Anything related to Lua meta-methods (a.k.a. metamethods), i.e. functions that alter how a Lua table behaves in the context of some operations (e.g. math operations, element access, garbage collection, etc.). Meta-methods can be used to implement object oriented programming and other powerful techniques using Lua tables.
Questions tagged [meta-method]
52 questions
8
votes
1 answer
Does next() look for a __pairs metamethod?
In general, the syntax:
for k, v in pairs(t) do
....
end
is equivalent to:
for k, v in next, t do
....
end
But what if t has a __pairs metamethod? Will the standard next() function check for this? If not, isn't it better to always use…
Siler
- 8,976
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7
votes
2 answers
what is actual implementation of lua __pairs?
Does anybody know actual implementation of lua 5.2. metamethod __pairs? In other words, how do I implement __pairs as a metamethod in a metatable so that it works exactly same with pairs()?
I need to override __pairs and want to skip some dummy…
user2872907
- 71
- 1
- 2
4
votes
1 answer
why can you set __index equal to a table
The index metamethod can be set equal to tables. From what I can tell
foo.__index = function(self, k)
return bar[k]
end
and
foo.__index = bar
are the same. Why is declaring functions this way allowed in this situation?
Ace shinigami
- 1,374
- 2
- 12
- 25
4
votes
1 answer
Is it possible to bypass __tostring the way rawget/set bypasses __index/__newindex in Lua?
For example:
local my_table = { name = "my table" }
local my_table_mt = {}
function my_table_mt.__tostring(tbl)
return "%s<%s>":format(tbl.name or "?", rawtostring(tbl))
end
Is something like this possible? I know the rawtostring method…
Llamageddon
- 3,306
- 4
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- 44
4
votes
3 answers
Metamethod when accessing a key as mutable
__index is called when accessing as immutable :
local foo = bar["foo"];
__newindex is called when access as mutable an index that doesn't exist :
local bar = { }
bar["foo"] = 123 -- calls __newindex
bar["foo"] = 456 -- does NOT call __newindex
Is…
Virus721
- 8,061
- 12
- 67
- 123
4
votes
2 answers
Lua __metatable bypass?
I was wondering if there is any way to escape the __metatable metamethod. I know there isn't one, but I'm trying to do something like this, but obviously __metatable blocks that from happening:
-- pretend that there is a __metatable field given…
Bertolt Hoover
- 71
- 3
4
votes
1 answer
How to fix this unpack issue?
I'm creating an Array class that adds more usage to tables. I have a metamethod that allows me to combine two tables, ex:
Array(5) .. Array(6, 10) should give you {5, 6, 10}
I'm aware that I can use two loops to do this, but I'm trying to make my…
David
- 693
- 1
- 7
- 20
4
votes
2 answers
Accessing deeply nested table without error?
For a field inside a deeply nested table, for example, text.title.1.font. Even if you use
if text.title.1.font then ... end
it would result in an error like "attempt to index global 'text' (a nil value)" if any level of the table does not actually…
Dionysian
- 1,195
- 2
- 13
- 24
4
votes
2 answers
How to save boolean conditions and evaluate later
I need to create a structure. The structure must contain an array of "boolean conditions". Something like this:
function ReturnStructure ()
local structure = {
{A < 10},
{B == "smth"},
…
user3454119
- 43
- 3
4
votes
1 answer
modify # operator in lua
I have made a lua console on the love2d engine which is irrelevant. I am trying to upgrade the metatables to be able to effect more things (the pairs function, change the metatable on another table instead of the targeted one etc.) and one of the…
KaoSDlanor
- 65
- 7
3
votes
2 answers
How to create a simple importable class in Lua?
I'd like to create MyClass class in Lua in a separate file myclass.lua which I can import and use later.
It should be working the following way:
local MyClass = require 'myclass'
tab = {1,2,3}
m = MyClass(tab)
However, following the code in Lua…
minerals
- 6,090
- 17
- 62
- 107
3
votes
1 answer
Doesn't Lua inheritance include metamethods?
I'm trying to implement simple inheritance in Lua as it is presented in PIL 16.2. However, I've come across a surprising behavior: metamethods don't seem to be inherited. In the following example, I create a Point class with x and y members, and…
Ludwik
- 2,247
- 2
- 27
- 45
3
votes
1 answer
Lua C++ userdata matrix access to elements
I have a matrix class in C++ and the constructor is as follows:
template CMatrix::CMatrix(unsigned int varrow,unsigned int varcolumn)
{
//Lets set member variables
this->m_row=varrow;this->m_column=varcolumn;
…
macroland
- 973
- 9
- 27
3
votes
3 answers
How do we change the way print displays a table
Assuming I have a piece of code such as the following
aTable = {aValue=1}
aTable_mt = {}
print(aTable)
What must I do to make Lua print something like aTable current aValue = 1 as opposed to table: 0x01ab1d2.
So far I've tried setting the…
No Name
- 116
- 7
3
votes
1 answer
Why does this cause a C stack overflow?
I'm aware that I can fix this problem by using rawset, but I'm just wondering why the following code causes a C stack overflow.
local mt = {
__newindex = function(self, key, value)
self[key] = value
end
}
local x =…
David
- 693
- 1
- 7
- 20