braddr from #d asked me to post this here, so here we go :)

This has probably been posted before, but it's useful to keep in an accessible place. When trying to cast a function pointer to an external C version of one, it requires this really awkward trick.

Placing the alias outside of a class at the module level inside an extern (C) will make an alias of a C function pointer. You can then use this alias to convert the function pointers. Here is an example:

-- Boilerplate types --

extern (C)
{
  alias void function() CFunc;
  void function() testVar;
}

alias void function() NotCFunc;

-- Usage example --

// This does NOT work and gripes about unable to convert to C
testVar = cast(NotCFunc)GetSomeCFuncPointer();

// This DOES work
testVar = cast(CFunc)GetSomeCFuncPointer();

-- End of Example --

From my observations the alias command will capture the external state as part of the alias, which means this is the only way to cast to C function pointers at this time.

-Joshua Cearley
BlackCrystal Software