What defines an opaque type in C, and when are they necessary and/or useful ?

Question

I've seen the concept of 'opaque types' thrown around a bit but I really haven't found a succinct answer as to what defines an opaque type in C and more importantly what problems they allow us to solve with their existence. Thanks

Answer 1

In my understanding, opaque types are those which allow you to hold a handle (i.e., a pointer) to an structure, but not modify or view its contents directly (if you are allowed to at all, you do so through helper functions which understand the internal structure).

Opaque types are, in part, a way to make C more object-oriented. They allow encapsulation, so that the internal details of a type can change--or be implemented differently in different platforms/situations--without the code that uses it having to change.

Answer 2

An opaque type is a type which is exposed in APIs via a pointer but never concretely defined.

Answer 3

It is the most generally used for library purpose. Since the implementation is hidden, you can modify the library without recompiling any program which depend on it (if the interface is respected)

eg: version 1:

// header file
struct s;

int s_init(struct s **x);
int s_f(struct s *x);
int s_g(struct s *x);

// implementation
struct s { int x; }

int s_init(struct s **x) { *x = malloc(...); }
int s_f(..) { ... }
int s_g(..) { ... }

version 2

// header file
struct s;

int s_init(struct s **x);
int s_f(struct s *x);
int s_g(struct s *x);

// implementation
struct s { int y; int x; }

int s_init(struct s **x) { *x = malloc(...); }
int s_f(..) { ... }
int s_g(..) { ... }

From your program side, nothing changed! and as said previously, no need to recompile every single program which rely on it.