It seems like there's know such thing as a reference to a number/boolean/lightuserdata in Lua. However, what would be the easiest way to set a global in Lua that points to a C++ native type (e.g. float) and have it update automatically when I change the corresponding global in Lua?
int foo = 2;
//imaginary lua function that does what I want
lua_pushnumberpointer(state,&foo)
lua_setglobal(state,"foo")
-- later, in a lua script
foo = 5;
The last line should automatically update foo on the C++ side. What would be the easiest way to achieve something like this?
Take a look at meta tables, especially the tags __index and __newindex.
If you set them to suitable functions, you have full control what happens when a new index is set / an unset index is queried.
That should allow you to do all you asked for.
It might be advantageous to only set __newindex to a custom function and save the interesting entries in a table set on __index.
Example handler for __newindex and companion definition of __index.
Consider implementing it on the native side though, for performance, and because __hidden_notify_world() is most likely native.
do
local meta = getmetatable(_ENV) or {}
setmetatable(_ENV, meta)
local backing = {}
_ENV.__index = backing
function _ENV:__newindex(k, v)
if backing[k] ~= v then
backing[k] = v
__hidden_do_notify_world(k, v)
end
end
end
If you are using Lua < 5.2 (2011), you must use _G instead of _ENV.
Related
I'm reviewing some toy examples from Lua and I found the following one over there with respect to environments:
M = {} -- the module
complex = {} -- global complex numbers registry
mt = {} --metatable for complex numbers
function new (r, i)
local cp = {}
cp = {r=r, i=i}
return setmetatable(cp,mt)
end
M.new = new -- add 'new' to the module
function M.op (...)
--Why does not it work?
local _ENV = complex
return ...
end
function M.add (c1, c2)
return new(c1.r + c2.r, c1.i + c2.i)
end
function M.tostring (c)
return string.format("(%g,%g)", c.r, c.i) --to avoid +-
end
mt.__tostring = M.tostring
mt.__add = M.add
complex.a = M.new(4,3)
complex.b = N.new(6,2)
--nil
M.op(a+b)
--It works
M,op(complex.a+complex.b)
The use of _ENV has no effect. However, if I use complex = _G, both lines work. How do set a local environment for M.op. I'm not asking for specific libraries, I just want to know why it does not work and how to fix it.
M.op(a+b)
This line doesn't do what you expect, because it uses values of a and b that are available when this method is called. It doesn't matter that you set _ENV value inside the method, as by the time the control gets there, the values referenced by a and b have already been retrieved and since both values are nil in your code, you probably get "attempt to perform arithmetic on global..." error.
how to fix it.
I'm not sure what exactly you want to fix, as you already reference the example that works. If you assign complex.a you can't assume that a will have the same value without mapping complex table to _ENV.
I have actually been wondering about this question for a long time, and I can recall reading something somewhere on the internet about loading all local variables into a table, but I could never find it again.
What I am wondering is if there is basically any way to dynamically create a local variable, similar to how you can dynamically create a global variable.
In Lua you can create dynamic global variables simply by saying:
local my_env = getfenv(1);
for i = 1, 10 do
my_env["OMG_DYNAMIC_GLOBAL_VARIABLE_"..i] = i * i;
end
But I have always wondered if there is a way to essentially do the same or a similar thing for local variables. Something like
for i = 1, 10 do
local["OMG_DYNAMIC_LOCAL_VARIABLE_"..i] = i * i;
end
would kind of be pseudo code for what I am trying to do.
As a second part to my question, I am wondering if there is any way to possibly iterate through every local variable in the current scope? I guess pseudo code for what I am trying to do would look something like:
local a = 123;
local b = 321;
LocalVars = {(local)}; --the table with the local variables in it.
for i,v in pairs(LocalVars) do print(i, v); end
I would appreciate any help regarding either part of my question.
Thanks
the table with the local variables in it
There is no table with local variables. Local variables do not exist in compiled Lua. There are simply locations on the Lua stack and/or upvalues. Even the names of local variables are lost during the compilation process.
Local variables are always static constructs of the Lua source code. They never exist within a table. If you need a non-static construct, use a table. That's what tables are for. That's (part of) why Lua globals are in a table.
I don't see the advantage of what you proposed over a regular local table except for syntactic sugar.
local locals = {}
locals.a = 123
locals.b = 321
for i,v in pairs(locals) do
print(i, v)
end
The syntax you proposed does not make the program clearer or easier to read in any way. In fact it even creates new problems. Does the following print c as well or does it not because it is in a nested local scope?
for i,v in pairs(LocalVars) do
local c = 1729
print(i, v)
end
In Programming in Lua (3rd Ed.) by Roberto Ierusalimschy it is stated that
A common idiom in Lua is
local foo = foo
This code creates a local
variable, foo, and initializes it with the value of the global
variable foo. (The local foo becomes visible only after its
declaration.) This idiom is useful when the chunk needs to preserve
the original value of foo even if later some other function changes
the value of the global foo; it also speeds up the access to foo.
Could someone explain this more in detail and provide a simple example?
At the moment, the only use I can think of for this idiom is to manage local variables (in a given block) that have the same names as global variables, so that the global variable is left unchanged after the block.
An example:
foo = 10
do
local foo = foo
foo = math.log10(foo)
print(foo)
end
print(foo)
this gives:
1
10
But the same could be accomplished without using the idiom at all:
bar = 10
do
local bar = math.log10(bar)
print(bar)
end
print(bar)
that gives the same result. So my explanation doesn't seem to hold.
I've seen this used more often as a optimization technique than as a way to preserve original values. With the standard Lua interpreter, every global variable access and module access requires a table lookup. Local variables, on the other hand, have statically-known locations at bytecode-compile time and can be placed in VM registers.
In more depth: Why are local variables accessed faster than global variables in lua?
The explanation is correct; I'm not sure why you are not satisfied with your example. To give you a real example:
local setfenv = setfenv
if not setfenv then -- Lua 5.2+
setfenv = function() ..... end
end
Another reason is to preserve the value as it is at this moment, so that other functions that use that value (in a file or a module) would have the same expectations about that value.
Wrapping a global:
do
local setmetatable = setmetatable
function _ENV.setmetatable(...)
-- Do your thing
return setmetatable(...)
end
end
Reducing overhead by using a local instead of doing a lookup in the globals-table (which is a local btw.):
local type = type
for k, v in next, bigtable do
if type(v) == "string" then
-- Do one thing
else
-- Do other thing
end
end
I think you're splitting hairs, unintentionally.
local bar = math.log10(bar)
is essentially the same as local bar = bar in spirit, but we it would be less useful to claim that the idiom is local bar = a(bar), because we may want to handle the local in some way other than passing it to a function first -- e.g. appending it to something.
This point is that we want to refer to the local bar, just as you say, not exactly how the conversion from global to local is done.
Sorry if this is too obvious, but I am a total newcomer to lua, and I can't find it in the reference.
Is there a NAME_OF_FUNCTION function in Lua, that given a function gives me its name so that I can index a table with it? Reason I want this is that I want to do something like this:
local M = {}
local function export(...)
for x in ...
M[NAME_OF_FUNCTION(x)] = x
end
end
local function fun1(...)
...
end
local function fun2(...)
...
end
.
.
.
export(fun1, fun2, ...)
return M
There simply is no such function. I guess there is no such function, as functions are first class citizens. So a function is just a value like any other, referenced to by variable. Hence the NAME_OF_FUNCTION function wouldn't be very useful, as the same function can have many variable pointing to it, or none.
You could emulate one for global functions, or functions in a table by looping through the table (arbitrary or _G), checking if the value equals x. If so you have found the function name.
a=function() print"fun a" end
b=function() print"fun b" end
t={
a=a,
c=b
}
function NameOfFunctionIn(fun,t) --returns the name of a function pointed to by fun in table t
for k,v in pairs(t) do
if v==fun then return k end
end
end
print(NameOfFunctionIn(a,t)) -- prints a, in t
print(NameOfFunctionIn(b,t)) -- prints c
print(NameOfFunctionIn(b,_G)) -- prints b, because b in the global table is b. Kind of a NOOP here really.
Another approach would be to wrap functions in a table, and have a metatable set up that calls the function, like this:
fun1={
fun=function(self,...)
print("Hello from "..self.name)
print("Arguments received:")
for k,v in pairs{...} do print(k,v) end
end,
name="fun1"
}
fun_mt={
__call=function(t,...)
t.fun(t,...)
end,
__tostring=function(t)
return t.name
end
}
setmetatable(fun1,fun_mt)
fun1('foo')
print(fun1) -- or print(tostring(fun1))
This will be a bit slower than using bare functions because of the metatable lookup. And it will not prevent anyone from changing the name of the function in the state, changing the name of the function in the table containing it, changing the function, etc etc, so it's not tamper proof. You could also strip the tables of just by indexing like fun1.fun which might be good if you export it as a module, but you loose the naming and other tricks you could put into the metatable.
Technically this is possible, here's an implementation of the export() function:
function export(...)
local env = getfenv(2);
local funcs = {...};
for i=1, select("#", ...) do
local func = funcs[i];
for local_index = 1, math.huge do
local local_name, local_value = debug.getlocal(2, local_index);
if not local_name then
break;
end
if local_value == func then
env[local_name] = local_value;
break;
end
end
end
return env;
end
It uses the debug API, would require some changes for Lua 5.2, and finally I don't necessarily endorse it as a good way to write modules, I'm just answering the question quite literally.
Try this:
http://pgl.yoyo.org/luai/i/tostring
tostring( x ) should hopefully be what you are looking for
If I am not wrong (and I probably will, because I actually never programmed in Lua, just read a bunch of papers and articles), internally there is already a table with function names (like locals and globals in Python), so you should be able to perform a reverse-lookup to see what key matches a function reference.
Anyway, just speculating.
But the fact is that looking at your code, you already know the name of the functions, so you are free to construct the table. If you want to be less error prone, it would be easier to use the name of the function to get the function reference (with eval or something like that) than the other way around.
I'm just getting started with Lua. In the example I'm learning from (the Ghosts & Monsters Corona open source), I see this pattern repeatedly.
local director = require("director")
local mainGroup = display.newGroup()
local function main()
mainGroup:insert(director.directorView)
openfeint = require ("openfeint")
openfeint.init( "App Key Here", "App Secret Here", "Ghosts vs. Monsters", "App ID Here" )
director:changeScene( "loadmainmenu" )
return true
end
main()
Is this some sort of convention experienced Lua programmers recommend or are there genuine advantages to doing it this way? Why wouldn't you just skip the function all together and do this:
local director = require("director")
local mainGroup = display.newGroup()
mainGroup:insert(director.directorView)
local openfeint = require ("openfeint")
openfeint.init( "App Key Here", "App Secret Here", "Ghosts vs. Monsters", "App ID Here" )
director:changeScene( "loadmainmenu" )
Is there some implicit benefit to the first style over the second? Thanks!
Is this some sort of convention experienced Lua programmers recommend or are there genuine advantages to doing it this way?
It's not typical. The advantage is that object state is private, but that's not enough of an advantage to recommend it.
I see this pattern repeatedly.
I've never seen it before, and it happens only once in the source you posted.
EDIT: Adding a response to a question asked in the comments below this post.
A function which accesses external local variables binds to those variables and is called a 'closure'. Lua (for historical reasons) refers to those bound variables as 'upvalues'. For example:
local function counter()
local i = 1
return function()
print(i)
i = i + 1
end
end
local a, b = counter(), counter()
a() a() a() b() --> 1 2 3 1
a and b are closures bound to different copies of i, as you can see from the output. In other words, you can think of a closure as function with it's own private state. You can use this to simulate objects:
function Point(x,y)
local p = {}
function p.getX() -- syntax sugar for p.getX = function()
return x
end
function p.setX(x_)
x = x_
end
-- for brevity, not implementing a setter/getter for y
return p
end
p1 = Point(10,20)
p1.setX(50)
print(p1.getX())
Point returns a table of closures, each bound to the locals x and y. The table doesn't contain the point's state, the closures themselves do, via their upvalues. An important point is that each time Point is called it creates new closures, which is not very efficient if you have large quantities of objects.
Another way of creating classes in Lua is to create functions that take a table as the first argument, with state being stored in the table:
function Point(x,y)
local p = {x=x,y=y}
function p:getX() -- syntax sugar for p.getX = function(self)
return self.x
end
function p:setX(x)
self.x = x
end
return p
end
p1 = Point(10,20)
p1:setX(50) -- syntax sugar for p1.setX(p1, 50)
print(p1:getX()) -- syntax sugar for p1.getX(p1)
So far, we're still creating new copies of each method, but now that we're not relying on upvalues for state, we can fix that:
PointClass = {}
function PointClass:getX() return self.x end
function PointClass:setX(x) self.x = x end
function Point(x,y)
return {
x = x,
y = y,
getX = PointClass.getX,
setX = PointClass.getY,
}
end
Now the methods are created once, and all Point instances share the same closures. An even better way of doing this is to use Lua's metaprogramming facility to make new Point instances automatically look in PointClass for methods not found in the instance itself:
PointClass = {}
PointClass.__index = PointClass -- metamethod
function PointClass:getX() return self.x end
function PointClass:setX(x) self.x = x end
function Point(x,y)
return setmetatable({x=x,y=y}, PointClass)
end
p1 = Point(10,20)
-- the p1 table does not itself contain a setX member, but p1 has a metatable, so
-- when an indexing operation fails, Lua will look in the metatable for an __index
-- metamethod. If that metamethod is a table, Lua will look for getX in that table,
-- resolving p1.setX to PointClass.setX.
p1:setX(50)
This is a more idiomatic way of creating classes in Lua. It's more memory efficient and more flexible (in particular, it makes it easy to implement inheritance).
I frequently write my own Lua scripts this way because it improves readability in this case:
function main()
helper1( helper2( arg[1] ) )
helper3()
end
function helper1( foo )
print( foo )
end
function helper2( bar )
return bar*bar
end
function helper3()
print( 'hello world!' )
end
main()
This way the "main" code is at the top, but I can still define the necessary global functions before it executes.
A simple trick, really. I can't think of any reason to do this besides readability.
The first style could be used too improove readability, but I would rather give the function some meaningful name instead of main or just go without the function.
By the way, I think it's always a good practice to name blocks of code, i.e. put them into functions or methods. It helps explain your intend with that piece of code, and encourages reuse.
I don't see much point to the first style as you've shown it. But if it said something like if arg then main() end at the bottom, the script might (just might) be useful as a loadable "library" in addition to being a standalone script. That said, having a main() like that smacks of C, not Lua; I think you're right to question it.