Adding a function pointer within a struct to print

Question

What would be the proper way to do the following in C?

typedef struct Book {
    char* title;
    unsigned int year;
    void // print ??;
} Book;

void print_book(Book *book)
{
    printf("{\n\ttitle: \"%s\",\n\tyear: %d\n}\n", book->title, book->year);
}

int main(int argc, char * argv[])
{
    Book romeo = {
        .title="Rome & Juliet",
        .year=2000
    };
    print_book(&romeo); // how can I do romeo.print() instead?
}

What would be the correct way to define the struct member print to point to the print_book function?

`romeo.print()` can't work in C, only in C++. In C the best you can do would require `romeo.print(&romeo)`, which isn't really better than `print_book(&romeo)`. And storing the function pointer would require extra space in every instance of the struct. — interjay, Nov 01 '20 at 19:49

Tom Karzes · Accepted Answer · 2020-11-01T20:20:29.440

3

The standard way to declare it would be:

typedef struct Book {
    char* title;
    unsigned int year;
    void (*func)(struct Book *book);
} Book;

I had to use struct Book instead of Book because the Book type has not yet been defined.

A better way to do it is:

typedef struct Book Book;
struct Book {
    char* title;
    unsigned int year;
    void (*func)(Book *book);
};

This creates the typedef before the structure definition, so the typedef name can then be used inside the structure.

To call it, you can do:

Book romeo;
romeo.func = print_book;
(*romeo.func)(&romeo);

In this context, you can optionally apply the & operator to print_book, e.g. romeo.func = &print_book;. The two are equivalent.

Note that I've called the function pointer func, but you can of course use any name you like (e.g. print, as in your post).

edited Nov 01 '20 at 20:20

answered Nov 01 '20 at 19:49

Tom Karzes

22,815
2
22
41

what does `(*func)` do? I've never seen that before in C. – carl.hiass Nov 01 '20 at 19:51
1

That's the traditional way to declare a function pointer (as opposed to an actual function definition). – Tom Karzes Nov 01 '20 at 19:52
@carl.hiass I suggest you read about **Function Pointers**. – gsamaras Nov 01 '20 at 19:52
@TomKarzes I see -- thank you. How would I then call the `print` member on the `romeo` object? – carl.hiass Nov 01 '20 at 19:53
@gsamaras sure can you provide a link with a good tutorial/documentation or something? – carl.hiass Nov 01 '20 at 19:54
@carl.hiass If you have `Book book;` and you set `book.func`, then you could call it as `(*book.func)(&book);` for example. The syntax has loosened up a bit in terms of including the `*`, but I like the call to match the declaration. That way it's clear that it's a function pointer. – Tom Karzes Nov 01 '20 at 19:55
@carl.hiass that is a plethora of options out there ;) Here is one I had used some years ago: https://www.cprogramming.com/tutorial/function-pointers.html – gsamaras Nov 01 '20 at 19:55
You'd need to call `romeo.print(&romeo)` or given a book pointer, `bookp->print(bookp)`. It isn't as canvas C++ where the `this` pointer is provided automatically. – Jonathan Leffler Nov 01 '20 at 19:58
@TomKarzes could you please show an example of the `func` function and how it would be called? I keep getting a seg fault when trying to access the `func` (which I've renamed from print). – carl.hiass Nov 01 '20 at 20:07
1

@carl.hiass Sorry for the name change, I hadn't realize that you had intended it to be called `print`, but you can use any name you like. The example call from my previous comment should work. I just added it to the answer - refresh the page and take a look. – Tom Karzes Nov 01 '20 at 20:11

score 0 · Answer 2 · answered Nov 01 '20 at 19:52

Generally speaking, you can't, because your print() function does not know about the Book containing it (or a pointer to it). It would have to be a function which takes a Book or const Book * as a parameter.

Also, it is not a good idea to mix-up the logic for printing a book with the fundamental definition of a book. Maybe in some other context, books are printed differently? They're still books. In most (?) programming languages supporting objects with methods, simple abstract objects don't include methods for printing themselves - that is done using stand-alone functions or other constructs.

Adding a function pointer within a struct to print

2 Answers2

Linked