前向声明为结构 vs 类

I found a mistake in a C++ forward declaration of a class, which was wrongly declared as struct Book instead of class Book. I think Book used to be a struct, got changed to a class but the forward declarations remained.

Book.h:

class Book {
    ...
};

库.h:

struct Book;

class Library {
    std::vector<Book*> books;
};

没有编译器警告，生成的程序似乎运行良好.这让我很好奇:理论上，这会导致问题吗?它只是一个指针，是的，但是例如，如果类有虚方法、多重继承，指针会不同吗?

Struct 'Book' was previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI