Forward declaration as struct vs class

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 {
    ...
};

Library.h:

struct Book;

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

There were no compiler warnings and the resulting program seemed to work fine. It made me curious: in theory, can this ever cause a problem? It's just a pointer, yes, but for example, if the class had virtual methods, multiple inheritance, could the pointers be different?

I know the differences between class/struct regarding default public/private but I'm asking specifically about the forward declarations and possible consequences of swapping them.

struct and class are completely interchangeable as far as forward declarations are concerned. Even for definitions, they only affect the default access specifier of the objects members, everything else is equivalent. You always define "classes" of objects.

The only place where struct must be used over class, is when forward declaring opaque data for c bindings.

Regarding your edit:

I know the differences between class/struct regarding default public/private but I'm asking specifically about the forward declarations and possible consequences of swapping them.

Visual C++ produces warning C4099. It does it, because the name decoration for its functions incorporates the keyword you used. Therefore, programs may fail to link properly. So perfectly standard compliant code may not link when using VC++ (A bonehead move on Microsoft's part, AFAIC).

A discussion of this warning, as well as why it can be ignored if you are disciplined, can be found here