|
|
| Line 1: |
Line 1: |
| {{User:Merlin11188/Templates/NoEdit}} | | {{User:Merlin11188/Templates/NoEdit}} |
|
| |
| ==[[Fields]] and [[Methods]]== | | ==[[Fields]] and [[Methods]]== |
| Object-Oriented Programming, often called ''OOP'', is a type of programming that focuses primarily on '''objects''', or [[Data_Types|data types]] that have fields and methods. Fields are like [[variables]], and [[methods]] are like functions; however, there is one fundamental difference: [[fields]] and [[methods]] are stored inside of an object, to be called through that object. This definition corresponds perfectly to [[tables]] in Lua. A class refers to the type of object. Let's say that we want to create an object class named {{`|account}}, with the field {{`|balance}}, and the methods {{`|input}} and {{`|withdraw}}. The first thing to do is to make a prototype with these fields and methods:
| |
| {{Code and output|fit=output|code=
| |
| account = {
| |
| balance = 0, -- Declare field 'balance'
| |
| input = function(self, amount) -- We want this to be a method, so we set it up like one
| |
| -- Test the validity of value passed to parameter 'amount'
| |
| amount = (type(amount) == "number" and amount) or (type(amount) == "string" and tostring(amount)) or error("Invalid amount", 2)
| |
| self.balance = self.balance + amount
| |
| end,
| |
| withdraw = function(self, amount) -- Method, set it up like one
| |
| -- Test the validity of value passed to parameter 'amount'
| |
| amount = (type(amount) == "number" and amount) or (type(amount) == "string" and tostring(amount)) or error("Invalid amount", 2)
| |
|
| |
| -- Is it possible to get this much money from your account?
| |
| if self.balance >= amount then
| |
| self.balance = self.balance - amount
| |
| return self.balance
| |
| else
| |
| error("Attempt to overdraw account!", 2)
| |
| end
| |
| end
| |
| }
| |
|
| |
| print(account.balance) -- Fields use periods
| |
| account:input(300) -- Methods use colons
| |
| print(account.balance)
| |
| account:withdraw(175)
| |
| print(account.balance)
| |
| |output=
| |
| 0
| |
| 300
| |
| 125
| |
| }}
| |
|
| |
|
| That worked! We input 300 and then withdrew 175. But there's a problem. What if we want to add a second account? We might do something like this:
| | Stuff. |
| {{Code and output|fit=output|code=
| | More stuff. |
| account:input(150)
| |
| print(account.balance)
| |
| |output=
| |
| 275}}
| |
| What happened there? {{`|account}} '''still refers to the other account'''. So, how do we solve this problem? Using '''constructors'''
| |
|
| |
|
| ==Constructors== | | ==Constructors== |
| Constructors are functions that are called when you ''create'' a new object. Using the previous example, a constructor might look like this:
| |
| <code lua>
| |
| accou
| |
|
| |
| ----
| |
| This definition creates a new function and stores it in field withdraw of the Account object. Then, we can call it as <br>
| |
|
| |
| <code lua>
| |
| Account.withdraw(100.00)
| |
| print(Account.balance)
| |
| </code>
| |
|
| |
| This kind of function is almost what we call a method. However, the use of the global name Account inside the function is a bad programming practice. First, this function will work only for this particular object. Second, even for this particular object the function will work only as long as the object is stored in that particular global variable; if we change the name of this object, withdraw does not work any more:
| |
|
| |
| <code lua>
| |
| a = Account; Account = nil
| |
| a.withdraw(100.00) -- ERROR!
| |
| </code>
| |
|
| |
| Such behavior violates the previous principle that objects have independent life cycles.
| |
| A more flexible approach is to operate on the receiver of the operation. For that, we would have to define our method with an extra parameter, which tells the method on which object it has to operate. This parameter usually has the name self or this:
| |
|
| |
| <code lua>
| |
| function Account.withdraw(self, v)
| |
| self.balance = self.balance - v
| |
| end
| |
| </code>
| |
|
| |
| Now, when we call the method we have to specify on which object it has to operate:
| |
|
| |
| <code lua>
| |
| a1 = Account; Account = nil
| |
| ...
| |
| a1.withdraw(a1, 100.00) -- OK
| |
| </code>
| |
|
| |
| With the use of a self parameter, we can use the same method for many objects:
| |
|
| |
| <code lua>
| |
| a2 = {balance=0, withdraw = Account.withdraw}
| |
| ...
| |
| a2.withdraw(a2, 260.00)
| |
| </code>
| |
|
| |
| This use of a self parameter is a central point in any object-oriented language. Most OO languages have this mechanism partly hidden from the programmer, so that she does not have to declare this parameter (although she still can use the name self or this inside a method). Lua can also hide this parameter, using the colon operator. We can rewrite the previous method definition as
| |
|
| |
| <code lua>
| |
| function Account:withdraw (v)
| |
| self.balance = self.balance - v
| |
| end
| |
| </code>
| |
|
| |
| and the method call as
| |
|
| |
| <code lua>
| |
| Account:withdraw(100.00)
| |
| </code>
| |
|
| |
| The effect of the colon is to add an extra hidden parameter in a method definition and to add an extra argument in a method call. The colon is only a syntactic facility, although a convenient one; there is nothing really new here. We can define a function with the dot syntax and call it with the colon syntax, or vice-versa, as long as we handle the extra parameter correctly:
| |
|
| |
| <code lua>
| |
| Account = {
| |
| balance=0;
| |
| withdraw = function (self, v)
| |
| self.balance = self.balance - v
| |
| end
| |
| }
| |
|
| |
| function Account:deposit (v)
| |
| self.balance = self.balance + v
| |
| end
| |
|
| |
| Account.deposit(Account, 200.00)
| |
| Account:withdraw(100.00)
| |
|
| |
| print(Account.balance)
| |
| </code>
| |
|
| |
| Now our objects have an identity, a state, and operations over this state.
| |
|
| |
| == Constructors ==
| |
| So far, we've shown how to create a single once-use object. However, we might want to manage multiple accounts. Here's how we do it:
| |
|
| |
| <code lua>
| |
| Account = {}
| |
| function Account.new(balance)
| |
| return setmetatable({balance = balance}, Account})
| |
| end
| |
| Account.__index = Account
| |
| </code>
| |
|
| |
| This creates the constructor {{`|Account.new(balance)}}, which returns a new account object. The {{`|setmetatable}} makes it so the new account will look in the {{`|Account}} table for its metamethods, and the metamethod {{`|Account.__index = Account}} makes it look in the {{`|Account}} table for its methods. Let's add some methods then!
| |
| <code lua>
| |
| function Account:withdraw(v)
| |
| self.balance = self.balance - v
| |
| end
| |
| function Account:deposit(v)
| |
| self.balance = self.balance + v
| |
| end
| |
| function Account:__tostring()
| |
| return "Account(" .. self.balance .. ")"
| |
| end
| |
| </code>
| |
|
| |
|
| To use it:
| | Construction. |
| | Yeah. |
|
| |
|
| {{code and output|code=
| | ==Inheritance== |
| local a = Account.new(100)
| |
| local b = Account.new(200)
| |
| print(a, b)
| |
|
| |
|
| a:withdraw(20)
| | Stuff about inheritance. |
| b:deposit(40)
| |
| print(a, b)
| |
| |output=
| |
| Account(100) Account(200)
| |
| Account(80) Account(240)
| |
| }}
| |
|
| |
|
| == See also == | | == See also == |
