朋友功能错误

Question

Book.h：

#ifndef BOOK_H
#define BOOK_H
#include <string>
class Author;

class Book {

public:
    Book(const std::string title, const std::string publisher, const Author& author, int isbn, double price);
    std::string getTitle() const;
    Author getAuthor() const;
    Book& setAuthor(const Author& author);
private:
    std::string b_title;
    std::string b_publisher;
    Author b_author;
    int b_isbn;
    double b_price;
};

Author.h：

#include <string>
#ifndef AUTHOR_H
#define AUTHOR_H

class Book;

class Author {
    friend Book::getAuthor() const;
    friend Book::setAuthor(const Author& author);
    public:
        Author(const std::string& name, const std::string& email, Gender gender);
        Author& setName(const std::string& name);
    private:
        std::string a_name;
        std::string a_email;
        int a_gender;
};

#endif

在author.h中的错误，在友元函数上：

friend Author Book::getAuthor() const;

错误是：

error: invalid use of incomplete type 'class Book'
error: forward declaration of 'class Book'

当然，你说这是一个重复的问题。我看到了你所写的内容，但我担心它没有涵盖错误。