1
00:00:00,580 --> 00:00:01,420
Welcome.

2
00:00:01,420 --> 00:00:05,890
In this lecture, we're going to take a closer look at meta tables and meta methods to make sure you

3
00:00:05,890 --> 00:00:07,120
understand everything.

4
00:00:07,120 --> 00:00:09,040
So let's just jump right into it.

5
00:00:09,040 --> 00:00:11,530
I'm first going to create a class for us.

6
00:00:11,530 --> 00:00:13,660
We're just going to call it bank account.

7
00:00:13,660 --> 00:00:16,330
This is going to be our bank account class.

8
00:00:16,780 --> 00:00:18,340
And what is the bank account need.

9
00:00:18,340 --> 00:00:20,230
Well a bank account needs an owner right.

10
00:00:20,230 --> 00:00:21,760
Who owns the bank account?

11
00:00:21,940 --> 00:00:23,650
The bank account needs a value.

12
00:00:23,650 --> 00:00:25,210
How much money is stored in it.

13
00:00:25,360 --> 00:00:28,030
And then we'll also need a Pin for the bank account.

14
00:00:28,900 --> 00:00:34,300
This table is going to act as our class and our meta table for any future bank account objects.

15
00:00:34,510 --> 00:00:37,150
Now, what are some functions that our bank account needs?

16
00:00:37,180 --> 00:00:42,460
Well, to create a new bank account of course we're going to need a new constructor.

17
00:00:43,510 --> 00:00:48,790
So if we ever want to create a new bank account, we'll have to pass an owner to this function.

18
00:00:48,790 --> 00:00:53,530
However much money we want to start by putting into our bank account, and then the pin to set for our

19
00:00:53,530 --> 00:00:54,400
bank account.

20
00:00:54,580 --> 00:01:01,600
We also need some functions that the bank account can use, such as depositing into our bank account.

21
00:01:04,440 --> 00:01:07,770
It may be a function to withdraw from our bank account.

22
00:01:12,560 --> 00:01:17,930
And with these functions, we can pass the amount we want to deposit as well as the pin required.

23
00:01:18,500 --> 00:01:21,350
And then we can do the same thing for withdrawing as well.

24
00:01:22,330 --> 00:01:27,580
So inside of our constructor, what we could do is we could create a new variable called self, and

25
00:01:27,580 --> 00:01:29,890
it'll be equal to the set Metatable function.

26
00:01:29,890 --> 00:01:35,650
We'll create a new table and attach the bank account as the meta table.

27
00:01:35,680 --> 00:01:41,770
Now in order for us to access the values that we put in here originally, these properties here, as

28
00:01:41,770 --> 00:01:46,420
well as the functions that are a part of the bank account, we have to set a meta method inside of the

29
00:01:46,420 --> 00:01:48,340
bank account the index meta method.

30
00:01:48,340 --> 00:01:52,900
So bank account dot underscore underscore index is equal to the bank account.

31
00:01:52,900 --> 00:01:59,860
That way whenever I let's say index this table here it's going to check to see hey we've got a bank

32
00:01:59,890 --> 00:02:00,910
account meta method.

33
00:02:00,910 --> 00:02:04,330
And let's see if that index belongs to this meta method.

34
00:02:04,330 --> 00:02:09,580
If it does like deposit or the owner or the value or the pin, then it'll be good to go.

35
00:02:10,290 --> 00:02:13,590
So this section we can call here for our meta methods.

36
00:02:15,030 --> 00:02:21,150
Now that we have this done, we can now access things like the value, the owner and the pin for this

37
00:02:21,150 --> 00:02:25,980
new bank account so we can set the owner equal to the owner that is passed to the function.

38
00:02:25,980 --> 00:02:29,400
We can set the value equal to the value passed to this function.

39
00:02:29,490 --> 00:02:33,120
And we can set the pin to the pin that is passed to this function.

40
00:02:33,480 --> 00:02:37,470
Then we just need to return this new table back to where this function was called.

41
00:02:37,470 --> 00:02:40,890
Now we are able to create new bank account objects.

42
00:02:41,130 --> 00:02:44,520
So if I go down here I can create a new account.

43
00:02:44,520 --> 00:02:48,120
I'll just call this account one equal to bank account dot new.

44
00:02:48,390 --> 00:02:50,910
Let's say the owner is Bob.

45
00:02:51,060 --> 00:02:53,520
Bob wants to put $100 into his bank account.

46
00:02:53,520 --> 00:02:55,950
And his pin will be one, two, three, four.

47
00:02:56,760 --> 00:03:01,950
So now we have a new bank account object and we can do another bank account object.

48
00:03:01,980 --> 00:03:03,840
We'll call this account two.

49
00:03:04,390 --> 00:03:10,630
We'll have a bank account new, and we'll have Jim be the owner of this bank account.

50
00:03:11,020 --> 00:03:14,920
He puts in $500 and he has a pin of four, three, two, one.

51
00:03:15,920 --> 00:03:18,980
Now, let's say I wanted to add these bank accounts together.

52
00:03:18,980 --> 00:03:23,300
Let's say remove value from one bank account and put it into the other one.

53
00:03:23,420 --> 00:03:30,440
So for example, in our first bank account, maybe I want to add the value that is inside of Jim's bank

54
00:03:30,470 --> 00:03:31,820
account account two.

55
00:03:32,720 --> 00:03:38,780
How could I add the value of this bank account, which is 500, and put it in this bank account, which

56
00:03:38,780 --> 00:03:44,300
has a value of 600, meaning we would subtract 500 from this bank account, making it zero, and add

57
00:03:44,300 --> 00:03:49,700
500 to this bank account, making it 600 just by using the addition operator.

58
00:03:49,970 --> 00:03:52,910
Well, this is where the add meta method comes into play.

59
00:03:53,030 --> 00:03:55,820
What we could do for our bank account.

60
00:03:55,820 --> 00:04:01,730
We can set the underscore underscore add meta method equal to a new function.

61
00:04:02,750 --> 00:04:07,280
And this add meta method automatically passes two arguments to our function.

62
00:04:07,280 --> 00:04:13,340
One is the first object we're trying to add to, and then the second object is the other object we want

63
00:04:13,340 --> 00:04:14,780
to add to the first object.

64
00:04:14,960 --> 00:04:16,910
So we can call this adding.

65
00:04:18,050 --> 00:04:23,060
Now, the first thing that I want to make sure of is that this thing that I'm trying to add to my table

66
00:04:23,060 --> 00:04:25,850
is also another bank account object.

67
00:04:25,850 --> 00:04:31,640
But how would I be able to differentiate this table from, let's say, other tables that are in my game?

68
00:04:31,760 --> 00:04:36,050
Well, what we could do for the bank account is we could add another property.

69
00:04:36,640 --> 00:04:42,850
And we could call it underscore, underscore type and set it to some unique type for our bank account.

70
00:04:42,850 --> 00:04:46,150
So let's say this is the type of bank account.

71
00:04:46,600 --> 00:04:50,620
This isn't an actual meta method, but we can just assume it is.

72
00:04:50,650 --> 00:04:53,440
And it's going to store the bank account type for us.

73
00:04:53,950 --> 00:04:59,440
So what we could do is we need to first verify that adding is not null.

74
00:04:59,440 --> 00:05:05,470
Because let's say I tried to do this, it would error, but I want to avoid that error.

75
00:05:05,470 --> 00:05:07,630
So we're going to have to make a check for it.

76
00:05:07,780 --> 00:05:11,560
So we can say if not adding.

77
00:05:11,560 --> 00:05:14,290
So if the value of adding is false.

78
00:05:14,710 --> 00:05:18,760
Or we could check if the type of adding.

79
00:05:19,930 --> 00:05:22,330
Is not equal to the type of table.

80
00:05:23,970 --> 00:05:33,090
And what we could do is we could raise an error and we could say, attempt to add type and we'll concatenate

81
00:05:33,090 --> 00:05:35,280
it with the type of this adding.

82
00:05:36,290 --> 00:05:39,740
And we'll say attempt to add this type to type.

83
00:05:40,640 --> 00:05:46,610
And then we can do self dot underscore underscore type which will reference the type right here.

84
00:05:46,610 --> 00:05:48,980
That is part of the bank account class.

85
00:05:48,980 --> 00:05:53,240
So it'll so if this um so let's say adding was nil.

86
00:05:53,240 --> 00:05:58,670
If it was nil then this would print attempt to add type nil to type bank account.

87
00:05:59,470 --> 00:06:04,930
The next thing we can check for is whether or not the type of this table is a bank account.

88
00:06:04,930 --> 00:06:12,220
So if the adding dot underscore underscore type is not equal to bank account.

89
00:06:14,170 --> 00:06:22,030
Then we can return self and end, and we'll actually need to do that right here as well.

90
00:06:24,870 --> 00:06:30,210
Now we have to make sure we return self, because when we call this addition operator, it's attempting

91
00:06:30,240 --> 00:06:34,830
to create or store a new variable on our account, one which is going to be different from the original.

92
00:06:34,830 --> 00:06:39,960
And we need so we have to return self back to where this function was called.

93
00:06:39,960 --> 00:06:45,390
Otherwise it's not going to work and we won't be able to add any amount to our table.

94
00:06:45,390 --> 00:06:52,440
But once we verify that, what we're trying to add to this table is a table and it is a bank account.

95
00:06:52,620 --> 00:06:54,270
What we could do now.

96
00:06:55,520 --> 00:07:01,130
Is we could use the deposit and withdrawal functions to our advantage.

97
00:07:01,130 --> 00:07:09,980
So for the adding bank account, what we could do is we could withdraw, um, the amount that is inside

98
00:07:09,980 --> 00:07:10,910
of this table.

99
00:07:10,910 --> 00:07:12,590
So adding dot value.

100
00:07:13,100 --> 00:07:17,750
And we'll also make sure to pass the pin to verify, you know, this is the bank account's owner or

101
00:07:17,750 --> 00:07:18,380
whatever.

102
00:07:18,950 --> 00:07:26,030
We could withdraw it from that, and then we could deposit it into the original table using self deposit.

103
00:07:27,250 --> 00:07:31,960
But then the issue here is, how are we going to know the exact amount of adding value?

104
00:07:31,960 --> 00:07:35,020
Because when we withdraw it, it's going to subtract from it.

105
00:07:35,020 --> 00:07:39,790
And we need to set that amount in the deposit well inside of these functions.

106
00:07:40,760 --> 00:07:48,200
What we could do is we could first check if the pin passed to this function is not equal to the pin

107
00:07:48,200 --> 00:07:53,360
of the bank account, then we're just going to return false, meaning the deposit failed.

108
00:07:53,750 --> 00:07:59,570
And then I'm going to return a number which signifies how much money was deposited or withdrawn.

109
00:07:59,570 --> 00:08:01,940
In this case it would be zero because we failed.

110
00:08:02,570 --> 00:08:08,090
We could also check if the amount that they're attempting to deposit into the account is negative for

111
00:08:08,090 --> 00:08:08,750
some reason.

112
00:08:08,750 --> 00:08:12,980
So if the amount is less than or let's say it's equal to zero.

113
00:08:12,980 --> 00:08:19,490
So if it's zero or less, then we're also going to return false and zero and end.

114
00:08:20,450 --> 00:08:27,050
Otherwise if all checks out then we can set self dot value plus equal the amount.

115
00:08:27,600 --> 00:08:29,190
And then we can return true.

116
00:08:29,220 --> 00:08:35,340
We successfully added this amount and returned the amount we added or deposited into the bank account.

117
00:08:36,010 --> 00:08:40,360
And again, remember we have access to self here is because we're using a cone.

118
00:08:40,360 --> 00:08:45,670
So whenever we call this function directly on a bank account table, the table automatically gets passed

119
00:08:45,670 --> 00:08:47,290
to this function as self.

120
00:08:48,140 --> 00:08:52,580
And then we can basically just copy this and do the same thing here.

121
00:08:52,970 --> 00:08:54,920
But instead what we're going to do.

122
00:08:55,580 --> 00:09:02,330
Is we're going to make a variable, I'll call it amount can withdraw and it's going to be equal to math

123
00:09:02,330 --> 00:09:03,350
dot min.

124
00:09:03,650 --> 00:09:06,920
And we'll pass self dot value and the amount.

125
00:09:07,160 --> 00:09:09,320
So let's say for some reason.

126
00:09:10,300 --> 00:09:14,530
The amount we were trying to withdraw is greater than the amount of money that we already have in the

127
00:09:14,530 --> 00:09:15,190
bank account.

128
00:09:15,190 --> 00:09:19,270
So let's say this was 50 and we only had $10 in our bank account.

129
00:09:19,270 --> 00:09:24,790
Well, this function is going to return to us the smallest number out of the list of numbers we give

130
00:09:24,790 --> 00:09:29,470
to it, which in this case would be $10, because that's all we can withdraw.

131
00:09:29,470 --> 00:09:32,050
And we want to be able to withdraw $50.

132
00:09:32,320 --> 00:09:39,940
So once we get that correct number then we can set self dot value minus equal amount can withdraw and

133
00:09:39,940 --> 00:09:44,320
we return true and we return the amount that we withdrew.

134
00:09:45,750 --> 00:09:46,260
Great.

135
00:09:46,260 --> 00:09:48,360
So now we can go back here.

136
00:09:48,360 --> 00:09:52,830
And what we could do is we could store the result from this function into two variables.

137
00:09:53,830 --> 00:09:58,720
Um, I'm not going to need the first one, but I will need the second one, which will be the amount

138
00:09:58,720 --> 00:10:00,880
that we withdrew out of this account.

139
00:10:01,180 --> 00:10:06,310
And then we can deposit this amount into the original table.

140
00:10:07,180 --> 00:10:10,120
And pass the pen for this table as well.

141
00:10:10,690 --> 00:10:16,270
And then last but not least, we have to make sure to return self at the very end.

142
00:10:17,170 --> 00:10:24,610
So what we've done now is that whenever we attempt to add two bank accounts together, just like this.

143
00:10:25,420 --> 00:10:27,970
It's going to call this ad meta method.

144
00:10:27,970 --> 00:10:32,680
It's going to pass the first table, which is account one, and pass the second table which is account

145
00:10:32,680 --> 00:10:33,190
two.

146
00:10:33,220 --> 00:10:39,160
It's going to make sure that the second table is a table and that it's a type of bank account.

147
00:10:39,160 --> 00:10:45,190
When it is then we're going to withdraw however much money we can out of the second bank account.

148
00:10:45,790 --> 00:10:50,500
And we're going to take that amount and deposit within the original bank account.

149
00:10:50,890 --> 00:10:52,180
That's all we're doing here.

150
00:10:52,180 --> 00:10:57,730
So that means after I add the amount of money that is in account two, to account one, this should

151
00:10:57,730 --> 00:11:01,420
be $600 and our account two should be $0.

152
00:11:01,420 --> 00:11:02,320
So let's find out.

153
00:11:02,320 --> 00:11:05,620
Let's print account one dot value.

154
00:11:05,680 --> 00:11:09,310
And then we can also print account two dot value.

155
00:11:09,310 --> 00:11:14,110
And then we can go and run our game and see what gets printed in the console.

156
00:11:15,240 --> 00:11:15,960
And there we go.

157
00:11:15,960 --> 00:11:22,260
Just like that, we got $600 in our first bank account, and now we have zero in the second bank account.

158
00:11:23,740 --> 00:11:24,730
Pretty neat.

159
00:11:24,730 --> 00:11:29,680
Well, let's say I wanted to subtract the bank accounts.

160
00:11:29,680 --> 00:11:34,810
So let's say I wanted to subtract money, however much I could out of this bank account and put it into

161
00:11:34,810 --> 00:11:36,040
the second bank account.

162
00:11:37,140 --> 00:11:40,140
Well, we have another meta method for that.

163
00:11:40,140 --> 00:11:42,180
And that meta method is called.

164
00:11:42,840 --> 00:11:46,650
Underscore underscore sub short for subtraction.

165
00:11:47,190 --> 00:11:49,230
And again we'll need a function for this.

166
00:11:49,230 --> 00:11:53,040
And it passes the first table we want to subtract from.

167
00:11:53,490 --> 00:11:56,430
And then it also passes us the second table.

168
00:11:56,430 --> 00:11:59,970
We'll call it removing which we want to take out of this.

169
00:11:59,970 --> 00:12:02,910
And I guess we could put it into this bank account.

170
00:12:03,630 --> 00:12:07,470
So what we could do is we could copy the same thing here.

171
00:12:07,530 --> 00:12:08,190
Right.

172
00:12:08,190 --> 00:12:15,270
We want to make sure that the second thing we're trying to subtract from the original table is a table,

173
00:12:15,270 --> 00:12:16,590
and that it's a bank account.

174
00:12:16,590 --> 00:12:20,010
So I'm going to replace these references.

175
00:12:20,400 --> 00:12:27,630
And then what I need to do is I need to call the withdraw function on self.

176
00:12:28,680 --> 00:12:32,880
And I need to withdraw however much money I can.

177
00:12:32,880 --> 00:12:43,110
In this case, I want to withdraw the removing dot value and then we'll make sure to pass self dot pin.

178
00:12:43,260 --> 00:12:48,210
Now we need to store whether or not this withdraw was successful or not.

179
00:12:48,210 --> 00:12:49,950
So success.

180
00:12:49,950 --> 00:12:51,330
And then the amount.

181
00:12:52,190 --> 00:12:54,770
Because let's say the second table had $500.

182
00:12:54,770 --> 00:12:58,730
I wanted to subtract from a table that only had $100.

183
00:12:58,730 --> 00:13:02,120
That means I withdraw would fail, right?

184
00:13:02,210 --> 00:13:04,430
In the case that our withdrawal fails.

185
00:13:04,430 --> 00:13:06,500
So if not successful.

186
00:13:07,300 --> 00:13:14,770
What we could do is the same thing set success amount equal to self withdraw and instead we'll just

187
00:13:14,770 --> 00:13:18,100
withdraw whatever money we have within this bank account.

188
00:13:18,100 --> 00:13:20,590
So dot value.

189
00:13:21,010 --> 00:13:26,860
So we are withdrawing all the money that we can out of self make sure to pass the pin.

190
00:13:28,910 --> 00:13:37,070
And once that's done, then we can go ahead and deposit that money into the second table or the removing

191
00:13:37,100 --> 00:13:37,610
table.

192
00:13:37,610 --> 00:13:38,900
So deposit.

193
00:13:40,270 --> 00:13:43,690
This amount and pass the pin as well.

194
00:13:44,050 --> 00:13:51,220
Otherwise, if we were successful in withdrawing the money, then we can just do the exact same thing

195
00:13:51,220 --> 00:13:52,060
here as well.

196
00:13:52,510 --> 00:13:57,430
And actually, there's a better way to format our code where we don't have to have these two same lines

197
00:13:57,430 --> 00:13:58,870
doing the exact same thing.

198
00:13:58,870 --> 00:14:02,410
What we could do is we could just have one of those lines.

199
00:14:02,410 --> 00:14:08,260
So this if statement will just check if we weren't successful, if we were not successful, then it

200
00:14:08,260 --> 00:14:09,700
updates these variables.

201
00:14:09,700 --> 00:14:15,460
And then once the variable gets updated, then we deposit the correct amount into that second table.

202
00:14:15,460 --> 00:14:22,240
So that means because I changed this to subtraction, what's going to happen is it's going to take the

203
00:14:22,240 --> 00:14:26,800
$100 from this account and put it into our second account.

204
00:14:26,800 --> 00:14:31,330
So the first account should be zero and the second account should be 600.

205
00:14:31,330 --> 00:14:32,260
It should flip.

206
00:14:32,260 --> 00:14:33,760
So let's run and find out.

207
00:14:35,300 --> 00:14:38,540
And we get an error attempt to index nil with value.

208
00:14:39,250 --> 00:14:41,830
Oh, and the reason it did that?

209
00:14:41,830 --> 00:14:42,400
Ha ha!

210
00:14:42,400 --> 00:14:48,670
We have to make sure to return self at the end here, because if we don't return self, it's not going

211
00:14:48,670 --> 00:14:53,230
to know what to update account one with and it's just going to set it to nil.

212
00:14:53,230 --> 00:14:55,600
So this is why we have to return self here.

213
00:14:55,600 --> 00:14:57,580
Otherwise it's going to end up as nil.

214
00:14:58,900 --> 00:15:00,070
So we got that fixed.

215
00:15:00,070 --> 00:15:02,650
We're going to save that and let's run it again.

216
00:15:02,920 --> 00:15:05,920
Should be there we go zero 600.

217
00:15:06,010 --> 00:15:11,860
So we've subtracted however much money we could out of the first account and placed it into the second

218
00:15:11,860 --> 00:15:12,490
account.

219
00:15:13,030 --> 00:15:20,440
Now, if I updated the money in the second account to only be $50, then what should happen is that

220
00:15:20,440 --> 00:15:25,030
it should subtract $50 from the first account and make this account $100.

221
00:15:25,030 --> 00:15:26,200
So let's run it.

222
00:15:26,830 --> 00:15:31,420
And there we go, $50 in the first account, $100 in the second account.

223
00:15:32,690 --> 00:15:38,600
So as you can see, this is how we can use tables and meta tables to imitate classes in Lua.

224
00:15:38,840 --> 00:15:44,330
We can also take all this functionality and store it in a different script, which would be our module

225
00:15:44,330 --> 00:15:44,780
script.

226
00:15:44,780 --> 00:15:46,820
So let's actually do that real quick.

227
00:15:46,850 --> 00:15:52,400
I'm going to go into server storage, and I actually already have a folder in here called classes.

228
00:15:52,400 --> 00:15:55,790
And what I'm going to do is I'm going to create a new module script in here.

229
00:15:56,030 --> 00:15:59,990
And this is going to be my bank account class.

230
00:16:01,530 --> 00:16:07,530
And because a module script returns a table, we can literally just use the table that we declared right

231
00:16:07,530 --> 00:16:11,310
here as the table to return in this module script.

232
00:16:11,580 --> 00:16:14,880
So actually let's go and copy all of this functionality.

233
00:16:15,480 --> 00:16:17,070
And we can delete this.

234
00:16:17,070 --> 00:16:22,110
And at the very end of our module script we can just return the bank account table.

235
00:16:22,650 --> 00:16:31,380
So now whenever we require this module script from a script in our server, we can get this bank account

236
00:16:31,380 --> 00:16:32,010
class.

237
00:16:32,010 --> 00:16:37,440
And then we can use the bank account new function to create new bank account objects.

238
00:16:37,440 --> 00:16:41,160
And we can use the same deposit and withdrawal functions.

239
00:16:41,160 --> 00:16:47,490
And we can use the same subtraction and addition meta methods that are inside of our bank account class.

240
00:16:47,880 --> 00:16:54,210
So this is kind of the core essential idea of meta methods and meta tables as classes in Lua.

241
00:16:55,200 --> 00:17:00,060
In the next lectures, we're going to go ahead and move forward into some other territory in object

242
00:17:00,060 --> 00:17:07,290
oriented programming such as inheritance, polymorphism, abstraction, and a bunch of other neat stuff.

243
00:17:07,290 --> 00:17:08,880
So I hope to see you there.