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- Okay, the other type of
string is the String class

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and that's much more dynamic.

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This is how you create
an instance of String.

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00:00:16,093 --> 00:00:20,070
The String structure has a
static method if you like.

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It's called an associated function from.

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String::from hello.

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So this here is a literal piece of text

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and what we're doing here

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is we're creating a String object,

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s is a String object, which
basically owns its own memory.

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When you say String::from
hello, the String object s

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will basically copy hello into here

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and it now owns that memory.

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So I'm gonna forget about
the String literal now.

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S has its own copy of that text

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and that's what a String
object looks like.

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If you wanted to, no particular reason,

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rather than using implicit
typeing for that string,

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you can use explicit typing
to say s is a String object.

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And that would give you the same effect.

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So internally, a String object holds text

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as a potentially growable vector of bytes.

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That's actually specified
in the documentation.

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Internally, a string
holds a vector of bytes

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to hold the text that you've assigned.

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So consider this example.

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I talk about memory allocation.

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This is how memory is allocated.

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The String variable s lives on the stack.

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So that's just a local
variable on the stack.

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We have this local variable s.

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And so I'm gonna draw
it as a String object.

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That's my String object s.

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The text that the String holds
is allocated on the heap.

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So the text that it holds, hello,

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that text is allocated on the heap.

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Hello.

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The String object has a
pointer onto the heap like so.

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So continuing our example

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where I've got my stack-based variable s,

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which has a pointer into the heap,

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and on the heap, it holds hello like so,

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let's consider what happens
at the end of the function.

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At the end of the function,

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this is how memory gets deallocated.

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So first of all, my String
object s is just a local variable

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and it goes out of scope
at the curly bracket.

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And when it goes out of scope,

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this object is going to be destroyed

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just because that's what always happens

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when you have objects
allocated locally on the stack.

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Just before the String object dies,

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it has a destructor effectively.

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There's a function we'll
have look at called drop,

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which is effectively a destructor.

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The drop function is automatically called

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on an object when it
goes our of scope here.

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And for the String structure,

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the drop function deallocates its buffer.

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So it'll deallocate the drop function,

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will automatically deallocate the buffer

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and then the String
object goes out of scope

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and the String object itself disappears.

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So it's very similar

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to the idea of destructors in C++.

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There's no garbage collection in Rust.

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That would be far too slow.

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It does it deterministically.

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An object cleans up its own allocation

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just as it's about to be dropped.

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So like I said, there is a Drop mechanism.

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There's actually a trait on
interface called Drop in Rust.

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And the trait called Drop
has a function called drop,

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which is similar to a destructor.

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It so happens that this happens,

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this drop function will
be called automatically

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at the end of an object's lifetime,

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very much like destructor in C++.

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And it gives the object a chance

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to deallocate its resources.

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00:04:15,870 --> 00:04:17,100
So luckily for us,

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the String structure
implements the Drop trait.

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It has a drop function
automatically called

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00:04:23,970 --> 00:04:27,120
when the String is about to be destroyed.

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The drop function deallocates
the buffer on the heap.

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00:04:33,900 --> 00:04:37,140
So I'm gonna have a look at
some mutable String objects.

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Then we'll have a look at the demo.

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You can modify text in a String as long

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as you've declared the String as mutable.

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So there are various methods

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that'll actually change
the value of the String.

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00:04:49,410 --> 00:04:52,020
You can invoke methods to modify contents.

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There are also methods to query contents

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as well obviously, like .length.

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Let's have a look at the documentation

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00:04:57,900 --> 00:05:01,560
of the String class or the
String structure I should say

97
00:05:01,560 --> 00:05:02,850
to see what's available.

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00:05:02,850 --> 00:05:04,613
And then we'll have a look at the demo.

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00:05:05,580 --> 00:05:09,300
This is the documentation
for these String structure

100
00:05:09,300 --> 00:05:13,683
in the String module, in the std crate.

101
00:05:14,580 --> 00:05:18,540
Lots of functions here,
which allow you to query

102
00:05:18,540 --> 00:05:20,940
and modify the contents of a String.

103
00:05:20,940 --> 00:05:24,630
Like push_str, for
example, is like an append.

104
00:05:24,630 --> 00:05:28,080
There's a push function, which
appends a single character

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00:05:28,080 --> 00:05:30,963
and a push_str, which depends a string.

106
00:05:32,133 --> 00:05:36,030
There are other methods
down here which are listed

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under a different section,
as_ptr, for example.

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You can get back the address

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00:05:42,690 --> 00:05:45,093
of the text that the string points to.

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So you'll probably find yourself
looking in here quite a lot

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00:05:50,130 --> 00:05:52,410
just to see what methods you have

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00:05:52,410 --> 00:05:55,083
and how they work and
what parameters they take.

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00:05:57,060 --> 00:05:59,850
So let's have a look at our
example again to see how to work

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00:05:59,850 --> 00:06:01,563
with the String structure.

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00:06:02,460 --> 00:06:04,350
Okay, so I'm back in the example.

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00:06:04,350 --> 00:06:07,530
I'm just going to reset the clock.

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Just get rid of all my
little windows here.

118
00:06:11,760 --> 00:06:13,620
So we're back into a clean state.

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00:06:13,620 --> 00:06:17,730
This is my demo_string_handling
in the demo.

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00:06:17,730 --> 00:06:21,120
My do_it function, we've
looked at string_literals.

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00:06:21,120 --> 00:06:22,200
So now we're going to look

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00:06:22,200 --> 00:06:25,050
at string_objects and how to
have mutable_string_objects.

123
00:06:27,240 --> 00:06:28,650
So here's my example here.

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00:06:28,650 --> 00:06:33,180
I've got a string_object, s3, wales.

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00:06:33,180 --> 00:06:34,920
I've got a string_object, s4.

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00:06:34,920 --> 00:06:37,350
I've typed this explicitly.

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00:06:37,350 --> 00:06:40,230
No particular benefit of doing that really

128
00:06:40,230 --> 00:06:42,270
but just for demonstration purposes.

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00:06:42,270 --> 00:06:44,970
And that String is cymru,
which is, as you know,

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00:06:44,970 --> 00:06:47,340
is Welsh for Wales.

131
00:06:47,340 --> 00:06:49,233
I can print out a string,

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00:06:51,150 --> 00:06:54,870
I can get the address of
the first byte in the text

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00:06:54,870 --> 00:06:56,820
and I can get the length of the string.

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00:06:56,820 --> 00:06:58,740
And I've done that for string three

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00:06:58,740 --> 00:07:00,990
and I've done it for string four.

136
00:07:00,990 --> 00:07:02,790
At the end of the function,

137
00:07:02,790 --> 00:07:05,643
these two string objects go out of scope.

138
00:07:06,840 --> 00:07:09,480
The drop function is called
on the String objects

139
00:07:09,480 --> 00:07:13,470
because String, the String structure,

140
00:07:13,470 --> 00:07:15,660
implements the Drop trait.

141
00:07:15,660 --> 00:07:18,030
In other words, it has a drop function,

142
00:07:18,030 --> 00:07:22,320
which will dynamically
effectively delete the memory

143
00:07:22,320 --> 00:07:23,153
on the heap.

144
00:07:25,140 --> 00:07:28,260
And then I've got a
mutable_string example.

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00:07:28,260 --> 00:07:31,500
I've got a mutable_string, s5,

146
00:07:31,500 --> 00:07:33,240
which has got some white space

147
00:07:33,240 --> 00:07:35,610
at the beginning deliberately, super.

148
00:07:35,610 --> 00:07:37,863
When you call the push_str function,

149
00:07:39,330 --> 00:07:41,790
the String object kind of expands

150
00:07:41,790 --> 00:07:43,950
its dynamic memory allocation.

151
00:07:43,950 --> 00:07:46,200
So it's the same String object,

152
00:07:46,200 --> 00:07:48,840
it's just that the buffer gets expanded.

153
00:07:48,840 --> 00:07:51,150
You can convert the string to_uppercase,

154
00:07:51,150 --> 00:07:55,770
trim the string to get rid
of leading and trailing space

155
00:07:55,770 --> 00:07:57,780
and then convert the String to_uppercase.

156
00:07:57,780 --> 00:08:02,760
So s6 will be an uppercase
version of super swans.

157
00:08:02,760 --> 00:08:03,993
Let's run it and see.

158
00:08:10,380 --> 00:08:15,380
So we have the details for s3 and s4.

159
00:08:15,870 --> 00:08:19,440
So s3 is wales, the
address of the first byte

160
00:08:19,440 --> 00:08:20,940
and the number of bytes.

161
00:08:20,940 --> 00:08:23,520
Likewise for s4.

162
00:08:23,520 --> 00:08:25,620
And then we have our mutable_string.

163
00:08:25,620 --> 00:08:28,500
S5, well numbered.

164
00:08:28,500 --> 00:08:33,500
So initially, that string
contains super swans.

165
00:08:34,050 --> 00:08:38,760
So s5 contains super swans with spaces.

166
00:08:38,760 --> 00:08:42,807
That's s5 and I've got
the address of it there.

167
00:08:42,807 --> 00:08:45,870
S6 is the kind of converted version.

168
00:08:45,870 --> 00:08:48,990
It trimmed it and then
converted it it to uppercase.

169
00:08:48,990 --> 00:08:53,670
So s6 is the trimmed uppercased
version of super swans.

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00:08:53,670 --> 00:08:54,503
Super.