/* managed_pointer.h

These macros allow wrapping an allocator/deallocator into an object that is

owned and managed by the Lua garbage collector.

Why? It is too easy to leak objects that need to be manually released, especially

when dealing with the Lua API which can throw errors from many operations.

USAGE

-----

For example, given an object that can be created or released with the following

functions:

  fancy_buffer* new_buffer();

  void free_buffer(fancy_buffer* p_buffer)

You could declare a managed version like so:

  MANAGED_POINTER_ALLOCATOR(new_managed_buffer, fancy_buffer*, new_buffer, free_buffer)

And then, when you need to create a new fancy_buffer in your code:

  fancy_buffer *my_buffer = new_managed_buffer(L);

NOTES

-----

Managed objects MUST NOT be freed manually. They will automatically be

freed during the next GC sweep after your function exits (even if via an error).

The managed object is pushed onto the stack, but should generally be ignored,

but you'll need to bear this in mind when creating managed pointers in the

middle of a sequence of stack operations.

*/

#define MANAGED_POINTER_MT(wrapped_type) #wrapped_type "_managedptr_mt"

#define MANAGED_POINTER_ALLOCATOR(name, wrapped_type, wrapped_alloc, wrapped_free) \

  static int _release_ ## name(lua_State *L) {                                \

  	wrapped_type *p = (wrapped_type*)lua_topointer(L, 1);                 \

  	if(*p != NULL) {                                                      \

	  	wrapped_free(*p);                                             \

	}                                                                     \

  	return 0;                                                             \

  }                                                                           \

  static wrapped_type name(lua_State *L) {                                    \

  	wrapped_type *p = (wrapped_type*)lua_newuserdata(L, sizeof(wrapped_type)); \

  	if(luaL_newmetatable(L, MANAGED_POINTER_MT(wrapped_type)) != 0) {     \

  		lua_pushcfunction(L, _release_ ## name);                      \

  		lua_setfield(L, -2, "__gc");                                  \

  	}                                                                     \

  	lua_setmetatable(L, -2);                                              \

  	*p = wrapped_alloc();                                                 \

  	if(*p == NULL) {                                                      \

  		lua_pushliteral(L, "not enough memory");                      \

  		lua_error(L);                                                 \

  	}                                                                     \

  	return *p;                                                            \

  }