Difference between revisions of "Metatable"
Superaxander (Talk | contribs) (Add an example of the __eq metatable property) |
|||
| Line 1: | Line 1: | ||
{{NeedsWork|Not enough info, perhaps incorrect info}} | {{NeedsWork|Not enough info, perhaps incorrect info}} | ||
| − | A metatable is an addition on to normal tables for adding metamethods, making classes as well as protecting functions. | + | A metatable is an addition on to normal tables for adding metamethods, making classes as well as protecting functions. When a certain action is performed on a table, lua checks whether the corresponding __metatable field exists. For example, when you want to access the index bar in the table foo, but foo.bar does not exist, if the table has __index as a __metatable field, lua will call the __index field. If it's value is another table, lua will search for the index in the new table; however, if the value is a function, lua will pass the table name and the index as arguments. |
<!-- Possibly more, I don't know about any other application, unfortunately --~~~~ --> | <!-- Possibly more, I don't know about any other application, unfortunately --~~~~ --> | ||
== Setting and getting a metatable == | == Setting and getting a metatable == | ||
Revision as of 21:46, 7 May 2015
| This page needs some serious TLC, stat! Please help us by cleaning it, fixing it up, or sparing it some love.
(Reason: Not enough info, perhaps incorrect info) |
A metatable is an addition on to normal tables for adding metamethods, making classes as well as protecting functions. When a certain action is performed on a table, lua checks whether the corresponding __metatable field exists. For example, when you want to access the index bar in the table foo, but foo.bar does not exist, if the table has __index as a __metatable field, lua will call the __index field. If it's value is another table, lua will search for the index in the new table; however, if the value is a function, lua will pass the table name and the index as arguments.
Setting and getting a metatable
You will use setMetatable and getMetatable to set and get metatables.
You use getMetatable(table) to get your metatable from a table. It will return the __metatable metafield in the table.
setMetatable(table, metatable) to set your table's metatable. It will error if the __metatable metafield is set.
However, if the table has a metatable with the metafield __metatable, it will error when you use setMetatable and return what it is set to when getMetatable is called.
Metamethods
A metamethod is a type of function which changes the default way a table acts. The table below demonstrates what each
one is called, and how each one is used.
| Name | Description | Arguments passed |
| __index | Used when table[variable] is called. | The variable. |
| __newindex | Used like __index, but it is only called when the variable matches an empty space in the table. | The variable. |
| __call | A metamethod to turn a table's name into a function, or whatever you want it to do. | Function, Tuple |
| __metatable | Used for protecting metatables, called when getmetatable() is used on the table, setmetatable() just errors. | Nothing (as far as Unit158 knows) |
| __mul | Used as an operator overloader for multiplication. | The two objects being "multiplied". |
| __div | Used as an operator overloader for division. | The two objects being "divided". |
| __sub | Used as an operator overloader for substraction. | The two objects being "subtracted". |
| __add | Used as an operator overloader for addition. | The two objects being "added". |
| __mod | Used as an operator overloader for modulo. | The two objects being "divided". |
| __pow | Used as an operator overloader for exponentiation operation. | The two objects being exponentiated. |
| __unm | I am not exactly sure how this one works, it is for negatives though. | The table being changed into a negative. |
| __eq | __eq is an operator overloader for comparison. | The two objects being compared |
| __lt | Less than. | The two objects being compared |
| __le | Less than or equal to. | The objects being compared |
| __concat | The objects being concatenated | The two objects being concatenated |
| __len | The # operator. | The object having the operation performed on it. |
Examples
 Example | |
| Use __eq to check if two tables are equal | |
| Code |
table1 = {x=0, y=1, z=0}
table2 = {x=1, y=1, z=0}
metatable = {__eq = function(operand1, operand2, type)
if operand1.x == operand2.x and operand1.y == operand2.y and operand1.z then --Note: In reality you might want to first check if your these variables actually exist
return true
else
return false
end }
setmetatable(table1, metatable)
setmetatable(table2, metatable)
if table1 == table2 then
print("Equal")
else
print("Not equal")
end
|
