Outer class can't create an object of my inner class [duplicate]

Question

I am so confused. I have a nested class where Book is the inner and Bookshelf is the outer class. When I try to create a Book object in my Bookshelf class like Book b; it will work IF i have an empty constructor in my Book class. But when I try to use my current Book constructor that takes in 3 values, I cant declare it in my Bookshelf class. Why is that?

This is the error Im getting:

d.cpp:48:17: error: expected identifier before string constant
   48 |         Book b ("Hello", "2", 12);
      |                 ^~~~~~~
d.cpp:48:17: error: expected ',' or '...' before string constant 

---

This is my code:

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <stdexcept>

using namespace std;

class Bookshelf
{
public:
Bookshelf() = default;
 class Book
    {
    private:
            string title {};
            string author {};
            int pages {};
            int readers {};
        
        public:
            Book(string const& t, string const& a, int const p)
                : title{t}, author{a}, pages{p}, readers{0}
            {
            }

            string get_title () const
            {
                return title;
            }

            void print() const
            {
                cout << "Title: " << title << "\nAuthor: " << author
                     << "\nPages: " << pages << "\nReaders: " << readers << endl;
            }


        };

    void add_book(Book const& b) 
    {
    bookshelf.push_back(b);
    }

    private:
        vector<Book> bookshelf {};
        Book b ("Hello", "2", 12);
};

int main()
{
    Bookshelf::Book book_1("Hej", "Me", 100);
    Bookshelf::Book book_2("Yo", "Me", 150);
    Bookshelf bookshelf_1;

    return 0;
}