Metatable - Revision history

MKlegoman357: Improved

2015-05-12T19:18:07Z

Improved

MKlegoman357: Took the intro from the PIL, it's a bit better. Also, note that it's 'Lua', not 'lua' or 'LUA'.

2015-05-12T18:32:57Z

Took the intro from the PIL, it's a bit better. Also, note that it's 'Lua', not 'lua' or 'LUA'.

Creator at 21:46, 7 May 2015

2015-05-07T21:46:32Z

Superaxander: Add an example of the __eq metatable property

2015-02-02T15:19:09Z

Add an example of the __eq metatable property

Turkey: /* Metamethods */ Adding external links section with link to official Lua metatable documentation

2014-09-14T10:28:51Z

‎Metamethods: Adding external links section with link to official Lua metatable documentation

Turkey: /* Metamethods */ Spelling corrections and added to tutorials category

2014-09-14T09:18:09Z

‎Metamethods: Spelling corrections and added to tutorials category

Unit158: Created page with "{{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. ..."

2013-04-17T13:15:13Z

Created page with "{{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. ..."

New page

{{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.

== 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. <br/>

{| class="wikitable"
|-
| 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 then.
| The two objects being compared
|-
| __le
| Less then or equal to.
| The objects being compared
|-
| __concat
| The objects being concat-ed 
| The two objects being concat-ed
|-
| __len
| The # operator.
| The object having the operation preformed on.
|-
|}

@@ Line 1: / Line 1: @@
 {{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. 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.
+Metatables allow us to change the behavior of a table. For instance, using metatables, we can define how Lua computes the expression a+b, where a and b are tables. Whenever Lua tries to add two tables, it checks whether either of them has a metatable and whether that metatable has an __add field. If Lua finds this field, it calls the corresponding value (the so-called metamethod, which should be a function) to compute the sum.
-<!-- Possibly more, I don't know about any other application, unfortunately --~~~~ -->
+<!-- Taken from the Lua PIL (http://www.lua.org/pil/13.html) --~~~~ -->
 == Setting and getting a metatable ==

@@ Line 90: / Line 90: @@
 |}
 == External links ==
 * [http://www.lua.org/manual/5.1/manual.html#2.8 Lua 5.1 Reference Manual: Metatables]
 [[Category:Tutorials]]

@@ Line 89: / Line 89: @@
 |-
 |}
 [[Category:Tutorials]]

@@ Line 1: / Line 1: @@
 {{NeedsWork|Not enough info, perhaps incorrect info}}
-Metatables allow us to change the behavior of a table. For instance, using metatables, we can define how Lua computes the expression a+b, where a and b are tables. Whenever Lua tries to add two tables, it checks whether either of them has a metatable and whether that metatable has an __add field. If Lua finds this field, it calls the corresponding value (the so-called metamethod, which should be a function) to compute the sum.
+'''Metatables''' allow us to change the behavior of a {{type|table}}. For instance, using metatables, we can define how Lua computes the expression <code>a+b</code>, where <var>a</var> and <var>b</var> are tables. Whenever Lua tries to add two tables, it checks whether either of them has a metatable and whether that metatable has an <var>__add</var> field. If Lua finds this field, it calls the corresponding value (the so-called metamethod, which should be a {{type|function}}) to compute the sum.
 <!-- Taken from the Lua PIL (http://www.lua.org/pil/13.html) --~~~~ -->
 == Setting and getting a metatable ==
-You will use setMetatable and getMetatable to set and get metatables.
+To set the metatable of a table use <code>setmetatable( table, metatable )</code>, where <var>table</var> is the table which's metatable you want to set and <var>metatable</var> is the new metatable of that table.
-You use getMetatable(table) to get your metatable from a table. It will return the __metatable metafield
+To get the metatable of a table use <code>getmetatable( table )</code>, where <var>table</var> is the table which's metatable you want to get.
-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
+A metamethod is a type of function which changes the default behavior of a table. The table below lists what each
-one is called, and how each one is used. <br/>
+one is called, and how each one is used.<br/>
 {| class="wikitable"
@@ Line 25: / Line 21: @@
 |-
 | __index
-| Used when table[variable] is called.
+| Can be either a {{type|function}} or a {{type|table}}. Triggered whenever a table is being indexed and there is no field under the indexed <var>key</var>. If set to a table then that table is indexed using the same <var>key</var>. If it's a function then that function is called with the <var>table</var> and the <var>key</var> as parameters and its return value is used as the value under that <var>key</var>.
-| The variable.
+| the <var>table</var> and the <var>key</var>
 |-
 | __newindex
-| Used like __index, but it is only called when the variable matches an empty space in the table.
+| Triggered whenever a new (nil, non-existent) field is being assigned in a table.
-| The variable.
+| the <var>table</var>, the <var>key</var> and the new <var>value</var>
 |-
 | __call
-| A metamethod to turn a table's name into a function, or whatever you want it to do.
+| Called whenever a table is being called as it was a function.
-| Function, Tuple
+| the <var>table</var> and the arguments that where passed the the table-call expression.
 |-
 | __metatable
-| Used for protecting metatables, called when getmetatable() is used on the table, setmetatable() just errors.
+| If present, <code>getmetatable</code> returns its value instead of the actual metatable, <code>setmetatable</code> raises an error.
-| Nothing (as far as Unit158 knows)
 |-
 | __mul
-| Used as an operator overloader for multiplication.
+| Triggered whenever a multiplication operation with a table occurs. Passes the two operands <var>a</var> and <var>b</var>, without telling which of the operands is the table and which is not.
-| The two objects being "multiplied".
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __div
-| Used as an operator overloader for division.
+| Triggered whenever a division operation with a table occurs. Behaves similar to <var>__mul</var> metamethod.
-| The two objects being "divided".
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __sub
-| Used as an operator overloader for substraction.
+| Triggered whenever a subtraction operation with a table occurs. Behaves similar to <var>__mul</var> metamethod.
-| The two objects being "subtracted".
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __add
-| Used as an operator overloader for addition.
+| Triggered whenever an addition operation with a table occurs. Behaves similar to <var>__mul</var> metamethod.
-| The two objects being "added".
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __mod
-| Used as an operator overloader for modulo.
+| Triggered whenever a modulo operation with a table occurs. Behaves similar to <var>__mul</var> metamethod.
-| The two objects being "divided".
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __pow
-| Used as an operator overloader for exponentiation operation.
+| Triggered whenever a exponentiation (power) operation with a table occurs. Behaves similar to <var>__mul</var> metamethod.
-| The two objects being exponentiated.
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __unm
-| I am not exactly sure how this one works, it is for negatives though.
+| Triggered whenever the unary ( <code>-table</code> ) operator is used on a table.
-| The table being changed into a negative. <!-- Feel free to change this one X] -->
+| the <var>table</var>
 |-
 | __eq
-| __eq is an operator overloader for comparison.
+| Triggered whenever an "equal" ( == ) operation is performed on two tables with the exact same metatable. If the metatable does not match - returns false.
-| The two objects being compared
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __lt
-| Less than.
+| Triggered whenever a "less than" ( < ), or "more than" ( > ), operation is performed on the table. Similarly to the <var>__mul</var> metamethod it is not known which parameter is which.
-| The two objects being compared
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __le
-| Less than or equal to.
+| Triggered whenever a "less than or equal to" ( <= ), or "less than or equal to" ( >= ), operation is performed on the table. Similarly to the <var>__mul</var> metamethod it is not known which parameter is which.
-| The objects being compared
+| operand <var>a</var> and operand <var>b</var>
 |-
 | __concat
-| The objects being concatenated
+| Triggered whenever a concatenation ( .. ) operation is performed on the table. Similarly to the <var>__mul</var> metamethod it is not known which parameter is which.
-| The two objects being concatenated
+| operand <var>a</var> and operand <var>b</var>
-|-
-| __len
-| The # operator.
-| The object having the operation performed on it.
-|-
 |}