Lua and OO bindings

Thomas Fjellstrom

This isn't actually a question. I finally managed to figure out the lua5.2 lib properly.

I thought I'd share what I came up with:

#SelectExpand
   1#include <cstdio>
   2#include <lua.hpp>
   3#include <cerrno>
   4#include <cstring>
   5#include <stdlib.h>
   6
   7#include <new>
   8
   9void lua_stack_dump(lua_State *L);
  10
  11#define _strify(str) #str
  12#define _concat(str1, str2) str1 ## str2
  13
  14#define register_class(state, class_name, meta_table, class_table) do { \
  15  luaL_newmetatable(state, #class_name); \ 
  16  lua_pushvalue(state, -1); \ 
  17  lua_setfield(state, -2, "__index"); \ 
  18  luaL_setfuncs(state, class_table, 0); \ 
  19  luaL_newlib(state, meta_table); \ 
  20  /* lua_pushvalue(state, -1);*/ \ 
  21  lua_setglobal(state, #class_name); \ 
  22  lua_stack_dump(state); \ 
  23} while(0) 
  24
  25#define create_lua_object(state, class_name, obj) do { \
  26  void *udata__ = lua_newuserdata(state, sizeof(_concat(Lua,class_name))); \ 
  27  _concat(Lua,class_name) *obj__ = ::new (udata__) _concat(Lua,class_name)(state, obj); \ 
  28  \ 
  29  luaL_getmetatable(state, #class_name); \ 
  30  lua_setmetatable(state, -2); \ 
  31  lua_stack_dump(state); \ 
  32} while(0) 
  33
  34#define get_lua_object(state, class_name) (_concat(Lua,class_name) *)luaL_checkudata(state, 1, #class_name)
  35
  36#define destroy_lua_object(state, class_name) do { \
  37  _concat(Lua, class_name) *udata__ = (_concat(Lua, class_name) *)luaL_checkudata(state, 1, #class_name); \ 
  38  udata__->~_concat(Lua, class_name)(); \ 
  39} while(0) 
  40
  41class Test
  42{
  43  public:
  44    Test() { printf("new Test!\n"); }
  45    ~Test() { printf("delete Test!\n"); }
  46    
  47    void it() { printf("TEST IT!\n"); }
  48};
  49
  50class LuaTest
  51{
  52  public:
  53    LuaTest(lua_State *, Test *test) : test(test) { printf("new LuaTest!\n"); }
  54    ~LuaTest() { printf("delete LuaTest!\n"); }
  55    void it() { test->it(); }
  56  private:
  57    Test *test;
  58};
  59
  60static int Test_destroy(lua_State *state)
  61{
  62  printf("Test_destroy\n");
  63  destroy_lua_object(state, Test);
  64  return 0;
  65}
  66
  67static int Test_it(lua_State *state)
  68{
  69  printf("Test_it\n");
  70  LuaTest *test = get_lua_object(state, Test);
  71  test->it();
  72  return 0;
  73}
  74
  75static int Test_new(lua_State *state)
  76{
  77  printf("Test_new begin\n");
  78  Test *newTest = new Test();
  79  create_lua_object(state, Test, newTest);
  80  printf("Test_new end\n");
  81  return 1;
  82}
  83
  84static const luaL_Reg test_meta_methods[] = {
  85  { "new", &Test_new },
  86  { "__gc", &Test_destroy },
  87  { 0, 0 }
  88};
  89
  90static const luaL_Reg test_methods[] = {
  91  { "it", &Test_it },
  92  { 0, 0 }
  93};
  94
  95int main(int argc, char **argv)
  96{
  97  printf("start\n");
  98  lua_State *state = luaL_newstate();
  99  luaL_openlibs(state);
 100  
 101  printf("call register_class\n");
 102  register_class(state, Test, test_meta_methods, test_methods);
 103  
 104  printf("open lua file\n");
 105  FILE *fh = fopen("luaobjtest.lua", "r");
 106  if(!fh)
 107  {
 108    printf("failed to open luaobjtest.lua: %s\n", strerror(errno));
 109    return 0;
 110  }
 111  
 112  fseek(fh, 0, SEEK_END);
 113  long len = ftell(fh);
 114  fseek(fh, 0, SEEK_SET);
 115  
 116  char *script = (char *)malloc(len+1);
 117  if(!script)
 118  {
 119    printf("failed to allocate enough memory to load luaobjtest.lua\n");
 120    return 0;
 121  }
 122  
 123  script[len] = 0;
 124  size_t ret = fread(script, 1, len, fh);
 125  if(ret != (size_t)len)
 126  {
 127    printf("failed to read entire luaobjtest.lua file, read %li bytes, expected %li bytes.\n", ret, len);
 128    return 0;
 129  }
 130  
 131  fclose(fh);
 132
 133  printf("call luaL_loadstring\n");
 134  if(luaL_loadstring(state, script) != LUA_OK)
 135  {
 136    const char *errstr = luaL_checklstring(state, 1, 0);
 137    printf("failed to load luaobjtest.lua: %s\n", errstr);
 138    free(script);
 139    return 0;
 140  }
 141
 142  printf("call lua_pcall\n");
 143  if(lua_pcall(state, 0, LUA_MULTRET, 0) != LUA_OK)
 144  {
 145    const char *errstr = luaL_checklstring(state, 1, 0);
 146    printf("failed to run luaobjtest.lua: %s\n", errstr);
 147    free(script);
 148    return 0;
 149  }
 150  
 151  printf("done!\n");
 152  
 153  return 0;
 154}
 155
 156void lua_stack_dump(lua_State *L)
 157{
 158  int i;
 159  int top = lua_gettop(L);
 160  for (i = 1; i <= top; i++) {  /* repeat for each level */
 161    int t = lua_type(L, i);
 162    switch (t) {
 163
 164      case LUA_TSTRING:  /* strings */
 165      printf("`%s'", lua_tostring(L, i));
 166      break;
 167
 168      case LUA_TBOOLEAN:  /* booleans */
 169      printf(lua_toboolean(L, i) ? "true" : "false");
 170      break;
 171
 172      case LUA_TNUMBER:  /* numbers */
 173      printf("%g", lua_tonumber(L, i));
 174      break;
 175
 176      default:  /* other values */
 177      printf("%s", lua_typename(L, t));
 178      break;
 179
 180    }
 181    printf("  ");  /* put a separator */
 182  }
 183  printf("\n");  /* end the listing */
 184}

Maybe it'll help someone here trying to embed lua in their own programs, who also want to use objects from C++.

append:

Forgot the actual lua script:

local test = Test.new();
test:it();

Dizzy Egg

Fjellstrom! This is...you are....nice one man!

Thomas Fjellstrom

Dizzy Egg said:

Fjellstrom! This is...you are....nice one man!

I take it you've been fighting with lua 5.2 as well? Most of my frustration was due to a typo/thinko in luaL_checkudata, passed 0 instead of 1 as the second argument. tried changing everything but that, thinking it was ok ::)

Dizzy Egg

Ahh...err...well I've been applying the 'copy & paste' approach (because I'm a lazy bastard), and have had not much luck so far with 5.2, I've hacked? together a solution, but compared to what you've just shared it looks like...well, a mess.

Thomas Fjellstrom

I.. Yeah, I tried the copy & paste method hoping it would get me at least part way, but almost none of the available code is for 5.2.

I may change up the api later to use templates and other magic to make things a bit easier and cleaner. Maybe..

Right now thats just ripped out of a project I started trying to integrate lua into while I was at my moms. I spent days if not a week (or two) trying to get it to work properly.

Dizzy Egg

It's much appreciated! And 'hi' to your mum! (and Jasper!)

Edgar Reynaldo

That stuff looks way over my head. I've got to break out of C++ one of these days

Thomas Fjellstrom

lua's api really could have been done better.

Edgar Reynaldo

So it's just a C api? I haven't looked at LUA in ages.

Raidho36

LUA's api isn't designed to be used in object-oriented fashion, provided LUA don't support objects. So it's as if you were complaining that OO-coding with plain assembly was hard and it could've done it better.

Vanneto

Nobody is complaining. The only one (again!) that is bitching and moaning is... You.

EDIT: And while the API for Lua is in C, the language itself supports OOP.

Raidho36

Hey, I'm not moaning. Also, that's not really an OOP, I mean you could do just the same with C, but that doesn't make C an object-oriented language.

Polybios

I did something quite similar, very much inspired by this wrapper solution here, which also provides some nice templates and ugly macros ( ) for passing arguments : Embedding Lua for Fun and Profit

Had to adapt it to 5.2, too, though. As far as I remember, it wasn't more than replacing luaL_openlib with luaL_setfuncs. (edit: There was another thing, in Lua 5.2, tables can have a __gc metamethod while in 5.1 they can't, so I had to allow its execution for the userdata only, as this wrapper uses an userdata object wrapped inside a table for its c++-objects.)
Also, this wrapper doesn't require separate wrapper classes, as yours.
But I like the separation of class-methods and meta-methods, which you do, very much.

The one thing I found quite complicated to do about binding lua and c++ was to track userdata objects which contain other user-data objects, when the container is managed on the C side. You have to keep around a lua reference in your container object on the C-side in order to prevent the contained objects from being garbage-collected...
Have you thought about how do deal with that already? I'd be interested in how you do it.

I also hacked the Lua core so I had to declare globals. I don't like the concept of treating everything that isn't anything else as globals implicitly (I mean, I don't think global by default is bad, but you should be required to declare them in order to prevent typos from crashing your program, just as an extra-check).

Raidho36 said:

I mean you could do just the same with C

Last time I checked, C didn't provide anything like meta-methods, which allow you to do constructors, destructors, operator overloading, ...

Raidho36

I was comparing it to LUA's "OO". Seriously, that ain't OO. It's just LUA's loose internal structure allows a lot of things to work, which makes it possible to run sort of OO there. But LUA by itself has no support for it so you'd have to kludge up some kludgingly kludgy kludges to make it work right, just like you'd have to with C.

Polybios

That's sort of right, but I'd still argue that it works better with Lua than with C because you have metatables and metamethods. You simply can't do this implicit- function-calling-OOP-magic with C!
If you need native-true-OOP support out of the box, you should use a different embeddable scripting language, not Lua.
I like the concept of Lua's general purpose tables which is why I use it.

Thomas Fjellstrom

It's about as good as C++'s OO support. It has some convenient syntax sugar to allow some OO concepts to work, just like lua does \o/

But yes, please keep this as on topic as possible, winging about random crap just because you can is looked down upon.

Polybios said:

Also, this wrapper doesn't require separate wrapper classes, as yours.

I started off without the wrapper classes. I added them thinking not using them was causing issues somehow? I tried a lot of things to get my code to work. But it turns out I want to keep the wrappers anyhow, so I can let lua properly dispose of stuff.

The wrapper classes in some instances will be aloud to dispose of their wrapped objects, and you can't really do that by storing a single pointer in the udata.

It actually wouldn't take much to rip out the need for the wrapper class in those macros..

Something like:

#SelectExpand
   1#include <cstdio>
   2#include <lua.hpp>
   3#include <cerrno>
   4#include <cstring>
   5#include <stdlib.h>
   6
   7#include <new>
   8
   9void lua_stack_dump(lua_State *L);
  10
  11#define _strify(str) #str
  12#define _concat(str1, str2) str1 ## str2
  13
  14#define register_class(state, class_name, meta_table, class_table) do { \
  15  luaL_newmetatable(state, #class_name); \ 
  16  lua_pushvalue(state, -1); \ 
  17  lua_setfield(state, -2, "__index"); \ 
  18  luaL_setfuncs(state, class_table, 0); \ 
  19  luaL_newlib(state, meta_table); \ 
  20  lua_setglobal(state, #class_name); \ 
  21} while(0)  
  22
  23#define create_lua_object(state, class_name, obj) do { \
  24  void **udata__ = (void **)lua_newuserdata(state, sizeof(class_name *)); \ 
  25  *udata__ = obj; \ 
  26  luaL_getmetatable(state, #class_name); \ 
  27  lua_setmetatable(state, -2); \ 
  28} while(0)  
  29
  30#define get_lua_object(state, class_name) *(class_name **)luaL_checkudata(state, 1, #class_name)
  31
  32#define destroy_lua_object(state, class_name) do { \
  33  class_name **udata__ = (class_name **)luaL_checkudata(state, 1, #class_name); \ 
  34  (*udata__)->~class_name(); \ 
  35} while(0)  
  36
  37class Test
  38{
  39  public:
  40    Test() { printf("new Test!\n"); }
  41    ~Test() { printf("delete Test!\n"); }
  42    
  43    void it() { printf("TEST IT!\n"); }
  44};
  45
  46static int Test_destroy(lua_State *state)
  47{
  48  printf("Test_destroy\n");
  49  destroy_lua_object(state, Test);
  50  return 0;
  51}
  52
  53static int Test_it(lua_State *state)
  54{
  55  printf("Test_it\n");
  56  Test *test = get_lua_object(state, Test);
  57  test->it();
  58  return 0;
  59}
  60
  61static int Test_new(lua_State *state)
  62{
  63  printf("Test_new begin\n");
  64  Test *newTest = new Test();
  65  create_lua_object(state, Test, newTest);
  66  printf("Test_new end\n");
  67  return 1;
  68}
  69
  70static const luaL_Reg test_meta_methods[] = {
  71  { "new", &Test_new },
  72  { "__gc", &Test_destroy },
  73  { 0, 0 }
  74};
  75
  76static const luaL_Reg test_methods[] = {
  77  { "it", &Test_it },
  78  { 0, 0 }
  79};
  80
  81int main(int argc, char **argv)
  82{
  83  printf("start\n");
  84  lua_State *state = luaL_newstate();
  85  luaL_openlibs(state);
  86  
  87  printf("call register_class\n");
  88  register_class(state, Test, test_meta_methods, test_methods);
  89  
  90  printf("open lua file\n");
  91  FILE *fh = fopen("luaobjtest.lua", "r");
  92  if(!fh)
  93  {
  94    printf("failed to open luaobjtest.lua: %s\n", strerror(errno));
  95    return 0;
  96  }
  97  
  98  fseek(fh, 0, SEEK_END);
  99  long len = ftell(fh);
 100  fseek(fh, 0, SEEK_SET);
 101  
 102  char *script = (char *)malloc(len+1);
 103  if(!script)
 104  {
 105    printf("failed to allocate enough memory to load luaobjtest.lua\n");
 106    return 0;
 107  }
 108  
 109  script[len] = 0;
 110  size_t ret = fread(script, 1, len, fh);
 111  if(ret != (size_t)len)
 112  {
 113    printf("failed to read entire luaobjtest.lua file, read %li bytes, expected %li bytes.\n", ret, len);
 114    return 0;
 115  }
 116  
 117  fclose(fh);
 118
 119  printf("call luaL_loadstring\n");
 120  if(luaL_loadstring(state, script) != LUA_OK)
 121  {
 122    const char *errstr = luaL_checklstring(state, 1, 0);
 123    printf("failed to load luaobjtest.lua: %s\n", errstr);
 124    free(script);
 125    return 0;
 126  }
 127
 128  printf("call lua_pcall\n");
 129  if(lua_pcall(state, 0, LUA_MULTRET, 0) != LUA_OK)
 130  {
 131    const char *errstr = luaL_checklstring(state, 1, 0);
 132    printf("failed to run luaobjtest.lua: %s\n", errstr);
 133    free(script);
 134    return 0;
 135  }
 136  
 137  printf("done!\n");
 138  
 139  return 0;
 140}
 141
 142void lua_stack_dump(lua_State *L)
 143{
 144  int i;
 145  int top = lua_gettop(L);
 146  for (i = 1; i <= top; i++) {  /* repeat for each level */
 147    int t = lua_type(L, i);
 148    switch (t) {
 149
 150      case LUA_TSTRING:  /* strings */
 151      printf("`%s'", lua_tostring(L, i));
 152      break;
 153
 154      case LUA_TBOOLEAN:  /* booleans */
 155      printf(lua_toboolean(L, i) ? "true" : "false");
 156      break;
 157
 158      case LUA_TNUMBER:  /* numbers */
 159      printf("%g", lua_tonumber(L, i));
 160      break;
 161
 162      default:  /* other values */
 163      printf("%s", lua_typename(L, t));
 164      break;
 165
 166    }
 167    printf("  ");  /* put a separator */
 168  }
 169  printf("\n");  /* end the listing */
 170}

The only issue with that, is if you really want lua calling your destructor? Probably not, so that should probably be removed.

Quote:

Have you thought about how do deal with that already? I'd be interested in how you do it.

I haven't considered that. Even slightly. I'm sure you can do it simply enough, possibly with a nice little "smart_lua_ptr" class to wrap those lua objects?

Polybios

Interesting.

You could use placement new directly on your C++-object/the userdata with (sizeof(YourClass) and call the destructor explicitly in the __gc method, can't you? Is there a difference between Lua - TestDestroy - ~LuaTest (explicitly) - ~Test (implicitly) and Lua - TestDestroy - ~Test (explicitly)? Isn't it just shorter?
At least that's how I do it. I found it to become really tedious to write the glue code, so I'd like to minimize that and avoid extra wrapper classes when possible. (Of course you can emit the __gc glue-function (e. g. TestDestroy) in one line using a macro).

But: In what situation would you want the Lua wrapper class to let go of the wrapped C++ object? Wouldn't that be insecure?

While a smart pointer is a good idea (of which I haven't thought ::) ), I think it doesn't solve the problem with the references on its own. You'd still need a place to store your lua_smart_ptrs; chances are that you don't want your C++-container-object to store them, because then, you couldn't use it with plain C++ objects anymore (?). Besides, I'd prefer to not have my C++ objects "contaminated" with Lua glue code. You definitively need those references though, because it will inevitably crash when you don't have them (speaking from experience here 8-) ). [As soon as Lua runs a GC-cycle, it will collect your CppObjects, because it is unable to "see" that they are "referenced" on the C++ side in your CppContainer.]

While I think about it, this container-object-reference-thing is actually the only case for which you definitively need a wrapper class, no?

Well, whatever, I just wanted to draw your attention to this problem, because it was the part which I found the most difficult.
I've also spent quite some time in order to get it to work; I guess I spent even more time searching for premade solutions - which either didn't work, were made for Lua 5.1 or were overly complicated and took ages to compile because of all the templates involved.

(By the way, is there a reason you don't use lua_dofile in the example? )

Thomas Fjellstrom

The main reason I use pointers (or the wrapper object) rather than explicit objects is, at least in the project I wrote the original code for, most objects will be created in the C++ code, and exposed to lua. And I don't want lua destroying my objects on me.

With the wrapper class, I have an extra step where I can put some smarter code to handle the lua destruction event. It could potentially do smarter things. Maybe.

Polybios said:

But: In what situation would you want the Lua wrapper class to let go of the wrapped C++ object? Wouldn't that be insecure?

Yeah, it could be. But its nice to allow it, say the lua code creates an object, and doesn't attach it to anything, and then exits? I'd like the orphans to go away if at all possible.

Quote:

(By the way, is there a reason you don't use lua_dofile in the example? )

Nope. I yanked the loadstring call from another example that loads from an array of strings. I just forgot about dofile and friends.

Polybios

Thomas Fjellstrom said:

most objects will be created in the C++ code, and exposed to lua. And I don't want lua destroying my objects on me.

Ah, okay, understand now.

Thomas Fjellstrom

In case you're interested, this is the other lua test I made:

#SelectExpand
  1#include <lua.hpp>
  2#include <cstring>
  3#include <cstdlib>
  4
  5struct Script {
const char *name;
const char *script;
bool loaded;
  9} scripts[] = {
{ "luatest1", "bar = 123; print(\"luatest1 run!\"); function foo() \n print(\"Hi from luatest1.foo!\");\nend", false },
{ "luatest2", "bar = 123; print(\"luatest2 run!\"); function foo() \n print(\"Hi from luatest2.foo!\");\nend", false },
{ "luatest3", "bar = 123; print(\"luatest3 run!\"); function foo() \n print(\"Hi from luatest3.foo!\");\nend", false },
{ "luatest4", "bar = 123; print(\"luatest4 run!\"); function foo() \n print(\"Hi from luatest4.foo!\");\nend", false }
 14};
 15
 16bool load_script(lua_State *state, const char *name, const char *str)
 17{
printf("load script %s\n", name);

if(luaL_loadstring(state, str) != LUA_OK) // 1
{
  const char *errstr = luaL_checklstring(state, 1, 0);
  printf("failed to load luatest: %s\n", errstr);
  return false;
}

lua_newtable(state); // script env  // 21
lua_setglobal(state, name); // set global name for new table // 1
 29
lua_getglobal(state, name); // 21
lua_newtable(state); // metatable // 321
lua_getglobal(state, "_G"); // 4321
lua_setfield(state, -2, "__index"); // set __index in metatable to _G // 321
lua_setmetatable(state, -2); // set metatable for script env // 21

lua_setupvalue(state, 1, 1); // set env for state // 21

printf("run script %s\n", name);
if(lua_pcall(state, 0, LUA_MULTRET, 0) != LUA_OK) // run script // 1
{
  const char *errstr = luaL_checklstring(state, 1, 0);
  printf("failed to run script %s: %s\n", name, errstr);
  return false;
}

return true;
 47}
 48
 49bool run_script(lua_State *state, const char *name)
 50{  
lua_getglobal(state, name);
lua_getfield(state, -1, "foo");
if(lua_pcall(state, 0, LUA_MULTRET, 0) != LUA_OK)
{
  const char *errstr = luaL_checklstring(state, 1, 0);
  printf("failed to call %s.foo: %s\n", name, errstr);
  return false;
}

return true;
 61}
 62
 63int main(int argc, char **argv)
 64{
lua_State *state = luaL_newstate();
luaL_openlibs(state);

printf("begin\n");

for(int i = 0; i < sizeof(scripts) / sizeof(Script); i++)
{
  if(!load_script(state, scripts[i].name, scripts[i].script))
  {
    printf("failed to load %s :(\n", scripts[i].name);
    continue;
  }
  
  scripts[i].loaded = true;
}

for(int i = 0; i < sizeof(scripts) / sizeof(Script); i++)
{
  if(scripts[i].loaded)
  {
    if(!run_script(state, scripts[i].name))
    {
      printf("failed to run %s :(\n", scripts[i].name);
      continue;
    }
  }
}

printf("done!\n");
return 0;
 95}

It loads each script into its own table/namespace in the same lua_State. I intend(ed) to use it to load a bunch of scripts that do certain things, and I can then just run them all in sequence without the overhead of separate states and the like. Oh, and they could potentially share some data if needed. They are partially sandboxed, so a bit safer than just loading a bunch of scripts into the same state, but not quite locked down, which you can do, but I didn't bother with the added complexity.

Thread #612599. Printed from Allegro.cc