Local opaque object in C

local_opaque_object.c

// library.h -------------------------------------------------------------------
#define opaque(Type) Type; int $sizeof_##Type(void);

#define opaque_impl(Type) int $sizeof_##Type(void) { return sizeof(Type); }

#define local(Type) ((Type *)alloca($sizeof_##Type()))

// foo.h -----------------------------------------------------------------------

typedef struct Foo opaque(Foo);

void 	foo_set_data(Foo *foo, int data);
int	foo_get_data(Foo *foo);

// foo.c -----------------------------------------------------------------------

struct Foo {
	int data;
};

opaque_impl(Foo)

void foo_set_data(Foo *foo, int data)
{
	foo->data = data;
}

int foo_get_data(Foo *foo)
{
	return foo->data;
}


 // main.c ---------------------------------------------------------------------

int main()
{
	// Allocates an opaque object Foo on the stack.
	Foo *foo = local(Foo);
	
	foo_set_data(foo, 42);
	
	return foo_get_data(foo);
}