Functions in Dart are first-class objects, allowing you to pass them to other objects or functions.
void main() {
var shout = (msg) => ' ${msg.toUpperCase()} ';
print(shout("yo"));
}
This made me wonder if there was a way to modify a function a run time, just like an object, prior to passing it to something else. For example:
Function add(int input) {
return add + 2;
}
If I wanted to make the function a generic addition function, then I would do:
Function add(int input, int increment) {
return add + increment;
}
But then the problem would be that the object I am passing the function to would need to specify the increment. I would like to pass the add function to another object, with the increment specified at run time, and declared within the function body so that the increment cannot be changed by the recipient of the function object.
The answer seems to be to use a lexical closure.
From here: https://dart.dev/guides/language/language-tour#built-in-types
A closure is a function object that has access to variables in its
lexical scope, even when the function is used outside of its original
scope.
Functions can close over variables defined in surrounding scopes. In
the following example, makeAdder() captures the variable addBy.
Wherever the returned function goes, it remembers addBy.
/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
return (int i) => addBy + i;
}
void main() {
// Create a function that adds 2.
var add2 = makeAdder(2);
// Create a function that adds 4.
var add4 = makeAdder(4);
assert(add2(3) == 5);
assert(add4(3) == 7);
}
In the above cases, we pass 2 or 4 into the makeAdder function. The makeAdder function uses the parameter to create and return a function object that can be passed to other objects.
You most likely don't need to modify a closure, just the ability to create customized closures.
The latter is simple:
int Function(int) makeAdder(int increment) => (int value) => value + increment;
...
foo(makeAdder(1)); // Adds 1.
foo(makeAdder(4)); // Adds 2.
You can't change which variables a closure is referencing, but you can change their values ... if you an access the variable. For local variables, that's actually hard.
Mutating state which makes an existing closure change behavior can sometimes be appropriate, but those functions should be very precise about how they change and where they are being used. For a function like add which is used for its behavior, changing the behavior is rarely a good idea. It's better to replace the closure in the specific places that need to change behavior, and not risk changing the behavior in other places which happen to depend on the same closure. Otherwise it becomes very important to control where the closure actually flows.
If you still want to change the behavior of an existing global, you need to change a variable that it depends on.
Globals are easy:
int increment = 1;
int globalAdder(int value) => value + increment;
...
foo(globalAdd); // Adds 1.
increment = 2;
foo(globalAdd); // Adds 2.
I really can't recommend mutating global variables. It scales rather badly. You have no control over anything.
Another option is to use an instance variable to hold the modifiable value.
class MakeAdder {
int increment = 1;
int instanceAdd(int value) => value + increment;
}
...
var makeAdder = MakeAdder();
var adder = makeAdder.instanceAdd;
...
foo(adder); // Adds 1.
makeAdder.increment = 2;
foo(adder); // Adds 2.
That gives you much more control over who can access the increment variable. You can create multiple independent mutaable adders without them stepping on each other's toes.
To modify a local variable, you need someone to give you access to it, from inside the function where the variable is visible.
int Function(int) makeAdder(void Function(void Function(int)) setIncrementCallback) {
var increment = 1;
setIncrementCallback((v) {
increment = v;
});
return (value) => value + increment;
}
...
void Function(int) setIncrement;
int Function(int) localAdd = makeAdder((inc) { setIncrement = inc; });
...
foo(localAdd); // Adds 1.
setIncrement(2);
foo(localAdd); // Adds 2.
This is one way of passing back a way to modify the local increment variable.
It's almost always far too complicated an approach for what it gives you, I'd go with the instance variable instead.
Often, the instance variable will actually represent something in your model, some state which can meaningfully change, and then it becomes predictable and understandable when and how the state of the entire model changes, including the functions referring to that model.
Using partial function application
You can use a partial function application to bind arguments to functions.
If you have something like:
int add(int input, int increment) => input + increment;
and want to pass it to another function that expects to supply fewer arguments:
int foo(int Function(int input) applyIncrement) => applyIncrement(10);
then you could do:
foo((input) => add(input, 2); // `increment` is fixed to 2
foo((input) => add(input, 4); // `increment` is fixed to 4
Using callable objects
Another approach would be to make a callable object:
class Adder {
int increment = 0;
int call(int input) => input + increment;
}
which could be used with the same foo function above:
var adder = Adder()..increment = 2;
print(foo(adder)); // Prints: 12
adder.increment = 4;
print(foo(adder)); // Prints: 14
In the Lua C API I can store a number or a string from the stack with lua_tostring().
How can a “reference” (if that is the correct term) to a Lua function be passed to C through the Lua API? So it can be called later from C, with lua_call(), without having to reference it by its name.
(It really needs to be like that, the C program will call the function somewhere in the future and the program doesn't know anything about the function because the functions to be passed are defined in the Lua program)
In C you can't refer to Lua functions directly but you can represent numbers and strings. So, for a function to "be called later", you can store this function in some table and refer to it by a numeric or string key of the table.
Here's a simpleminded mechanism to start with:
On the Lua side:
funcs = {}
local function register_hanlder(key, fn)
funcs[key] = fn
end
register_handler("on_mouse_click", function()
print "You clicked me!"
end)
On the C side:
/* code not tested */
lua_getglobal(L, "funcs");
lua_getfield(L, -1, "on_mouse_click");
if (!lua_isnil(L, -1)) {
lua_call(L, 0, 0);
else {
// nothing registered
}
Instead of registering the functions in a global table you can register them in the registry table (see luaL_ref). You'll get some integer (that's the key in the registry table where the function value is) that you can pass around in you C code.
Note that if you don't need to store a Lua function "for use later" you don't need any of this: if your C function has some Lua function passed to it via argument you can call it outright.
== Edit:
As I mentioned, instead of using a global variable (the funcs above) you can store the reference to the function in the "registry". Conceptually there's no difference between this method and the previous one.
Let's re-use the previous example: you want the Lua programmer to be able to register a function that would be fired whenever a mouse is clicked in your application.
The Lua side would look like this:
register_mouse_click_handler(function()
print "the mouse was clicked!"
end)
On the C side you define register_mouse_click_handler:
static int the_mouse_click_handler = 0;
static int register_mouse_click_handler(lua_State* L) {
the_mouse_click_handler = luaL_ref(L, LUA_REGISTRYINDEX);
return 0;
}
(...and expose it to Lua.)
Then, in your application, when the mouse is clicked and you want to call the Lua function, you do:
...
if (the_mouse_click_handler != 0) {
lua_rawgeti(L, LUA_REGISTRYINDEX, the_mouse_click_handler);
lua_call(L, 0, 0);
} else {
// No mouse handler was registered.
}
...
(I may have typos in the code.)
How do you create a Lua object that only exposes its attributes and not its methods? For example:
local obj = {
attr1 = 1,
attr2 = 2,
print = function(...)
print("obj print: ", ...)
end,
}
Produces:
> for k,v in pairs(obj) do print(k, v) end
attr1 1
attr2 2
print function: 0x7ffe1240a310
Also, is it possible to not use the colon syntax for OOP in Lua? I don't need inheritance, polymorphism, only encapsulation and privacy.
I started out with the above question and after chasing down the rabbit hole, I was surprised by the limited number of examples, lack of examples for the various metamethods (i.e. __ipairs, __pairs, __len), and how few Lua 5.2 resources there were on the subject.
Lua can do OOP, but IMO the way that OOP is prescribed is a disservice to the language and community (i.e. in such a way as to support polymorphism, multiple inheritance, etc). There are very few reasons to use most of Lua's OOP features for most problems. It doesn't necessarily mean there's a fork in the road either (e.g. in order to support polymorphism there's nothing that says you have to use the colon syntax - you can fold the literature's described techniques in to the closure-based OOP method).
I appreciate that there are lots of ways to do OOP in Lua, but it's irritating to have there be different syntax for object attributes versus object methods (e.g. obj.attr1 vs obj:getAttr() vs obj.method() vs obj:method()). I want a single, unified API to communicate internally and externally. To that end, PiL 16.4's section on Privacy is a fantastic start, but it's an incomplete example that I hope to remedy with this answer.
The following example code:
emulates a class's namespace MyObject = {} and saves the object constructor as MyObject.new()
hides all of the details of the objects inner workings so that a user of an object only sees a pure table (see setmetatable() and __metatable)
uses closures for information hiding (see Lua Pil 16.4 and Object Benchmark Tests)
prevents modification of the object (see __newindex)
allows for methods to be intercepted (see __index)
lets you get a list of all of the functions and attributes (see the 'key' attribute in __index)
looks, acts, walks, and talks like a normal Lua table (see __pairs, __len, __ipairs)
looks like a string when it needs to (see __tostring)
works with Lua 5.2
Here's the code to construct a new MyObject (this could be a standalone function, it doesn't need to be stored in the MyObject table - there is absolutely nothing that ties obj once its created back to MyObject.new(), this is only done for familiarity and out of convention):
MyObject = {}
MyObject.new = function(name)
local objectName = name
-- A table of the attributes we want exposed
local attrs = {
attr1 = 123,
}
-- A table of the object's methods (note the comma on "end,")
local methods = {
method1 = function()
print("\tmethod1")
end,
print = function(...)
print("MyObject.print(): ", ...)
end,
-- Support the less than desirable colon syntax
printOOP = function(self, ...)
print("MyObject:printOOP(): ", ...)
end,
}
-- Another style for adding methods to the object (I prefer the former
-- because it's easier to copy/paste function()'s around)
function methods.addAttr(k, v)
attrs[k] = v
print("\taddAttr: adding a new attr: " .. k .. "=\"" .. v .. "\"")
end
-- The metatable used to customize the behavior of the table returned by new()
local mt = {
-- Look up nonexistent keys in the attrs table. Create a special case for the 'keys' index
__index = function(t, k)
v = rawget(attrs, k)
if v then
print("INFO: Successfully found a value for key \"" .. k .. "\"")
return v
end
-- 'keys' is a union of the methods and attrs
if k == 'keys' then
local ks = {}
for k,v in next, attrs, nil do
ks[k] = 'attr'
end
for k,v in next, methods, nil do
ks[k] = 'func'
end
return ks
else
print("WARN: Looking up nonexistant key \"" .. k .. "\"")
end
end,
__ipairs = function()
local function iter(a, i)
i = i + 1
local v = a[i]
if v then
return i, v
end
end
return iter, attrs, 0
end,
__len = function(t)
local count = 0
for _ in pairs(attrs) do count = count + 1 end
return count
end,
__metatable = {},
__newindex = function(t, k, v)
if rawget(attrs, k) then
print("INFO: Successfully set " .. k .. "=\"" .. v .. "\"")
rawset(attrs, k, v)
else
print("ERROR: Ignoring new key/value pair " .. k .. "=\"" .. v .. "\"")
end
end,
__pairs = function(t, k, v) return next, attrs, nil end,
__tostring = function(t) return objectName .. "[" .. tostring(#t) .. "]" end,
}
setmetatable(methods, mt)
return methods
end
And now the usage:
-- Create the object
local obj = MyObject.new("my object's name")
print("Iterating over all indexes in obj:")
for k,v in pairs(obj) do print('', k, v) end
print()
print("obj has a visibly empty metatable because of the empty __metatable:")
for k,v in pairs(getmetatable(obj)) do print('', k, v) end
print()
print("Accessing a valid attribute")
obj.print(obj.attr1)
obj.attr1 = 72
obj.print(obj.attr1)
print()
print("Accessing and setting unknown indexes:")
print(obj.asdf)
obj.qwer = 123
print(obj.qwer)
print()
print("Use the print and printOOP methods:")
obj.print("Length: " .. #obj)
obj:printOOP("Length: " .. #obj) -- Despite being a PITA, this nasty calling convention is still supported
print("Iterate over all 'keys':")
for k,v in pairs(obj.keys) do print('', k, v) end
print()
print("Number of attributes: " .. #obj)
obj.addAttr("goosfraba", "Satoshi Nakamoto")
print("Number of attributes: " .. #obj)
print()
print("Iterate over all keys a second time:")
for k,v in pairs(obj.keys) do print('', k, v) end
print()
obj.addAttr(1, "value 1 for ipairs to iterate over")
obj.addAttr(2, "value 2 for ipairs to iterate over")
obj.addAttr(3, "value 3 for ipairs to iterate over")
obj.print("ipairs:")
for k,v in ipairs(obj) do print(k, v) end
print("Number of attributes: " .. #obj)
print("The object as a string:", obj)
Which produces the expected - and poorly formatted - output:
Iterating over all indexes in obj:
attr1 123
obj has a visibly empty metatable because of the empty __metatable:
Accessing a valid attribute
INFO: Successfully found a value for key "attr1"
MyObject.print(): 123
INFO: Successfully set attr1="72"
INFO: Successfully found a value for key "attr1"
MyObject.print(): 72
Accessing and setting unknown indexes:
WARN: Looking up nonexistant key "asdf"
nil
ERROR: Ignoring new key/value pair qwer="123"
WARN: Looking up nonexistant key "qwer"
nil
Use the print and printOOP methods:
MyObject.print(): Length: 1
MyObject.printOOP(): Length: 1
Iterate over all 'keys':
addAttr func
method1 func
print func
attr1 attr
printOOP func
Number of attributes: 1
addAttr: adding a new attr: goosfraba="Satoshi Nakamoto"
Number of attributes: 2
Iterate over all keys a second time:
addAttr func
method1 func
print func
printOOP func
goosfraba attr
attr1 attr
addAttr: adding a new attr: 1="value 1 for ipairs to iterate over"
addAttr: adding a new attr: 2="value 2 for ipairs to iterate over"
addAttr: adding a new attr: 3="value 3 for ipairs to iterate over"
MyObject.print(): ipairs:
1 value 1 for ipairs to iterate over
2 value 2 for ipairs to iterate over
3 value 3 for ipairs to iterate over
Number of attributes: 5
The object as a string: my object's name[5]
Using OOP + closures is very convenient when embedding Lua as a facade or documenting an API.
Lua OOP can also be very, very clean and elegant (this is subjective, but there aren't any rules with this style - you always use a . to access either an attribute or a method)
Having an object behave exactly like a table is VERY, VERY useful for scripting and interrogating the state of a program
Is extremely useful when operating in a sandbox
This style does consume slightly more memory per object, but for most situations this isn't a concern. Factoring out the metatable for reuse would address this, though the example code above doesn't.
A final thought. Lua OOP is actually very nice once you dismiss most of the examples in the literature. I'm not saying the literature is bad, btw (that couldn't be further from the truth!), but the set of sample examples in PiL and other online resources lead you to using only the colon syntax (i.e. the first argument to all functions is self instead of using a closure or upvalue to retain a reference to self).
Hopefully this is a useful, more complete example.
Update (2013-10-08): There is one notable drawback to the closure-based OOP style detailed above (I still think the style is worth the overhead, but I digress): each instance must have its own closure. While this is obvious in the above lua version, this becomes slightly problematic when dealing with things on the C-side.
Assume we're talking about the above closure style from the C-side from here on out. The common case on the C side is to create a userdata via lua_newuserdata() object and attach a metatable to the userdata via lua_setmetatable(). On face value this doesn't appear like a problem until you realize that methods in your metatable require an upvalue of the userdata.
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|
}
Note how the table created with lua_createtable() didn't get associated with a metatable name the same as if you would have registered the metatable with luaL_getmetatable()? This is 100% a-okay because these values are completely inaccessible outside of the closure, but it does mean that luaL_getmetatable() can't be used to look up a particular userdata's type. Similarly, this also means that luaL_checkudata() and luaL_testudata() are also off limits.
The bottom line is that upvalues (such as userdata above) are associated with function calls (e.g. LI_MyType__tostring) and are not associated with the userdata itself. As of now, I'm not aware of a way in which you can associate an upvalue with a value such that it becomes possible to share a metatable across instances.
UPDATE (2013-10-14) I'm including a small example below that uses a registered metatable (luaL_newmetatable()) and also lua_setuservalue()/lua_getuservalue() for a userdata's "attributes and methods". Also adding random comments that have been the source of bugs/hotness that I've had to hunt down in the past. Also threw in a C++11 trick to help with __index.
namespace {
using FuncArray = std::vector<const ::luaL_Reg>;
static const std::string MYTYPE_INSTANCE_METAMETHODS{"goozfraba"}; // I use a UUID here
static const FuncArray MyType_Instnace_Metamethods = {
{ "__tostring", MyType_InstanceMethod__tostring },
{ "__index", MyType_InstanceMethod__index },
{ nullptr, nullptr }, // reserve space for __metatable
{ nullptr, nullptr } // sentinel
};
static const FuncArray MyType_Instnace_methods = {
{ "fooAttr", MyType_InstanceMethod_fooAttr },
{ "barMethod", MyType_InstanceMethod_barMethod },
{ nullptr, nullptr } // sentinel
};
// Must be kept alpha sorted
static const std::vector<const std::string> MyType_Instance___attrWhitelist = {
"fooAttr",
};
static int MyType_ClassMethod_newInstance(lua_State* L) {
// You can also use an empty allocation as a placeholder userdata object
// (e.g. lua_newuserdata(L, 0);)
auto userdata = static_cast<MyType*>(lua_newuserdata(L, sizeof(MyType)));
new(userdata) MyType(); // Placement new() FTW
// Use luaL_newmetatable() since all metamethods receive userdata as 1st arg
if (luaL_newmetatable(L, MYTYPE_INSTANCE_METAMETHODS.c_str())) { // |userdata|metatable|
luaL_setfuncs(L, MyType_Instnace_Metamethods.data(), 0); // |userdata|metatable|
// Prevent examining the object: getmetatable(MyType.new()) == empty table
lua_pushliteral(L, "__metatable"); // |userdata|metatable|literal|
lua_createtable(L, 0, 0); // |userdata|metatable|literal|table|
lua_rawset(L, -3); // |userdata|metatable|
}
lua_setmetatable(L, -2); // |userdata|
// Create the attribute/method table and populate with one upvalue, the userdata
lua_createtable(L, 0, funcs.size()); // |userdata|table|
lua_pushvalue(L, -2); // |userdata|table|userdata|
luaL_setfuncs(L, funcs.data(), 1); // |userdata|table|
// Set an attribute that can only be accessed via object's fooAttr, stored in key "fooAttribute"
lua_pushliteral(L, "foo's value is hidden in the attribute table"); // |userdata|table|literal|
lua_setfield(L, -2, "fooAttribute"); // |userdata|table|
// Make the attribute table the uservalue for the userdata
lua_setuserdata(L, -2); // |userdata|
return 1;
}
static int MyType_InstanceMethod__tostring(lua_State* L) {
// Since we're using closures, we can assume userdata is the first value on the stack.
// You can't make this assumption when using metatables, only closures.
luaL_checkudata(L, 1, MYTYPE_INSTANCE_METAMETHODS.c_str()); // Test anyway
auto myTypeInst = static_cast<MyType*>(lua_touserdata(L, 1));
lua_pushstring(L, myTypeInst->str()); // |userdata|string|
return 1; // |userdata|string|
}
static int MyType_InstanceMethod__index(lua_State* L) {
lua_getuservalue(L, -2); // |userdata|key|attrTable|
lua_pushvalue(L, -2); // |userdata|key|attrTable|key|
lua_rawget(L, -2); // |userdata|key|attrTable|value|
if (lua_isnil(L, -1)) { // |userdata|key|attrTable|value?|
return 1; // |userdata|key|attrTable|nil|
}
// Call cfunctions when whitelisted, otherwise the caller has to call the
// function.
if (lua_type(L, -1) == LUA_TFUNCTION) {
std::size_t keyLen = 0;
const char* keyCp = ::lua_tolstring(L, -3, &keyLen);
std::string key(keyCp, keyLen);
if (std::binary_search(MyType_Instance___attrWhitelist.cbegin(),
MyType_Instance___attrWhitelist.cend(), key))
{
lua_call(L, 0, 1);
}
}
return 1;
}
static int MyType_InstanceMethod_fooAttr(lua_State* L) {
// Push the uservalue on to the stack from fooAttr's closure (upvalue 1)
lua_pushvalue(L, lua_upvalueindex(1)); // |userdata|
lua_getuservalue(L, -1); // |userdata|attrTable|
// I haven't benchmarked whether lua_pushliteral() + lua_rawget()
// is faster than lua_getfield() - (two lua interpreter locks vs one lock + test for
// metamethods).
lua_pushliteral(L, "fooAttribute"); // |userdata|attrTable|literal|
lua_rawget(L, -2); // |userdata|attrTable|value|
return 1;
}
static int MyType_InstanceMethod_barMethod(lua_State* L) {
// Push the uservalue on to the stack from barMethod's closure (upvalue 1)
lua_pushvalue(L, lua_upvalueindex(1)); // |userdata|
lua_getuservalue(L, -1); // |userdata|attrTable|
// Push a string to finish the example, not using userdata or attrTable this time
lua_pushliteral(L, "bar() was called!"); // |userdata|attrTable|literal|
return 1;
}
} // unnamed-namespace
The lua script side of things looks something like:
t = MyType.new()
print(typue(t)) --> "userdata"
print(t.foo) --> "foo's value is hidden in the attribute table"
print(t.bar) --> "function: 0x7fb560c07df0"
print(t.bar()) --> "bar() was called!"
how do you create a lua object that only exposes its attributes and not its methods?
If you don't expose methods in any way, you can't call them, right? Judging from your example, it sounds like what you really want is a way to iterate through the attributes of an object without seeing methods, which is fair.
The simplest approach is just to use a metatable, which puts the methods in a separate table:
-- create Point class
Point = {}
Point.__index = Point
function Point:report() print(self.x, self.y) end
-- create instance of Point
pt = setmetatable({x=10, y=20}, Point)
-- call method
pt:report() --> 10 20
-- iterate attributes
for k,v in pairs(pt) do print(k,v) end --> x 10 y 20
is it possible to not use the colon syntax for OOP in Lua?
You can use closures instead, but then pairs is going to see your methods.
function Point(x, y)
local self = { x=x, y=y}
function pt.report() print(self.x, self.y) end
return self
end
pt = Point(10,20)
pt.report() --> 10 20
for k,v in pairs(pt) do print(k,v) end --> x 10 y 20 report function: 7772112
You can fix the latter problem by just writing an iterator that shows only attributes:
function nextattribute(t, k)
local v
repeat
k,v = next(t, k)
if type(v) ~= 'function' then return k,v end
until k == nil
end
function attributes (t)
return nextattribute, t, nil
end
for k,v in attributes(pt) do print(k,v) end --> x 10 y 20
I don't need inheritance, polymorphism
You get polymorphism for free in Lua, without or without classes. If your zoo has a Lion, Zebra, Giraffe each of which can Eat() and want to pass them to the same Feed(animal) routine, in a statically typed OO languages you'd need to put Eat() in a common base class (e.g. Animal). Lua is dynamically typed and your Feed routine can be passed any object at all. All that matters is that the object you pass it has an Eat method.
This is sometimes called "duck typing": if it quacks like a duck and swims like a duck, it's a duck. As far as our Feed(animal) routine is concerned, if it Eats like an animal, it's an animal.
only encapsulation and privacy.
Then I think exposing data members while hiding methods is the opposite of what you want to do.
Hello I have the following bit of code which seems to work, but I'm not sure why - I've built a testclass as follows
class testclass {
int ivalue;
public:
int getivalue();
void setivalue(int &v);
};
and then registered the testclass (bits left out for the actual functions but they're pretty basic). It's the registration of the metatables I'm not following. (etivalue and setivalue are c functions that call the class functions of the same name)
static const struct luaL_Reg arraylib_f [] = {
{"new", new_testclass},
{NULL, NULL}
};
static const struct luaL_Reg arraylib_m [] = {
{"set", setivalue},
{"get", getivalue},
{NULL, NULL}
};
int luaopen_testclass (lua_State *L) {
luaL_newmetatable(L, "LuaBook.testclass");
lua_pushvalue(L, -1); /* duplicates the metatable */
lua_setfield(L, -2, "__index");
luaL_register(L, NULL, arraylib_m);
luaL_register(L, "testclass", arraylib_f);
return 1;
}
The bit I don't understand is I'm adding the functions to the __index for the metatable but
when I run
a = testclass.new()
a:set(10)
print(a:get())
Then it works as expected. The bit I don't understand is why the set is being called when I think I've loaded it in the __index metatable? Is that what I've done or something else?
tia
int luaopen_testclass (lua_State *L) {
luaL_newmetatable(L, "LuaBook.testclass"); //leaves new metatable on the stack
lua_pushvalue(L, -1); // there are two 'copies' of the metatable on the stack
lua_setfield(L, -2, "__index"); // pop one of those copies and assign it to
// __index field od the 1st metatable
luaL_register(L, NULL, arraylib_m); // register functions in the metatable
luaL_register(L, "testclass", arraylib_f);
return 1;
}
That code is equivalent to the example Lua code:
metatable = {}
metatable.__index = metatable
metatable.set = function() --[[ stuff --]] end
metatable.get = function() --[[ stuff --]] end
I assume that 'new_testclass' C function sets the metatable "LuaBook.testclass" for the returned table.
In your code you dont add functions to the metatable __index field. You assign pointer to metatable to that metatable's field named __index, and you register set and get functions to it.
Now, if you set that metatable to the value returned from 'new_testclass' function (which I assume you do) - lets call that value 'foo', and you call foo:set(10), than Lua:
checks that there is no such field as 'set' in 'foo'
sees that 'foo' has a metatable
looks at that metatable's __index field - sees it's a table
checks if that table assigned to __index field has a field 'set' and it's value is a function
calls 'set' method passing 'foo' as self parameter
I hope that this will help you figure out whats going on here.
If I understand your question, you are asking how the set() get() get invoked through the __index metamethod.
The code can be expressed in pure lua:
local o = {}
function o.get(self)
return self.ivalue
end
function o.set(self, val)
self.ivalue = val
end
a = {}
mt = {
__index = function(t, n)
return o[n]
end
}
setmetatable(a, mt)
print(a:get())
a:set(10)
print(a:get())
results:
nil
10
In this example the mt table is set as the a table's metatable. The __index metamethod is invoked for both get and set since neither get or set currently exist in table a.
If this example is changed instead to this:
local o = {}
function o.get(self)
return self.ivalue
end
function o.set(self, val)
self.ivalue = val
end
a = {}
function a.get(self)
print('here')
return self.ivalue
end
mt = {
__index = function(t, n)
return o[n]
end
}
setmetatable(a, mt)
print(a:get())
a:set(10)
print(a:get())
results:
here
nil
here
10
In this case the __index metamethod is NOT invoked for get() since a get index already exists in the a table.
Many interesting constructs can be created using metamethods, once you understand how they work. I suggest reading 13.4.1 - The __index Metamethod in PiL and work through a few more examples. All of the above can also be done from the c api.