My code does not compile for a strange reason [closed]

C4700 is a warning, not an error. Your code compiles fine. It is just telling you that the members m_vitesse, m_portes, and m_prix are left uninitialized. If you want to initialize them, you’ll need to give those classes their own constructors, such as:

class Vehicule {
public:
    // This constructor initializes m_prix to 0
    Vehicule() : m_prix(0) { }

    void affiche() const {
        cout << "Ceci est un vehicule." << endl;
    }

protected:
    int m_prix;
};