1
00:00:05,300 --> 00:00:08,290
In this video, we'll talk more about the default constructor.

2
00:00:08,950 --> 00:00:13,150
A default constructor is a constructor that doesn't expect any arguments.

3
00:00:13,150 --> 00:00:15,510
It's also called the no args constructor.

4
00:00:16,309 --> 00:00:20,110
C++ has to have a way to construct and initialize objects

5
00:00:20,110 --> 00:00:22,910
if you don't provide any initialization information at all.

6
00:00:23,570 --> 00:00:26,230
So if you don't provide any constructors at all,

7
00:00:26,230 --> 00:00:29,830
c++ will generate a default constructor automatically.

8
00:00:30,330 --> 00:00:34,930
This default constructor does nothing, but it's still generated by c++.

9
00:00:35,730 --> 00:00:38,730
When you create objects with no initialization information,

10
00:00:38,730 --> 00:00:42,930
this is the constructor that's called. Let's take a look at the account class again.

11
00:00:44,290 --> 00:00:47,590
In this account class, we've defined no constructors at all.

12
00:00:47,890 --> 00:00:51,790
So c++ will automatically generate a default constructor

13
00:00:51,790 --> 00:00:55,990
that allows us to create objects with no initialization information.

14
00:00:55,990 --> 00:00:57,590
Let's see a few examples.

15
00:00:58,890 --> 00:01:02,250
Here we're creating two local objects Frank account and Jim account.

16
00:01:02,850 --> 00:01:05,650
The compiler provided default constructor will be used since we didn't provide

17
00:01:05,650 --> 00:01:08,010
since we didn't provide any constructors at all.

18
00:01:08,810 --> 00:01:10,910
In the case of a pointer to an object,

19
00:01:10,910 --> 00:01:14,210
the same thing happens except that the object is created on the heap.

20
00:01:15,010 --> 00:01:18,310
Since the compiler generated default constructor does nothing,

21
00:01:18,310 --> 00:01:23,210
the class member attributes of the account class could contain garbage since they haven't been initialized.

22
00:01:24,310 --> 00:01:27,990
Of course, we're free to provide our own user-defined no args constructor.

23
00:01:27,990 --> 00:01:30,210
In fact it's best practice to do so.

24
00:01:30,870 --> 00:01:35,070
In this case, we're defining our own user-defined no argus constructor.

25
00:01:35,070 --> 00:01:39,170
This constructor will be called when no initialization information is provided.

26
00:01:39,170 --> 00:01:43,630
And in this constructor, we set the account name to none and the account balance to 0.

27
00:01:43,630 --> 00:01:48,530
So we're initializing the account member attributes to values we know about, not garbage data.

28
00:01:49,230 --> 00:01:52,890
Now let's see what happens when we define a constructor that expects arguments.

29
00:01:55,190 --> 00:01:59,550
In this case, we're defining a constructor that expects a string and a double,

30
00:01:59,550 --> 00:02:03,750
and we initialize our member attributes to the values passed into the constructor.

31
00:02:03,750 --> 00:02:05,750
This is very commonly done.

32
00:02:05,750 --> 00:02:09,250
However, once we define a constructor for our class

33
00:02:09,250 --> 00:02:14,240
c++ will now not generate the no args default constructor automatically.

34
00:02:14,600 --> 00:02:17,960
If we still need it, then we must explicitly define it ourselves.

35
00:02:18,840 --> 00:02:22,740
If you have code that creates objects with no initialization information,

36
00:02:22,740 --> 00:02:26,100
then that code will no longer compile since the default constructor

37
00:02:26,100 --> 00:02:28,390
is no longer generated by c++.

38
00:02:29,290 --> 00:02:33,540
So in this example, we can no longer create account objects with no arguments,

39
00:02:33,540 --> 00:02:38,040
again, since the default constructor is no longer automatically generated by the compiler.

40
00:02:38,700 --> 00:02:42,900
So we'll get three compiler errors when we try to create Frank, Jim and Mary's account.

41
00:02:43,300 --> 00:02:45,660
However, we can still create account objects,

42
00:02:45,660 --> 00:02:49,460
but we have to use the constructor we provided that expects a string and a double.

43
00:02:49,820 --> 00:02:53,610
So we can create Bill's account since it provides the initialization information

44
00:02:53,610 --> 00:02:55,210
required by the constructor.

45
00:02:55,810 --> 00:02:58,210
Let's go through some of these examples in the IDE.

46
00:02:58,610 --> 00:03:02,210
Okay. So I'm in the IDE. I'm in the section 13 workspace

47
00:03:02,210 --> 00:03:06,010
in the default constructor project. And what I've got here is

48
00:03:06,010 --> 00:03:10,710
the same player class that we've been using where we've got the name the health and the xp for a player

49
00:03:10,710 --> 00:03:12,710
which are private instance variables,

50
00:03:12,710 --> 00:03:16,010
and we've also got a couple of member methods here:

51
00:03:16,010 --> 00:03:20,910
set name and get name that's just the setter and the getter for that name attribute right there.

52
00:03:21,790 --> 00:03:25,790
What's important to understand here, and of course, I would do the same thing for health or xp,

53
00:03:25,790 --> 00:03:29,690
but i don't want to clutter up the code, I just want to talk about the default constructors here.

54
00:03:29,690 --> 00:03:34,430
You'll notice that this player class has no constructor declared.

55
00:03:35,030 --> 00:03:38,030
C++ has to have a way to create objects.

56
00:03:38,030 --> 00:03:41,470
So if you don't provide any constructors,

57
00:03:41,470 --> 00:03:43,830
it will provide a system generated

58
00:03:43,830 --> 00:03:47,430
no args constructor that will allow you to create objects.

59
00:03:47,430 --> 00:03:50,630
So in this case, I don't have any constructors declared,

60
00:03:50,630 --> 00:03:53,230
but yet, I'm able to create

61
00:03:54,000 --> 00:03:57,000
player objects with no problem,

62
00:03:57,000 --> 00:04:00,800
I could say Frank.set name to let's say Frank.

63
00:04:03,700 --> 00:04:05,300
And I could output the

64
00:04:06,300 --> 00:04:08,960
value of Frank's name by using get name.

65
00:04:11,760 --> 00:04:14,660
Okay. Something real simple like that. And what we'll do is we'll walk through this,

66
00:04:14,660 --> 00:04:18,560
we'll step through it using the debugger. So I'll put a break point right here,

67
00:04:18,560 --> 00:04:20,560
and we can debug this real quickly.

68
00:04:21,459 --> 00:04:25,260
And we really don't care about the output. We just want to see that object.

69
00:04:25,260 --> 00:04:28,260
So you can see here that I'm on line 24,

70
00:04:28,510 --> 00:04:33,110
and I'm going to instantiate that Frank object, which is a player.

71
00:04:33,510 --> 00:04:35,010
There is no constructor.

72
00:04:35,510 --> 00:04:39,110
So behind the scenes, a default no args constructor is

73
00:04:39,110 --> 00:04:40,910
generated that does nothing,

74
00:04:40,910 --> 00:04:43,570
but it does allow c++ to create that object.

75
00:04:43,570 --> 00:04:45,170
So I'm going to go next here.

76
00:04:45,970 --> 00:04:48,570
At this point, Frank is created.

77
00:04:49,370 --> 00:04:51,370
You can see the object right up here.

78
00:04:51,370 --> 00:04:55,270
And I executed the set name, which set the attribute name to Frank.

79
00:04:55,870 --> 00:04:56,420
That's it.

80
00:04:56,420 --> 00:04:59,020
The next statement would obviously print it to the console.

81
00:04:59,020 --> 00:05:02,220
So you can see c++ has to have a way to create these objects.

82
00:05:02,220 --> 00:05:04,880
If you don't provide a way to initialize this

83
00:05:04,880 --> 00:05:07,780
using any kind of no args constructor,

84
00:05:07,780 --> 00:05:10,280
you get one automatically by the compiler.

85
00:05:10,780 --> 00:05:15,380
That's great. Now of course -- I'm going to stop this. Of course, we can create our own

86
00:05:15,380 --> 00:05:19,880
no args constructor. So let's do that. I'll do it down here so it's closer to the main.

87
00:05:20,880 --> 00:05:24,200
So what we can do is we can create our constructor. There it is. It's called player,

88
00:05:24,200 --> 00:05:26,700
same name as the class, no args,

89
00:05:27,060 --> 00:05:29,360
and we'll implement it right here.

90
00:05:29,360 --> 00:05:33,660
Now what's the default behavior I want when I don't provide any initialization information,

91
00:05:34,210 --> 00:05:37,110
anything reasonable whatever makes sense for your application.

92
00:05:37,110 --> 00:05:39,410
In this case, let's just say that the name is none.

93
00:05:40,770 --> 00:05:44,370
We'll say that the health is, let's say, a 100.

94
00:05:48,070 --> 00:05:50,270
And let's say the xp is 3.

95
00:05:52,770 --> 00:05:54,970
That's it. That's my initializer.

96
00:05:54,970 --> 00:05:58,470
That's also my default initializer. That's what's important.

97
00:05:58,470 --> 00:06:01,130
This will now be called right here. And I'll put a break

98
00:06:01,130 --> 00:06:04,730
point right in there so we can see it being called, and we'll debug this again.

99
00:06:09,130 --> 00:06:11,630
And here we go. What you can see happening is

100
00:06:11,990 --> 00:06:13,870
we're right here on line 30.

101
00:06:13,870 --> 00:06:16,070
And now when i step through this,

102
00:06:16,070 --> 00:06:19,430
notice how it transfers control to line 22,

103
00:06:19,430 --> 00:06:21,430
that's exactly what we expected.

104
00:06:21,430 --> 00:06:24,930
We didn't see that before because we didn't provide a default constructor,

105
00:06:24,930 --> 00:06:29,920
but now we're providing one. So ours will be called rather than the compiler created one.

106
00:06:30,470 --> 00:06:34,070
And this will just step through and it'll set the properties to name

107
00:06:34,670 --> 00:06:35,970
100 and 3.

108
00:06:36,770 --> 00:06:40,770
Now I'm back here on line 31. And you can see right here

109
00:06:40,770 --> 00:06:42,970
that we've got none, 100 and 3.

110
00:06:42,970 --> 00:06:45,870
And now I can change the nun to Frank right here on this line.

111
00:06:46,970 --> 00:06:49,630
And I'll refresh this, and you can see here it's Frank

112
00:06:50,430 --> 00:06:52,430
right there, the string is Frank.

113
00:06:52,430 --> 00:06:54,730
Okay. So there you go.

114
00:06:54,730 --> 00:06:59,090
If you don't provide any constructors, you get a default constructor for free if you will.

115
00:06:59,090 --> 00:07:03,390
And if you provide your own default constructor that's the one that will be used.

116
00:07:03,750 --> 00:07:06,050
Now let's do one more thing here.

117
00:07:06,050 --> 00:07:10,450
What happens if you create -- let's let me comment this out right here.

118
00:07:12,110 --> 00:07:14,010
And suppose that I want to create

119
00:07:14,560 --> 00:07:17,920
a different constructor, an overloaded constructor. So I'm going to say player

120
00:07:18,580 --> 00:07:22,180
and I want to provide the name.

121
00:07:22,680 --> 00:07:27,080
I want to provide all of those values. So this is my name val. I want health

122
00:07:29,960 --> 00:07:33,620
and I want the xp as well. I want to provide all three of these values.

123
00:07:34,320 --> 00:07:38,680
And in the body of this constructor, I'm just going to say name equals name val,

124
00:07:40,680 --> 00:07:42,280
health equals health val

125
00:07:46,280 --> 00:07:48,880
and xp equals xp val.

126
00:07:51,780 --> 00:07:55,980
Okay. So that's where we're at. Now we have a bit of a problem now.

127
00:07:56,780 --> 00:07:59,180
In c++ -- and this is true of many object-

128
00:07:59,180 --> 00:08:01,680
oriented languages, including java and others.

129
00:08:02,180 --> 00:08:05,480
You get that default constructor as long as you don't provide

130
00:08:05,480 --> 00:08:09,580
any other constructor. Well, what we just did here is we provided a constructor.

131
00:08:09,940 --> 00:08:13,140
In this case, this constructor expects three arguments.

132
00:08:13,140 --> 00:08:15,800
Now that compiler generated

133
00:08:15,800 --> 00:08:19,160
no args, default constructor will no longer be generated.

134
00:08:19,160 --> 00:08:23,060
So this line of code right here on line 35 won't compile anymore.

135
00:08:23,720 --> 00:08:27,080
Let me run the debugger here. You'll see the error right here,

136
00:08:27,080 --> 00:08:30,740
it says no matching function call for the no args constructor.

137
00:08:31,400 --> 00:08:35,200
This is perfectly normal. This is exactly the way it's supposed to work.

138
00:08:35,530 --> 00:08:39,429
The idea being that now you've provided a way to construct your object,

139
00:08:39,429 --> 00:08:42,730
so c++ note will no longer provide a default way,

140
00:08:42,730 --> 00:08:47,230
right,maybe you don't want a default way. So this won't compile.

141
00:08:47,230 --> 00:08:48,430
I'll comment that out,

142
00:08:49,420 --> 00:08:51,620
and I'll move the breakpoint down to here.

143
00:08:52,280 --> 00:08:57,270
So now the way we need to construct this object is to explicitly call that other constructor.

144
00:08:57,270 --> 00:09:01,370
So I'm going to say player Frank, and we'll initialize that to Frank

145
00:09:02,370 --> 00:09:05,570
100 and let's say 13 or something.

146
00:09:08,170 --> 00:09:12,770
Now what's going to happen is when this line executes let me get rid of fearless break points,

147
00:09:13,130 --> 00:09:16,430
when this line executes on 36, what's going to happen is

148
00:09:16,430 --> 00:09:19,430
this constructor is going to be called, the one that i just created.

149
00:09:19,430 --> 00:09:22,030
So I'll put a break point there so you can see that happening.

150
00:09:22,830 --> 00:09:26,930
Okay. So let's debug this again. We'll walk through it one more time,

151
00:09:27,920 --> 00:09:30,120
and here we are. We're on line 36.

152
00:09:31,220 --> 00:09:35,520
When I step through it, you'll see control will be transferred up to line 27 up here.

153
00:09:35,920 --> 00:09:39,580
So there we are. We're in the constructor now

154
00:09:39,580 --> 00:09:44,380
so we're explicitly calling that 3 arg constructor that we just created.

155
00:09:44,380 --> 00:09:48,980
Remember, once I create any kind of constructor, the default constructor

156
00:09:49,280 --> 00:09:52,580
that's automatically generated won't be generated anymore.

157
00:09:52,980 --> 00:09:54,280
We'll walk through this.

158
00:09:55,160 --> 00:09:59,160
And when we're done, we'll refresh this. And you can see Frank 113 has

159
00:09:59,160 --> 00:10:01,660
been created just as we expected.

160
00:10:01,660 --> 00:10:04,320
Now of course, we can

161
00:10:05,520 --> 00:10:08,820
create both of these, right. So now I can create,

162
00:10:10,480 --> 00:10:12,680
let's say, a Hero object.

163
00:10:13,980 --> 00:10:15,780
So now what we've got is we've got

164
00:10:15,780 --> 00:10:20,080
two constructors that we provided. We provided a no argus constructor,and we provided

165
00:10:20,080 --> 00:10:23,280
this overloaded constructor that expects the three arguments.

166
00:10:23,280 --> 00:10:27,530
In this case, Hero, we're not providing any initialization information.

167
00:10:27,530 --> 00:10:31,530
So the no args constructor, this guy right here on line 21 will be called.

168
00:10:32,190 --> 00:10:35,690
And in the case of Frank, it'll be just like before this one would be called.

169
00:10:35,690 --> 00:10:37,690
So let's step through that,

170
00:10:40,250 --> 00:10:43,550
and let me do that again, I'll put a break point on Hero.

171
00:10:45,910 --> 00:10:50,900
Let's try that one more time, and you can see what's happening here is. I'm right here on line 35,

172
00:10:50,900 --> 00:10:53,400
so I'm going to construct that Hero object.

173
00:10:53,950 --> 00:10:56,950
And you can see transfers control right up to here.

174
00:10:56,950 --> 00:11:00,750
So there's no args constructor is being called, that's what we expect.

175
00:11:00,750 --> 00:11:05,250
That's our constructor, right. That's not compiler generated because we wrote it ourselves.

176
00:11:05,250 --> 00:11:07,240
So that's going to execute.

177
00:11:07,600 --> 00:11:12,000
Now I come back to Frank, and this constructor will be called, so you'll see

178
00:11:12,300 --> 00:11:14,600
control will be transferred over here to 27.

179
00:11:15,260 --> 00:11:19,360
And we'll run through that, you can see it happening right here on line 29 that's where I'm at now.

180
00:11:20,160 --> 00:11:23,960
And at this point, we've got both those objects constructed

181
00:11:23,960 --> 00:11:26,120
and we can see them right here.

182
00:11:26,520 --> 00:11:31,120
There's Frank and there's Hero. Hero uses none 103,

183
00:11:31,120 --> 00:11:35,320
and Frank is using Frank 113, you see them right here.

184
00:11:35,920 --> 00:11:39,920
Okay. So that's it. That's the behavior for default constructors in c++.