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C ++ implicit and explicit call to inheritance constructor

I have a question about implicit and explicit calls to the base constructor. If we have a class hierarchy:

class Person{
    protected:
        std::string m_name;
    public:
        Person(std::string& _name) : m_name(_name){std::cout << "A person is being constructed." << std::endl;}
};

class Baby : public Person{
    private:
        int m_no_of_nappies;
    public:
        Baby(std::string& _name, int& _no_of_nappies) : m_no_of_nappies(_no_of_nappies), Person(_name) {std::cout << "A baby is being constructed." << std::endl ;}

};

According to my lectures, the call to “Baby” is basically like this:

std::string babyname = "Robert";
int nappies = 5;

Baby baby(babyname, nappies);

Invokes the following:

  • Since an explicit call to Person is made in the Baby initialization list: The Baby initialization list is received , but no_of_nappiesinitialized.
  • Next, a call to the Person constructor is created and the Person initialization list is called . Initialized m_name.
  • Then the body of the constructor Person is called.
  • The body of the child’s constructor is finally called.

This makes sense, however, what if there were implicit calls made to the default constructor of the parent class, for example:

class Vehicle{
    protected:
        int m_no_wheels;
    public:
        Vehicle() : m_no_wheels(0) { std::cout << "A vehicle is being constructed." << std::endl; }
};

class Bicycle : public Vehicle{
    protected:
        bool m_is_locked;
    public:
        Bicycle() : m_is_locked(false) { std::cout << "A bicycle is being constructed." << std::endl; }
};

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