On 11/24/2010 11:17 AM, Jes Sorensen wrote:
On 11/24/10 18:11, Avi Kivity wrote:
On 11/24/2010 07:06 PM, Jes Sorensen wrote:
Sorry but that is utterly and completely bogus! The enforcement is only
as good as the developers and maintainers make it,
class File {
public:
virtual ~File() {}
virtual void read(...) = 0;
virtual void write(...) = 0;
};
Anyone wishing to implement this interface is forced to implement read
and write methods (callbacks) with exactly the right signature. The
compiler will complain if they don't. So if File is a good interface,
we can make the compiler force people to use it correctly.
We can emulate this in C with ->ops-> things, but that's just
boilerplate and more places for people to get things wrong, or lazy and
take shortcuts.
In the mean time we spend our time debugging the runtime because the
virtual functions don't behave as expected. In C we know what is going
on, in C++ it is pray and hope.
The alternative in C is:
struct file_operations {
void (*release)(void);
void (*read)(...);
void (*write)(...);
};
struct file {
struct file_operations *ops;
};
If I do:
static file_operations posix_file_ops = {
.read = posix_file_read,
.write = posix_file_write,
};
struct posix_file {
struct file parent;
int fd;
};
void posix_file_init(struct posix_file *obj)
{
obj->parent.ops = posix_file_ops;
obj->fd = ...;
};
The compiler won't generate an error. Only upon a call to
file_release() will a null pointer dereference happens whereas in C++,
because this paradigm is structured in the language, the compiler can
help assist you.
Regards,
Anthony Liguori
Jes
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