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Introduction to Lua userdata

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Note – There are two kinds of userdata.

Light userdata- Here the block of memory is managed by the host.

Full userdata – Here the block of memory is managed by Lua.

We can create a userdata from C/C++ API, using the lua_newuserdata() method, that creates and push userdata to the stack and return the pointer to its content to initialize it.

Working of the userdata type in Lua programming Examples

Example of userdata type in Lua programming to show how to create the userdata in C++ –

Program Example #1

extern “C” { } namespace { void PushSimpleValue(lua_State* lua, int value) { auto& udata = *(int*)lua_newuserdata(lua, sizeof(int)); udata = value; lua_newtable(lua); lua_pushstring(lua, “__call”); lua_pushcfunction(lua, [](lua_State* lua){ auto& udata = *(int*)lua_touserdata(lua, 1); lua_pushinteger(lua, udata); return 1; }); lua_settable(lua, -3); lua_setmetatable(lua, -2); } struct Struct { int x = 0; int y = 0; }; void PushSimpleStruct(lua_State* lua, const Struct& s) { auto& udata = *(Struct*)lua_newuserdata(lua, sizeof(Struct)); udata = s; lua_newtable(lua); lua_pushstring(lua, “__index”); lua_pushcfunction(lua, [](lua_State* lua){ auto& udata = *(Struct*)lua_touserdata(lua, 1); const std::string key = lua_tostring(lua, 2); if (key == “x”) { lua_pushinteger(lua, udata.x); } else if (key == “y”) { lua_pushinteger(lua, udata.y); } else { lua_pushnil(lua); } return 1; }); lua_settable(lua, -3); lua_setmetatable(lua, -2); } struct TObject { int v; std::string s; }; void OwnedObjectPush(lua_State* lua, TObject&& t) { auto udata = (TObject*)lua_newuserdata(lua, sizeof(TObject)); new (udata) TObject(std::move(t)); lua_newtable(lua); lua_pushstring(lua, “__gc”); lua_pushcfunction(lua, [](lua_State* lua){ (void)printf(“Releasing shared object.n”); auto udata = (TObject*)lua_touserdata(lua, 1); return 0; }); lua_settable(lua, -3); lua_pushstring(lua, “__index”); lua_pushcfunction(lua, [](lua_State* lua){ auto& udata = *(TObject*)lua_touserdata(lua, 1); const std::string key = lua_tostring(lua, 2); if (key == “v”) { lua_pushinteger(lua, udata.v); } else if (key == “s”) { lua_pushlstring(lua, udata.s.c_str(), udata.s.length()); } else { lua_pushnil(lua); } return 1; }); lua_settable(lua, -3); lua_setmetatable(lua, -2); } lua, ); lua_newtable(lua); lua_pushstring(lua, “__gc”); lua_pushcfunction(lua, [](lua_State* lua){ (void)printf(“Shared object releasing.n”); return 0; }); lua_settable(lua, -3); lua_pushstring(lua, “__index”); lua_pushcfunction(lua, [](lua_State* lua){ const std::string key = lua_tostring(lua, 2); if (key == “v”) { lua_pushinteger(lua, urdata.v); } else if (key == “s”) { lua_pushlstring(lua, urdata.s.c_str(), urdata.s.length()); } else { lua_pushnil(lua); } return 1; }); lua_settable(lua, -3); lua_setmetatable(lua, -2); } void WithLua(lua_State* lua, const char* chunk, int nargs) { lua_insert(lua, -nargs – 1); (void)lua_call(lua, nargs, 0); } } int main() { const auto lua = luaL_newstate(); lua_gc(lua, LUA_GCSTOP, 0); luaL_openlibs(lua); lua_gc(lua, LUA_GCRESTART, 0); PushSimpleValue(lua, 100); WithLua(lua, R”lua( local urdata = … print(“Simple value: ” .. urdata()) )lua”, 1); TObject Tbobj1; Tbobj1.v = 99; Tbobj1.s = “Hello,”; OwnedObjectPush(lua, std::move(Tbobj1)); WithLua(lua, R”lua( local urdata = … print(“An owned object: (v:” .. urdata.v .. “, s:'” .. urdata.s .. “‘)”) )lua”, 1); Struct st; st.x = 20; st.y = 60; PushSimpleStruct(lua, st); WithLua(lua, R”lua( local urdata = … print(“THe Simple struct: (x:” .. urdata.x .. “, y:” .. urdata.y .. “)”) )lua”, 1); new TObject(), [&](TObject* p){ delete p; (void)printf(“The shared object was destroyed!n”); } }; PushShared(lua, Tbobj2); Tbobj2.reset(); WithLua(lua, R”lua( local urdata = … print(“The Shared object: (v:” .. urdata.v .. “, s:'” .. urdata.s .. “‘)”) )lua”, 1); (void)printf(“Destroy Lua instance.n”); lua_close(lua); (void)printf(“Done.n”); return EXIT_SUCCESS; }

Output:

Conclusion

The userdata is one of the basic types in Lua. Userdata represents an arbitrary C/C++ data that can be used in Lua.