std::ostream : class is inaccesible [C++]

I am getting this error in my implementation:

struct bookdatabase::Bookdatabase::Book

class "bookdatabase::BookDatabase::Book" is inaccessible

None of the following solutions solved my problem:

• std::basic_ostream is inaccessible in C++
• cannot access private members in friend ostream
• My ostream and istream friend function can't access private class members
• member of class inaccessible
• Inaccessible Members ?
  
  

Here's a picture of what visual studio has issue with in the .cpp file.

Here's a picture of the declaration in the header file.

Database.h

#include <string>
#include <vector>


#ifndef BOOKDATABASE_H
#define BOOKDATABASE_H

namespace bookdatabase {
    class BookDatabase {
    private:
        struct Book {
        private:
            std::string authorFirstName, authorLastName, authorFullName, bookTitle, pubDate;
        public:
            Book(const std::string &authFirst, const std::string &authLast, const std::string &title, const std::string &date);
            std::string getAuthor() const;
            std::string getBookTitle() const;
            std::string getPubDate() const;
            bool operator < (const Book &rhs) const;
            friend std::ostream& operator << (std::ostream& out, const bookdatabase::BookDatabase::Book& book);
        };
        void sortBooks();
        std::vector<Book>::iterator search(const std::string &title);
    public:
        BookDatabase();
        void printBookList();
        std::vector<Book> getDatabase() const;
        void removeBook(const std::string &title);

        void addBook(const std::string &authFirst, const std::string &authLast, const std::string &title, const std::string &date);
    private:
        std::vector<Book> database;
    };
}

#endif // BOOKDATABASE_H

Database.cpp

std::ostream & bookdatabase::operator<<(std::ostream & out, const bookdatabase::BookDatabase::Book & book) {
    out << authorFullName << ". " << bookTitle << ". " << pubDate;
    return out;
}

Am I having this issue because the Book class is a nested class?