How can I hide the declaration of a struct in C?

Question

In the question Why should we typedef a struct so often in C?, unwind answered that:

In this latter case, you cannot return the Point by value, sinc

Answer 1

Have a look at this example of a library, using a public header file, a private header file and an implementation file.

In file public.h:

struct Point;

struct Point* getSomePoint();

In file private.h:

struct Point
{
    int x;
    int y;
}

In file private.c:

struct Point* getSomePoint()
{
    /* ... */
}

If you compile these three files into a library, you only give public.h and the library object file to the consumer of the library.

getSomePoint has to return a pointer to Point, because public.h does not define the size of Point, only that is a struct and that it exists. Consumers of the library can use pointers to Point, but can not access the members or copy it around, because they do not know the size of the structure.

Regarding your further question: You can not dereference because the program using the library does only have the information from private.h, that does not contain the member declarations. It therefore can not access the members of the point structure.

You can see this as the encapsulation feature of C, just like you would declare the data members of a C++ class as private.