Use C struct declaration without knowing definition

[Ali Çehreli]

On 07/24/2019 05:35 PM, Ben Ogles wrote:
 > Is there a way to use a C header that contains a struct declaration but
 > no definition?
 >
 > For example, the C header may contain:
 >
 > typedef struct some_type_s some_type;
 >
 > Which is only fully defined in the implementing C file:
 >
 > struct some_type_s { ... };

And that type is traded as pointer to some_type on the C API, right? 
Usually there is a factory method that returns a pointer to a 
dynamically allocated object.

 > I am trying to wrap the header in D:
 >
 > extern(C)
 > {
 >    struct some_type_s;
 >    alias some_type = some_type_s;
 > }
 >
 > And then use it in another D file:
 >
 > some_type s;
 >
 > But dmd does not know the size of the struct so it can't compile. Is
 > there a work around to this other than moving the struct definition into
 > the C header file?

Indeed. The only option is to use some_type* on the D API as well:

extern(C) {
   struct S;
   S* C_API_allocate();
   void C_API(const(S) *);
}

// D API
void foo(S* s)  {
   C_API(s);
}

void main() {
   auto s = C_API_allocate();
   foo(s);
}

Note: That program fails to link because C API functions are missing 
definitions.

You can wrap the pointer i a D struct and dispatch member function calls 
to the C API.

Ali