Lua - set a function from a class [duplicate] - c++

I am trying to register a c++ function in Lua.
But getting this error:
CScript.cpp|39|error: argument of type 'int (CScript::)(lua_State*)' does not match 'int (*)(lua_State*)'|
EDIT:
int CApp::SetDisplayMode(int Width, int Height, int Depth)
{
this->Screen_Width = Width;
this->Screen_Height = Height;
this->Screen_Depth = Depth;
return 0;
}
int CScript::Lua_SetDisplayMode(lua_State* L)
{
// We need at least one parameter
int n = lua_gettop(L);
if(n < 0)
{
lua_pushstring(L, "Not enough parameter.");
lua_error(L);
}
int width = lua_tointeger(L, 1);
int height = lua_tointeger(L, 2);
int depth = lua_tointeger(L, 3);
lua_pushinteger(L, App->SetDisplayMode(width, height, depth));
return 0;
}
And in main:
lua_register(L, "setDisplayMode", Lua_SetDisplayMode);

You can not use a method of a class as a normal function, unless it is declared static. You have to define a normal function, which finds out what object you want the method to be called in, and then call the method.
The main reason it's not possible to use a class method as a callback from a C function (and remember that the Lua API is a pure C library), is because the computer doesn't know which object the method should be called on.

The answer is actually surprisingly simple; if you use lua_pushcclosure instead of lua_pushcfunction, you can pass parameters to your called function:
lua_pushlightuserdata(_state, this);
lua_pushcclosure(_state, &MyClass::lua_static_helper, 1);
int MyClass::lua_static_helper(lua_State *state) {
MyClass *klass = (MyClass *) lua_touserdata(state, lua_upvalueindex(1));
return klass->lua_member_method(state);
}

You cannot directly register a C++ non-static member function in Lua using just the basic Lua C API.
However, any of the various mechanisms that exist for easily associating C++ code with Lua will allow you to do so. toLua++, SWIG, Luabind, etc. If you're serious about using C++ objects with Lua, I suggest picking one of those and using it, rather than writing your own version. I personally use Luabind (most of the time; SWIG has its place in the toolbox), as it is the one that doesn't have some form of code generation. It's all done purely in C++, so there's no pre-pass step that generates a C++ source file.

You can work around the limitation by storing your active this pointer in a static variable as well. This introduces the problem of being unable to have two of these classes operating at the same time, but it works.
static CScript* luaThis; // This is a private variable inside CScript.
Then, inside your CScript constructor (or some kind of 'activate' function), you can just specify:
luaThis = this;
Then, when your static functions are called (they can even be private functions if they are registered from within the class), you have access to all your member information via the luaThis pointer.
lua_pushinteger(L, luaThis->App->SetDisplayMode(width, height, depth));
The problem, as I said, is that this restricts you to one active CScript at a time (since another callback from another Lua state would use luaThis while it is pointing to the wrong things). If you need multiple active instances ever, you can come up with some lookup mechanism using the incoming lua_State* as a key.
std::map<lua_State*, CScript*> lookups; // Just an idea, if it comes to this.
Hope that helps!

Related

Using C++ member function to handle a callback that takes simple static function pointer

I haven't used C++ in ages. Between what I've forgotten and what has changed in C++ over time, I'm really banging my head against the wall trying to do something that would be trivially easy in JavaScript, or any other language where functions are objects, and not just simple pointers.
I think I understand the basic problem: A class member function only exists in once place in memory (there isn't a different copy of it for each class instance). The only way the function knows what "this" is is because an instance pointer is passed along as an invisible first argument to every function call. A plain-old C-style callback isn't going to know anything about passing that instance pointer.
What I need is a new function that is somehow bound to my class instance, one which knows how to pass "this" along to the member function. That's the function I need to use as a callback.
But I don't know for sure how to dynamically create such a function. I think the code below is on the right track (except for casting pointer types), but it does bother me a bit because it seems like that there'd have to be some dynamic memory allocation going on, and if so, some way to track that allocation and do clean-up later.
class SignalMonitor {
int dataPin;
unsigned short timings[RING_BUFFER_SIZE];
unsigned long lastSignalChange = 0;
int dataIndex = 0;
int syncCount = 0;
void signalHasChanged();
public:
SignalMonitor(int);
};
SignalMonitor::SignalMonitor(int dataPin) {
this->dataPin = dataPin;
function<void()> callback = bind(&SignalMonitor::signalHasChanged, this);
wiringPiISR(dataPin, INT_EDGE_BOTH, callback);
}
void SignalMonitor::signalHasChanged() {
unsigned long now = micros();
int duration = (int) min(now - this->lastSignalChange, 10000ul);
this->lastSignalChange = now;
cout << duration << '\n';
}
I feel like this is close to what I want, but I'm getting this error:
acu-rite-433Mhz-reader.cpp:58:72: error: invalid cast from type ‘std::function<void()>’ to type ‘void*’
wiringPiISR(dataPin, INT_EDGE_BOTH, reinterpret_cast<void *>(callback));
^
Here's the call signature of the function I'm trying to pass this callback to:
int wiringPiISR (int pin, int edgeType, void (*function)(void))
I've found a number of similar issues discussed when searching on this topic, but they either don't quite match what I'm trying to do, or assume much more familiarity with C++ than I currently possess. (All I remember about function pointer types is that they can get hellishly ugly very quickly!)
I tried to use lambda function as a solution, but that led to an error (besides a type mismatch error) about something being "temporary", which I'm assuming meant that the lambda function's scope was temporary.
This is a far from ideal solution (I'm beginning to think there are no ideal solutions here), but it works for me in this particular case where there aren't likely to be very many instances of my SignalMonitor class in use at the same time.
First, I turned my signalHasChanged class method into a static method that takes an instance as an argument. (I could have kept the method as a class method by going through some hairy type-casting, but it wasn't worth it.)
Then I made ten almost-identical indirect callback functions:
void smCallback0() { SignalMonitor::signalHasChanged(monitors[0]); }
void smCallback1() { SignalMonitor::signalHasChanged(monitors[1]); }
void smCallback2() { SignalMonitor::signalHasChanged(monitors[2]); }
void smCallback3() { SignalMonitor::signalHasChanged(monitors[3]); }
void smCallback4() { SignalMonitor::signalHasChanged(monitors[4]); }
void smCallback5() { SignalMonitor::signalHasChanged(monitors[5]); }
void smCallback6() { SignalMonitor::signalHasChanged(monitors[6]); }
void smCallback7() { SignalMonitor::signalHasChanged(monitors[7]); }
void smCallback8() { SignalMonitor::signalHasChanged(monitors[8]); }
void smCallback9() { SignalMonitor::signalHasChanged(monitors[9]); }
Then I stuck all of those functions into an array:
void (*_smCallbacks[MAX_MONITORS])() = {
smCallback0, smCallback1, smCallback2, smCallback3, smCallback4,
smCallback5, smCallback6, smCallback7, smCallback8, smCallback9
};
Along with the monitors array, which is an array of SignalHandler pointers, this gives me ten available callback slots. (_smCallbacks is copied into smCallbacks as a way to get around foreward reference problems.)
The init method for SignalMonitor simply searches for an available slot, plugs itself in, then sets the callback:
void SignalMonitor::init() {
for (int i = 0; i < MAX_MONITORS; ++i) {
if (monitors[i] == NULL) {
callbackIndex = i;
monitors[i] = this;
break;
}
}
if (callbackIndex < 0)
throw "Maximum number of SignalMonitor instances reached";
wiringPiISR(dataPin, INT_EDGE_BOTH, smCallbacks[callbackIndex]);
}
There's also a destructor to free up the callback slots:
SignalMonitor::~SignalMonitor() {
if (callbackIndex >= 0)
monitors[callbackIndex] = NULL;
}
It may help to consider the traditional way of handling a similar issue. Other APIs have been designed where instead of void(*function)(void), wiringPiISR would expect a function void(*function)(void *). This allows the use of
static void signalHasChanged(void *p) {
static_cast<SignalMonitor*>(p)->signalHasChanged();
}
This is not a general solution, but because Raspberry Pi has a limited number of GPIO pins, and you can't have more callback functions than you have pins, you might be able to create one callback function per pin. Then, you need a global data structure that maps the interrupt pin to which SignalMonitor instance (or instances) it should signal. The constructor would register the 'this' object to a specific pin, then set the appropriate callback function based on the pin.
The callback functions would be able to pass a pin argument to a general function, which could then look up the specific SignalMonitor instance and call a class function.
I wouldn't want to do it for 1000 pins, 1000 instances, but this hack should work for anything running on a Pi.

Lua checkudata type C++

I've been to the end of Google and back trying to solve this problem.
I have a few userdata objects that I push from C++ to Lua.
I have a function that should get the X value of either a 2D or 3D object.
When I try to get the userdata object, taking into consideration that it could be either a 2D element or 3D object, I need to be able to get the X for whichever the user chooses.
Here is what I tried:
int getX(lua_State* L)
{
Object3D* a = static_cast<Object3D*>(luaL_checkudata(L, 1, "Object3D"));
if (!a)
{
Object2D* b = static_cast<Object2D*>(luaL_checkudata(L, 1, "Object2D"));
if (b)
{
lua_pushnumber(L, b:getX());
}
else
{
lua_pushnil(L);
}
}
else
{
lua_pushnumber(L, a:getX());
}
return 1;
}
Unfortunately if the userdata type is not Object3D, it fails and exits on an lua error without continuing to try Object2D.
Therefore, it will only work in the above code if the object being passed is of type Object3D.
luaL_testudata
void *luaL_testudata (lua_State *L, int arg, const char *tname);
This function works like luaL_checkudata, except that, when the test fails, it returns NULL instead of raising an error.
The lua(L)_check* functions throw Lua errors on failure, the lua(L)_to* functions return NULL. For whatever reason, this one deviates from the naming convention and is named lua(L)_test* instead, which makes it a bit harder to find.
Your code is incomplete and doesn't compile as-is so I can't be bothered to check, but if I'm not mistaken, just replacing luaL_checkudata with luaL_testudata should make it work as intended.
Solved by using rawequal to see which class the registry matches.

In lua is there a way to bind an upvalue to a userdata value instead of a function?

In the following example a userdata value is created of type MyType and a table is created with a metafunction __tostring which calls LI_MyType__tostring. The code creates a closure-based lua OOP. My gripe with the example provided is it appears as though there is only one way to associate userdata with a method call, via upvalues. In and of itself, this isn't problematic unless I want to share the same metatable across instances.
In an ideal world - and what I'm hoping to unearth with this question - is there a way to associate an upvalue with a value (e.g. userdata) without associating it with a function call via an upvalue? I'm hoping there is a trick that will let me continue to use closure-based lua OOP and share the same metatable across instances. I'm not optimistic, but I figured I'd ask to see if someone has a suggestion or a non-obvious trick.
using FuncArray = std::vector<const ::luaL_Reg>;
static const FuncArray funcs = {
{ "__tostring", LI_MyType__tostring },
};
int LC_MyType_newInstance(lua_State* L) {
auto userdata = static_cast<MyType*>(lua_newuserdata(L, sizeof(MyType)));
new(userdata) MyType();
// Create the metatable
lua_createtable(L, 0, funcs.size()); // |userdata|table|
lua_pushvalue(L, -2); // |userdata|table|userdata|
luaL_setfuncs(L, funcs.data(), 1); // |userdata|table|
lua_setmetatable(L, -2); // |userdata|
return 1;
}
int LI_MyType__tostring(lua_State* L) {
// NOTE: Blindly assume that upvalue 1 is my userdata
const auto n = lua_upvalueindex(1);
lua_pushvalue(L, n); // |userdata|
auto myTypeInst = static_cast<MyType*>(lua_touserdata(L, -1));
lua_pushstring(L, myTypeInst->str()); // |userdata|string|
return 1; // |userdata|string|
}
I'm hoping there's a way of performing something like (this is pseudo-code!):
// Assume that arg 1 is userdata
int LI_MyType__tostring(lua_State* L) {
const int stackPosition = -1;
const int upvalueIndex = 1;
const auto n = lua_get_USERDATA_upvalue(L, stackPosition, upvalueIndex);
lua_pushvalue(L, n); // |userdata|
auto myTypeInst = static_cast<MyType*>(lua_touserdata(L, -1));
lua_pushstring(L, myTypeInst->str()); // |userdata|string|
return 1; // |userdata|string|
}
I know this is similar to how things would be for the "normal" metatable style of OOP, but I want to keep things closure based and avoid introducing the colon syntax.
Another way of asking this question would be, is there a way to share metatables across userdata instances while using a closure-based OOP? Using lua's syntax from the scripting side of things, I don't think it's possible, but I'm hoping there's something that can be done on the C side of things.
UPDATE (2013-10-10): Based on #lhf's answer to use lua_setuservalue() and lua_getuservalue() the protocol I've settled on which allows me to reuse metatables is this:
Register a single metatable object using luaL_newmetatable(). This metatable can now be shared across userdata instances because no upvalues are used when registering the metatable.
Create a userdata value (lua_newuserdata()).
Assign the correct metatable to the userdata value (lua_setmetatable()).
Create and populate an instance method calls/attributes table with one upvalue, the userdata.
Use lua_setuservalue() on userdata to store a reference to the per-instance attribute/method table.
Change various metamethods (e.g. __index) to use the userdata's uservalue table.
As a consequence:
upvalues are never used in metamethods
upvalues are only used in a value's instance methods
there is only one extra table per instance of a given class
It's still not possible to escape creating a method/attribute table per userdata, but that overhead is nominal. It would be nice if obj.myMethod() would pass obj to function myMethod() somehow without using :, but that's exactly what : does because this isn't possible another way (unless you do make use of an upvalue).
lua_setuservalue seems to be exactly what you need. There is also of course lua_getuservalue.
(I'm skipping the C++ code and answering the question in the title.)
I don't think you should be trying to do it exactly, for a few reasons.
If you call object.method(), and you try to infer the object from the instance that was created, you're blocking your ability to pass function pointers around that behave on any object given.
You have cyclic references to objects that will never get garbage collected (each instance's function pointing back to the instance).
Just get the object from slot 1, and check its type matches your userdata. (luaL_checkudata)
If its not an object and tostring is called for example, just output that its a class of object name, instead of the instance details. It makes far more sense, and may well make debugging simpler if the object reports what it actually is, rather than trying to be too clever and misleading you.

Init values by using (somewhat) global variables vs. static function variables?

I have some small helper functions needed throughout the code.
To work, they need to be initialized with some data once.
Where should I store the init data?
I've come up with two methods:
I create static variables in the scope of the helper.cpp file which I set with a dedicated setter function and then use in my helper function.
static int _initData = 0;
void initHelpMe(int initData)
{
_initData = initData;
}
void helpMe()
{
doSomethingWith(_initData);
}
Or I use a static function variable inside the original helper function and a default parameter to it.
void helpMe(int initData = 0)
{
static int _initData = 0;
if (initData != 0)
_initData = initData;
doSomethingWith(_initData);
}
(Lets asume that 0 is outside of the valid data range of initData and that I've not shown additional code to ensure an error is raised when the function is called for the first time without initiating it first.)
What are the advantages / disadvantages of those two methods and is there an even better way of doing it?
I of course like the second method, because it keeps all the functionality in one place. But I already know it is not thread-safe (which is not an issue a.t.m.).
And, to make this more interesting, albeit being C++ this is not to be used in object-oriented but in procedural code. So please no answers proposing objects or classes. Just imagine it to be C with the syntax of C++.
I was going to suggest that you wrap your data into an object, until I realized that you are asking for a C solution with a C++ tag...
Both of your solutions have their benefits.
The second one is the one I'd prefer, assuming we just go by "what it looks like/maintainability". However, there is a drawback if helpMe is called MANY times with initData == 0, because of the extra if, which isn't present in the first case. This may or may not be an issue if doSomethingWith() is long enough a function and/or the compiler has the ability to inline helpMe (and initData is constant).
And of course, something in the code will have to call initHelpMe too, so it may turn out to be the same anyway.
In summary: Prefer the second one, based on isolation/encapsulation.
I clearly prefer the second! Global static data in different compilation units are initialized in unspecified order (In one unit in order, though). Local static data of a function is initialized at first call.
Example:
If you have two translation units A and B. The unit A calls during initialization the function helpMe of unit B. Assume the order of initialization is A, B.
The first solution will set the zero initialized _initData to some initData. After that the initialization of unit B resets _initData back to zero and may produce a memory leak or other harm.
There is a third solution:
void helpMe(int initData = 0)
{
static std::once_flag once;
static int _initData = 0;
std::call_once(once, [&] {
_initData = initData;
}
doSomethingWith(_initData);
}
I feel strongly both ways.
Prefer option 2 for the isolation, but option 1 lends itself to porting to a C++ class. I've coded both ways. It comes down to the SW architecture.
Let me offer another point.
Both options down side: You have not limited initialization to one occurrence. "need to be initialized with some data once". It appears OP's conditions insure a proper initialization of initHelpMe(123) or HelpMe(123) followed by helpMe(), but do not prevent/detect a secondary initialization.
Should a secondary need to be prevented/detected, some additional code could be used.
// Initialization
if (_initData != 0) {
; // Handle error
}
_initData = initData;
Another paradigm I've used follows. It may not be realizable in you code as it does not pass initData as a parameter but magically can get it.
void helpMe(void) {
static int Initialized = 0;
if (!Initialized) {
Initialized = 1;
_initData = initData();
}
doSomethingWith(_initData);
}

Register C++ function in Lua?

I am trying to register a c++ function in Lua.
But getting this error:
CScript.cpp|39|error: argument of type 'int (CScript::)(lua_State*)' does not match 'int (*)(lua_State*)'|
EDIT:
int CApp::SetDisplayMode(int Width, int Height, int Depth)
{
this->Screen_Width = Width;
this->Screen_Height = Height;
this->Screen_Depth = Depth;
return 0;
}
int CScript::Lua_SetDisplayMode(lua_State* L)
{
// We need at least one parameter
int n = lua_gettop(L);
if(n < 0)
{
lua_pushstring(L, "Not enough parameter.");
lua_error(L);
}
int width = lua_tointeger(L, 1);
int height = lua_tointeger(L, 2);
int depth = lua_tointeger(L, 3);
lua_pushinteger(L, App->SetDisplayMode(width, height, depth));
return 0;
}
And in main:
lua_register(L, "setDisplayMode", Lua_SetDisplayMode);
You can not use a method of a class as a normal function, unless it is declared static. You have to define a normal function, which finds out what object you want the method to be called in, and then call the method.
The main reason it's not possible to use a class method as a callback from a C function (and remember that the Lua API is a pure C library), is because the computer doesn't know which object the method should be called on.
The answer is actually surprisingly simple; if you use lua_pushcclosure instead of lua_pushcfunction, you can pass parameters to your called function:
lua_pushlightuserdata(_state, this);
lua_pushcclosure(_state, &MyClass::lua_static_helper, 1);
int MyClass::lua_static_helper(lua_State *state) {
MyClass *klass = (MyClass *) lua_touserdata(state, lua_upvalueindex(1));
return klass->lua_member_method(state);
}
You cannot directly register a C++ non-static member function in Lua using just the basic Lua C API.
However, any of the various mechanisms that exist for easily associating C++ code with Lua will allow you to do so. toLua++, SWIG, Luabind, etc. If you're serious about using C++ objects with Lua, I suggest picking one of those and using it, rather than writing your own version. I personally use Luabind (most of the time; SWIG has its place in the toolbox), as it is the one that doesn't have some form of code generation. It's all done purely in C++, so there's no pre-pass step that generates a C++ source file.
You can work around the limitation by storing your active this pointer in a static variable as well. This introduces the problem of being unable to have two of these classes operating at the same time, but it works.
static CScript* luaThis; // This is a private variable inside CScript.
Then, inside your CScript constructor (or some kind of 'activate' function), you can just specify:
luaThis = this;
Then, when your static functions are called (they can even be private functions if they are registered from within the class), you have access to all your member information via the luaThis pointer.
lua_pushinteger(L, luaThis->App->SetDisplayMode(width, height, depth));
The problem, as I said, is that this restricts you to one active CScript at a time (since another callback from another Lua state would use luaThis while it is pointing to the wrong things). If you need multiple active instances ever, you can come up with some lookup mechanism using the incoming lua_State* as a key.
std::map<lua_State*, CScript*> lookups; // Just an idea, if it comes to this.
Hope that helps!