Как реализовать вовод в поток класса, вектора переменной C++

#ifndef LINE_H
#define LINE_H
#include <string>
#include <vector>
#include <iostream>
#include <istream>

struct Date {
    int day{ 01 };
    int month{ 01 };
    int year{ 1999 };
    Date(int day, int month,int year ) {
        this->day = day;
        this->month = month;
        this->year = year;
    }
    Date() = default;
    friend std::istream& operator >>(std::istream& is, Date& date) {
        is >> date.day >> date.month >> date.year;
        return is;
    }
    friend std::ostream& operator <<(std::ostream& os, Date& date) {
        os << date.day << "." << date.month << "." << date.year;
        return os;
    }
};

class letter;

class Email {
    std::vector<letter> letters;
public:
    std::string userName{ "None" };
    Email(std::string userName) ;
    Email() = default;
    friend std::istream& operator >>(std::istream& is, Email& mail) {
        is >> mail.userName;
        return is;
    }
    friend std::ostream& operator<<(std::ostream& os, Email& mail) {
        os << "Имя пользователя: " << mail.userName << std::endl;
        os << "Содержимое вектора letters:" << std::endl;
        for (const letter& letterObj : mail.letters) {
            os << "Отправитель: " << letterObj.sender.userName << std::endl;
            os << "Сообщение: " << letterObj.message << std::endl;
            os << "Дата: " << letterObj.when << std::endl;
            os << std::endl;
        }
        return os;
    }
    std::vector<letter>& getLetters() {
        return letters;
    }
    void setLetters(const std::vector<letter>& newLetters) {
        letters = newLetters;
    }
};

class letter {
    Email sender;
    std::string message;
    Date when;
public:
    letter() = default;
    letter(std::string userName, std::string message, Date when);
    friend std::istream& operator>>(std::istream& is, letter& mes) {
        std::string email;
        is >> email;
        char quote;
        is >> std::ws >> quote;
        std::getline(is, mes.message, '"');
        is >> mes.when;
        return is;
    }
};
#endif

В классе Email нужно реализовать вывод в поток мой вектор letters