main.cpp

class MyBase
{
public:
    MyBase(int _x) : x(_x) {}
    virtual ~MyBase() {}
    virtual void say() = 0;
    int x = 0;
private:
    
};

class MyClassUser
{
public:
    MyClassUser(MyBase& _b) : b(_b) {}
    MyBase& b;
};

class MyDerived : public MyBase
{
public:
    MyDerived() : MyBase(42), user(*this) {}
    void say() override
    {
        std::cout << "MyDerived says " << x << "\n";
    }
    MyClassUser user;
};

void test_initlist()
{
    MyDerived d;
    d.say();
}