Class constructor using object of another class

Question

In this code, I would like to use an object of class birthday, the constructors of class Date are:

    Date(unsigned int y, unsigned int m, unsigned int d);
    Date(string yearMonthDay); // yearMonthDay must be in format "yyyy/mm/dd"

But I'm getting an error: "No default constructor exist for class Date"

class Person {

public:
    Person(std::string name, char gender, Date birthday);
    string getName();
    char getGender();
    int getYear();
    int getMonth();
    int getDay();

private:
    Date birthday;
    std::string name;
    char gender;
};

Person::Person(std::string name, char gender, Date birthday){
    Person::name = name;
    Person::gender = gender;
    Person::birthday = birthday;
}

Class Date:

class Date {
public:
    Date(unsigned int y, unsigned int m, unsigned int d);
    Date(string yearMonthDay); // yearMonthDay must be in format "yyyy/mm/dd"
    void setYear(unsigned int y);
    void setMonth(unsigned int m);
    void setDay(unsigned int d);
    void setDate(unsigned int y, unsigned int m, unsigned int d);
    unsigned int getYear() const;
    unsigned int getMonth() const;
    unsigned int getDay() const;
    string getDate() const; // returns the date in format "yyyy/mm/dd"

private:
    unsigned int year;
    unsigned int month;
    unsigned int day;
};

Answer 1

Use a constructor initializer list :

Person::Person(std::string name, char gender, Date birthday){
    : name(name), gender(gender), birthday(birthday)
{
    // Empty
}

That will initialize the members only once using the arguments, calling the appropriate constructors of the members.

Without a constructor initializer list, the members will be default initialized (default constructed) and then in the body of the constructor you use normal assignment to the members.

Answer 2

When you write any of your own constructors, the no-argument constructor is no longer supplied by the compiler. You can reinstate it (ask for it to be supplied) though:

Date() = default;

To make sure that you can default construct a Date safely, you should give proper defaults to its members:

private:
    unsigned int year{2000};
    unsigned int month{4};
    unsigned int day{30};