View and Pure Functions.srt

1
00:00:00,410 --> 00:00:02,580
functions can be declared to have

2
00:00:02,580 --> 00:00:05,609
certain visibility like public private

3
00:00:05,609 --> 00:00:09,719
internal and external functions can also

4
00:00:09,719 --> 00:00:11,759
be declared to have certain behavior

5
00:00:11,759 --> 00:00:16,320
like boo peer and payable in this video

6
00:00:16,320 --> 00:00:18,900
we're gonna go over what beyou and peer

7
00:00:18,900 --> 00:00:22,859
functions are functions declared with

8
00:00:22,859 --> 00:00:24,990
the keyword view promised not to modify

9
00:00:24,990 --> 00:00:28,230
the state of the blockchain according to

10
00:00:28,230 --> 00:00:30,689
the solidity documentation the following

11
00:00:30,689 --> 00:00:32,549
are considered to modify the state of

12
00:00:32,549 --> 00:00:34,800
the blockchain writing to state

13
00:00:34,800 --> 00:00:39,090
variables emitting events also known as

14
00:00:39,090 --> 00:00:41,160
logging this is a topic for another

15
00:00:41,160 --> 00:00:44,450
video creating other contracts using

16
00:00:44,450 --> 00:00:47,070
self-destruct this is a function to call

17
00:00:47,070 --> 00:00:49,140
when you want to delete your contract

18
00:00:49,140 --> 00:00:51,390
from the blockchain sending either by

19
00:00:51,390 --> 00:00:53,489
calls in other words calling the

20
00:00:53,489 --> 00:00:55,350
function to send either to another

21
00:00:55,350 --> 00:00:58,140
address calling any function not marked

22
00:00:58,140 --> 00:01:00,780
as view or peer using low-level calls

23
00:01:00,780 --> 00:01:03,750
and using inline assembly that contains

24
00:01:03,750 --> 00:01:05,970
certain op codes so if a function is

25
00:01:05,970 --> 00:01:08,430
marked as Bude then that function cannot

26
00:01:08,430 --> 00:01:10,260
contain any code that does any of

27
00:01:10,260 --> 00:01:13,020
actions mentioned here let's go over

28
00:01:13,020 --> 00:01:15,780
some examples in remix first we will

29
00:01:15,780 --> 00:01:17,820
walk through an example of a ballad view

30
00:01:17,820 --> 00:01:20,130
function next we're going to try

31
00:01:20,130 --> 00:01:22,320
compiling the function declared as view

32
00:01:22,320 --> 00:01:24,479
but inside the function we're going to

33
00:01:24,479 --> 00:01:27,360
modify the state variable and we're also

34
00:01:27,360 --> 00:01:29,729
going to try to compile function again

35
00:01:29,729 --> 00:01:31,770
declare this view but inside the

36
00:01:31,770 --> 00:01:33,210
function we're going to call another

37
00:01:33,210 --> 00:01:35,700
function which is neither Beauty nor

38
00:01:35,700 --> 00:01:38,759
peer first let's see an example of AB

39
00:01:38,759 --> 00:01:41,009
you function that does keep the promise

40
00:01:41,009 --> 00:01:43,409
not to modify the state here we have a

41
00:01:43,409 --> 00:01:45,930
state variable named X initialized to

42
00:01:45,930 --> 00:01:48,450
the value 1 we also have a function

43
00:01:48,450 --> 00:01:51,810
named add to X add to X takes in an

44
00:01:51,810 --> 00:01:54,479
integer variable named Y as input and

45
00:01:54,479 --> 00:01:57,990
returns a sum of X plus y where X is the

46
00:01:57,990 --> 00:02:00,420
state variable this is a ballot view

47
00:02:00,420 --> 00:02:02,340
function since it reads the state

48
00:02:02,340 --> 00:02:05,820
variable X but does not update it we can

49
00:02:05,820 --> 00:02:07,740
verify that calling this function does

50
00:02:07,740 --> 00:02:10,739
not update the state variable X compile

51
00:02:10,739 --> 00:02:13,940
and deploy the contract

52
00:02:13,940 --> 00:02:16,820
next let's check the value of X it

53
00:02:16,820 --> 00:02:18,740
should be one which is what we

54
00:02:18,740 --> 00:02:20,960
initialized it to we'll call this

55
00:02:20,960 --> 00:02:23,450
function later again and verify that the

56
00:02:23,450 --> 00:02:25,940
state variable was not modified next

57
00:02:25,940 --> 00:02:28,970
call add to X with some input you'll get

58
00:02:28,970 --> 00:02:31,970
the sum of X plus your input at the

59
00:02:31,970 --> 00:02:34,130
bottom of the remix console you'll

60
00:02:34,130 --> 00:02:36,020
notice that calling this function did

61
00:02:36,020 --> 00:02:38,810
not create any transactions as long as

62
00:02:38,810 --> 00:02:40,370
the function does not create any

63
00:02:40,370 --> 00:02:43,250
transactions there is no way to change

64
00:02:43,250 --> 00:02:45,710
the state of the blockchain so calling

65
00:02:45,710 --> 00:02:48,710
add to X did not modify any state to be

66
00:02:48,710 --> 00:02:51,530
100% sure let's double check on the

67
00:02:51,530 --> 00:02:53,810
state variable X and make sure that it

68
00:02:53,810 --> 00:02:58,070
hasn't changed it's still 1 what would

69
00:02:58,070 --> 00:03:00,230
happen if we declare a function as view

70
00:03:00,230 --> 00:03:02,780
but inside the function modify the state

71
00:03:02,780 --> 00:03:05,630
variable let's give it a try create a

72
00:03:05,630 --> 00:03:08,360
function called update X and declare as

73
00:03:08,360 --> 00:03:11,120
view we're going to violate the promise

74
00:03:11,120 --> 00:03:13,640
not to modify the state inside the

75
00:03:13,640 --> 00:03:16,250
function update the state variable X by

76
00:03:16,250 --> 00:03:19,970
incrementing it and hit compile you'll

77
00:03:19,970 --> 00:03:22,520
see a compilation error like this this

78
00:03:22,520 --> 00:03:24,320
is a really good feature of solidity

79
00:03:24,320 --> 00:03:26,660
that it can detect potential state

80
00:03:26,660 --> 00:03:29,660
changes a function cannot be AB u

81
00:03:29,660 --> 00:03:31,490
function if it calls another function

82
00:03:31,490 --> 00:03:34,730
which is neither bill nor peer here we

83
00:03:34,730 --> 00:03:37,250
have a function called foo and it is not

84
00:03:37,250 --> 00:03:39,530
declared as view we also have a function

85
00:03:39,530 --> 00:03:41,780
called invalid view func which is

86
00:03:41,780 --> 00:03:44,300
declared as view inside the function

87
00:03:44,300 --> 00:03:46,760
invalid be func it's going to call

88
00:03:46,760 --> 00:03:49,010
another function foo which is neither

89
00:03:49,010 --> 00:03:52,010
Beauty nor peer if you try to compile

90
00:03:52,010 --> 00:03:53,810
this code you'll get a error like this

91
00:03:53,810 --> 00:03:56,150
stating that invalid view func

92
00:03:56,150 --> 00:03:58,459
contains code that potentially modifies

93
00:03:58,459 --> 00:04:00,790
the state

94
00:04:00,790 --> 00:04:03,610
view functions declare not to modify the

95
00:04:03,610 --> 00:04:05,890
state peer functions make a stronger

96
00:04:05,890 --> 00:04:08,349
promise peer functions promise not to

97
00:04:08,349 --> 00:04:11,530
modify state like B functions in

98
00:04:11,530 --> 00:04:13,870
addition they also promise not to read

99
00:04:13,870 --> 00:04:15,930
the state according to the solidity

100
00:04:15,930 --> 00:04:18,040
documentation the following are

101
00:04:18,040 --> 00:04:19,720
considered reading from the state

102
00:04:19,720 --> 00:04:22,570
reading from state variables accessing

103
00:04:22,570 --> 00:04:25,030
the ether balance urban address reading

104
00:04:25,030 --> 00:04:28,000
from special variables like blog TX and

105
00:04:28,000 --> 00:04:30,760
message calling any function not marked

106
00:04:30,760 --> 00:04:33,280
peer and using inline assembly

107
00:04:33,280 --> 00:04:36,160
containing certain op codes so a peer

108
00:04:36,160 --> 00:04:38,560
function cannot contain any code that

109
00:04:38,560 --> 00:04:40,780
does any of the actions mentioned here

110
00:04:40,780 --> 00:04:44,260
and it can also modify the state like a

111
00:04:44,260 --> 00:04:47,139
b function let's see some examples of

112
00:04:47,139 --> 00:04:50,139
peer function in remix first we'll go

113
00:04:50,139 --> 00:04:52,240
over an example of a ballot peer

114
00:04:52,240 --> 00:04:55,210
function next we'll see some examples of

115
00:04:55,210 --> 00:04:58,300
invalid peer functions here is an

116
00:04:58,300 --> 00:05:00,910
example of a ballot peer function here

117
00:05:00,910 --> 00:05:03,430
we have a function named ad ad is a

118
00:05:03,430 --> 00:05:06,070
function that takes two inputs I and J

119
00:05:06,070 --> 00:05:09,310
and returns I plus J this is a ballot

120
00:05:09,310 --> 00:05:11,410
peer function since it doesn't read any

121
00:05:11,410 --> 00:05:14,650
state and modify any state next let's

122
00:05:14,650 --> 00:05:16,599
see some examples of invalid peer

123
00:05:16,599 --> 00:05:19,539
functions revisit the add two function

124
00:05:19,539 --> 00:05:22,150
back at the top of the contract this is

125
00:05:22,150 --> 00:05:25,030
a ballad view function however since it

126
00:05:25,030 --> 00:05:27,190
reads from the state variable this

127
00:05:27,190 --> 00:05:30,010
cannot be a peer function let's declare

128
00:05:30,010 --> 00:05:33,700
as peer anyways and hit compile you'll

129
00:05:33,700 --> 00:05:35,830
see a similar error to the ones we got

130
00:05:35,830 --> 00:05:38,110
when we try to compile invalid view

131
00:05:38,110 --> 00:05:39,740
function

132
00:05:39,740 --> 00:05:42,470
similar to how AB you function can only

133
00:05:42,470 --> 00:05:45,530
call other view or peer functions appear

134
00:05:45,530 --> 00:05:47,750
functions can only call other peer

135
00:05:47,750 --> 00:05:50,900
function so a function declared as peer

136
00:05:50,900 --> 00:05:53,840
but if it calls a non peer fortune then

137
00:05:53,840 --> 00:05:56,330
it is invalid code let's see an example

138
00:05:56,330 --> 00:05:59,360
here we have a function foo which is

139
00:05:59,360 --> 00:06:02,300
neither Beauty nor peer and we also have

140
00:06:02,300 --> 00:06:05,150
a function invalid peer func declared as

141
00:06:05,150 --> 00:06:08,030
peer inside this function foo is called

142
00:06:08,030 --> 00:06:10,820
since foo is neither Beauty nor peer

143
00:06:10,820 --> 00:06:13,730
this contract will not compile the code

144
00:06:13,730 --> 00:06:16,160
becomes valid when you declare foo as a

145
00:06:16,160 --> 00:06:19,010
peer function so we've just seen that a

146
00:06:19,010 --> 00:06:21,110
peer function can only call other peer

147
00:06:21,110 --> 00:06:24,740
functions it's easy to forget to declare

148
00:06:24,740 --> 00:06:27,680
a function as B or peer the good news is

149
00:06:27,680 --> 00:06:29,870
that the solidity compiler is smart

150
00:06:29,870 --> 00:06:32,120
enough to warn you that you might have

151
00:06:32,120 --> 00:06:34,010
forgotten to put these keywords on your

152
00:06:34,010 --> 00:06:37,070
functions when you compile your contract

153
00:06:37,070 --> 00:06:39,740
and remix you'll get warning stating

154
00:06:39,740 --> 00:06:42,410
that functions can be declared as view

155
00:06:42,410 --> 00:06:45,110
or peer so don't forget to carefully

156
00:06:45,110 --> 00:06:47,210
check the warnings and fix your code if

157
00:06:47,210 --> 00:06:49,600
necessary

158
00:06:49,600 --> 00:06:51,940
in this video we cover beer and peer

159
00:06:51,940 --> 00:06:54,460
functions beautiful chains promise not

160
00:06:54,460 --> 00:06:57,010
to modify the state peer functions make

161
00:06:57,010 --> 00:06:59,020
a stronger promised and view functions

162
00:06:59,020 --> 00:07:01,450
they promise neither to modify the state

163
00:07:01,450 --> 00:07:04,450
nor read the state thank you so much for

164
00:07:04,450 --> 00:07:08,310
watching and I'll see you next time