GameObject gameObject;
    Spaceship spaceship;
    display(gameObject);
    display(spaceship);
}

Understanding the output

In this puzzle, let’s say we’ve attempted to add logging to our game to keep track of the creation and rendering of game objects. We’ve decided to use the template method design pattern, which can be a great way to achieve runtime polymorphic behavior in our design. In this pattern, a base class defines common behavior in a concrete function, which internally calls virtual functions that can be overridden in derived classes to fill in the details. For example, the preceding render() function is a template method that calls the virtual getType() function to fill in the details of the specific derived type.

This works perfectly for the render() function, but something goes wrong in the constructor; it prints Created a GameObject both when creating a GameObject and when creating a Spaceship. What exactly is going on, and could this be undefined behavior?

Constructor’s behavior

In main, when initializing an object of type Spaceship, the GameObject base class part of the object is initialized first, and then the Spaceship part of the object is ...