The error shows: no matching functions for call to birthday::birthday(). How else should I declare the objects of one class in another class?

#include <iostream>

using namespace std;

class birthday
{

public:

    birthday(int d,int m,int y)
    {
        date=d;
        month=m;
        year=y;
    }
    void printdate()
    {
        cout<<date<<"/"<<month<<"/"<<year<<endl;
    }

private:

    int date;
    int month;
    int year;
};

class person
{

public:

    person(string x)
    {
        name=x;
    }
    void printname()
    {
        cout<< "the birthday person is "<< name << " whose birthday is on "<< day.printdate();
    }

private:

    birthday day;
    string name;
};

int main()
{
   birthday b(10,11,2007);
   person p("Jessica");
   p.printname();
   return 0;
}

CodePudding user response：

Just define a default constructor so you can make an object of type birthday without initializing it.

or you can default initialize person type private data member day.

CodePudding user response：

While the answers given regarding adding a default c'tor for birthday are technically correct, I think that they miss the real issue. Given how you declare the variables b and p in main(), it looks as if your intent is for the birthday variable b to be added to the person variable, which implies that what you actually want is to modify the person() constructor as such:

    person(string x, birthday b): name(x), day(b)
    { }

This will ensure that a birthday object is defined for a given person object when the person is constructed.

This isn't to say that adding a default c'tor for either birthday or person is wrong; indeed, it would probably be advisable. But on its own it might not have the desired effect.

CodePudding user response：

The problem is that your class birthday has a user-defined constructor and therefore the compiler will not synthesize the default constructor birthday::birthday() for your class.

So when you wrote,

class person
{
    //other members here
    private:
         birthday day; //this need the default constructor of birthday. But since class birthday has no default constructor this statement gives error
};

In the above snippet, the statetment, birthday day; needs the default constructor of class birthday but since there isn't one, so this statement results in the mentioned error.

Solution

To solve this you can add a default constructor for your class birthday as shown below:

class birthday
{

public:
    //add default constructor
    birthday() = default;
    birthday(int d,int m,int y)
    {
        date=d;
        month=m;
        year=y;
    }
    void printdate()
    {
        cout<<date<<"/"<<month<<"/"<<year<<endl;
    }

private:

    //using in class initializers give some default values to the data members according to your needs
    int date = 0;
    int month =0;
    int year = 0;
};

class person
{

public:

    
    person(string x)
    {
        name=x;
    }
    void printname()
    {
        cout<< "the birthday person is "<< name << " whose birthday is on "; 
        
        day.printdate();//moved this out of the cout because printdat() does not return anything
    }

private:

    birthday day;
    string name;
};

int main()
{
   birthday b(10,11,2007);
   person p("Jessica");
   p.printname();
   return 0;
}

Some of the changes that i made include:

1. Added default constructor for class birthday.
2. Used in-class initializers for data member date, year and month inside class birthday.
3. There was another problem with your code. Originally you had a statement cout<<day.printdate() in your program. But since printdate() has return type void, so the statement cout<<day.printdate() will give you error. To solve this, i removed the cout from this statement.

The output of the modified program can be seen here.