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[No Audio]

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In this video tutorial, we will be covering

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four topics. These topics include scope of a

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trait, trait methods with same name, super

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traits, marker traits, and auto traits.

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Pretty much exciting material to cover, so

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let's get started. First, we will talk about

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the scope of a trait. The scope of a trait

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refers to the portion of the code, where the

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trait is visible, and where its methods are

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accessible. When a trait is defined in a

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module, it is only visible and usable within

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that module, and any nested modules within

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that particular module. If the trait is

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marked as pub, it can, it can also be used by

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other modules outside of its scope.

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Understanding the scope of a trait is

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important for organizing Rust code, and for

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ensuring that traits and their methods are

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only used where they are intended to be used,

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and not accidentally exposed to other parts

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of the code where they shouldn't be exposed.

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With this background, let us dive into an

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example for illustrating the scope of a trait.

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I will start by adding a library crate.

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[No Audio]

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I will declare a module of basic.

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[No Audio]

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This module will have a public personal_info trait,

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which will contain a function of

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how_old which will return the age as a u8.

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[No Audio]

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Next, I will define a public structure of

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Student containing a couple of fields of name and age.

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[No Audio]

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Next, we will implement the personal_info.

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[No Audio]

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Let us include the definition

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of how_old function now.

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[No Audio]

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Next, we will define some_fn, which will

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make use of the how_old function

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within the same module.

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[No Audio]

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I will define an instance with some values.

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[No Audio]

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Next, I will call the function

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of how_old on this instance.

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[No Audio]

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Now if I remove the pub keyword

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from the trait, the trait is still in scope,

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and therefore there won't be any issues.

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However, if I keep the trait private to the

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module, and use the same function outside the

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module, the compiler will not be happy. Let

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me copy the code of the function and paste it

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outside the module.

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[No Audio]

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I will first bring the relevant items

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from the module into scope of

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the function. This is because, they will not

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be in scope by default, because they are

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residing in a different module. You may note

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that there are errors indicating to us that

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the trait personal_info is private. This

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means that by default, the trait is only

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available to us within the module or sub

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modules of the main module in which it is

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defined. Let us go ahead and make the trait public.

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[No Audio]

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The compiler is now happy and has no issues.

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It is not always necessary and

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mandatory to have the trait definition and

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its implementation for a specific type to be

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always in the same module. The trait

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definition may be in one module, and its

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implementation may be in another module.

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We just need to make sure that module in which

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we intend to have implementation of a trait

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for a specific type must bring the trait into

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its scope. For instance, I will define another module.

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[No Audio]

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I will bring the personal_info trait into scope first.

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[No Audio]

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Next I will define another struct called Person.

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[No Audio]

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I will implement the personal_info for Person.

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[No Audio]

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Finally, I will add the definition of how_old function.

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[No Audio]

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This will work fine, as long as the

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personal_info is public. However,

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if we make the personal_info private,

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then it will not work. Let me make it private.

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The compiler is indicating to us some errors.

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The summary is that like structs, functions,

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and enums, we can also make the traits public

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or private for controlling its scope and

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visibility inside the code. To get around the

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error in this case, we can either make the

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trait public or move the module inside the

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basic module. Both the options are available

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and are dependent on your underlying design,

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and you have to decide it carefully for

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proper organization of your code. Let us make

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it public for now. Finally, the same rules

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will apply if we intend to use the trait of

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personal_info in the main program.

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This is quite obvious, so we will leave the

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details of that and we'll move to the next topic.

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The next topic is with regards to

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trait methods with the same name for the

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type. When we have a trait with a method

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which is implemented for a specific type, and

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the type itself also has an implementation

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for the same method, then the compiler

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defaults to calling the method that is

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directly implemented on the type itself.

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Let us see the details with an example. Consider a

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Pilot trait with a method of fly.

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[No Audio]

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Let us define another trait of Wizard also

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containing the fly method.

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[No Audio]

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Next, let us define a Human

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struct with no fields.

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[No Audio]

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Let us now implement the two traits for the Human.

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First, we will implement the Pilot trait for Human.

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[No Audio]

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Let us say the fly method definition now.

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[No Audio]

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Next, we will implement the Wizard for Human.

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[No Audio]

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Let us say the function of fly.

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[No Audio]

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Finally, we will directly implement the fly

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inside the implementation of Human.

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[No Audio]

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You may have noticed that, now we have three

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implementations of the same function of fly

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for Human in three different contexts of Pilot,

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Wizard, and a direct implementation inside the

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Human. As pointed out earlier, when the type

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itself implements a method, so by default, its

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own version will be invoked. In this case, we

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are expecting the fly within the Human

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implementation to be invoked. Let us see if

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this is the case. In the main, I will first

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create an instance of the Human.

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[No Audio]

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Next, I will call the fly on the Human.

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Let us execute to see which function

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will be invoked in this case.

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[No Audio]

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As expected, the function inside the

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implementation of the type itself has been

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invoked. To invoke the specific

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implementations for the type. We will not use

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the dot notation, but rather will use the full

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information of the function with proper

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inputs. For instance, to invoke the

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implementation provided by the pilot trait, we

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will use this syntax.

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[No Audio]

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In the same way, to invoke the

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implementation inside the Wizard

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trait, we will use this syntax.

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[No Audio]

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Let us now move to the

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next topic of super traits, but first

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I will comment out the code.

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[No Audio]

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Rust doesn't have inheritance, but you can

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define a trait as being a super trait of

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another trait. Super traits are a feature of

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Rust's trait system that allows one trait to

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inherit methods and associated types from

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another trait. When a trait inherits from

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another trait, it gains access to all the

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methods and associated types of the parent

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trait. This is similar to inheritance in

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object oriented programming in other

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languages, where a subclass can inherit

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methods and properties from a parent class.

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Let us see some examples to illustrate the

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key ideas. I will define a trait of person

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containing a function of name.

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[No Audio]

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Next, I will define another trait called

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Student with a Person being it's super trait.

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[No Audio]

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The colon followed by the trait after the

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name of the trait, is the syntax for

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mentioning a super trait for the trait. In

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this case the Person is a super trait for the

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Student trait. The key requirement for any

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type now that implements the Student trait

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is that, it also needs to have an

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implementation for the Person trait. This

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means that, any type which implements the

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Student trait must also implement the Person

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trait. Let us include a function of complete_info

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inside the Student trait with an

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input of self and an output of a tuple.

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[No Audio]

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Let us now see how we can implement the trait

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for a specific type. I will define a

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university_student struct.

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[No Audio]

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The structure will have three fields.

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[No Audio]

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Next we will implement the Student for university student.

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[No Audio]

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Let us include the function of complete_info.

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[No Audio]

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The function will return a tuple with the first value being the

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name, the second value being the age, and the

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third value being the university of the Student.

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[No Audio]

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You may note that we do not have the name

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method implemented for the Student for now.

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This method will be used from the parent

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trait implementation, so this means that we

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also need to implement the parent trait for

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the type of university_student.

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There is also an error that is being

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indicated by the compiler, let us see the

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details. It says the trait bound uni_student: Person

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is not satisfied, the trait person is not

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implemented for uni_student.

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The compiler is also indicating to

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use the Person trait which is not implemented

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for the type. Another way of thinking about

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the parent trait is that, when we declare a

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trait to be a parent trait, then in some

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sense, we are associating some extra

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properties with the type, which also needs to

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be satisfied for that particular type.

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In other words, the type must also have these

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additional properties, let us implement the parent trait now.

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[No Audio]

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Next, let us also include the method of name

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which will return the name of the Student.

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[No Audio]

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Now, the compiler is no more complaining as

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the requirements are being fulfilled, that is,

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any type which implements the child trait

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must also implement a parent trait or super

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trait. Let us use these implementations in

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the main now. I will create an instance of

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the university Student initialized from

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some suitable values.

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[No Audio]

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I will now call the complete_info

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function, which will enable us to use the

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parent track method of name.

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[No Audio]

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I can also directly call the parent trait methods,

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since they are also implemented for the type.

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[No Audio]

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Now, any type which is Student can access the

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super trait Person methods. To see this more

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clearly, I will define a function called info.

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[No Audio]

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This function we'll use a generic type s

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which will have the Student trait.

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[No Audio]

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The input to the function will be a reference to s.

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[No Audio]

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Inside the function, I will access the

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complete_info which is implemented for the

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Student inside the print statement.

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[No Audio]

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I will also access the name function which is not

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part of the Student trait, but it's part of

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the super trait Person.

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[No Audio]

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This means now, that the

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super trait methods are also

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accessible to us, although the generic is of type

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Student and not that of a Person. Let us call the info

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function in the main now with the reference to s.

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[No Audio]

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Let us cargo run this now.

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[No Audio]

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With regards to the super traits, there is a

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closely related concept of combo traits.

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Combo traits refers to the use of multiple

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traits and capacity of super traits. Let us

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extend the current example to explain this.

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Let me add one more trait called Programmer,

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which will have a single function of favorite language.

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[No Audio]

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Next, I will define another trait

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called computer science student, and we'll

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mention multiple traits as its super traits.

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[No Audio]

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The plus means that the trait computer

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science student will have both the Programmer

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and the Student as its super traits. This is

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also referred to as combo traits, which is

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used to mention multiple traits as super

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traits. Please note now that in this case,

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implementing the computer science student for

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a certain type will require us to implement

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both the super traits of Programmer and

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Student. Moreover, since Student has a super

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trait of person, so implementing type must

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also have the implementation for the Person trait.

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Let us now move to the topic of marker

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traits, but first I will comment out the code.

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[No Audio]

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A marker trait is a trait that doesn't require

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any methods to be implemented. Moreover, it

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also does not have any trait items. Instead,

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it serves as a way to add metadata or

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constraints to a type. Marker traits are

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useful in Rust, because they allow us to

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commute additional information

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about a type to the compiler without

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requiring any actual functionality to be

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implemented. This can be helpful in cases

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where we need to specify certain behaviors or

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properties of a type without actually

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providing any concrete implementations.

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Let us see an example of this. I will define a

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trait called Some_properties,

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which will have Clone, PartialEq, and

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Default as its super traits.

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[No Audio]

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The plus is used to mention the combo traits

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and the colon in the syntax for mentioning

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the super traits. This means now that

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all these traits are the super traits

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have the Some_properties traits.

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Now, as pointed out a few

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times earlier also, any type which has the

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Some_properties trait will also

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have the Clone, PartialEq, and the

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Default traits. This means now that by

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declaring a type to have Some_properties,

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we are putting a restriction on

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this type or marking the type that it must

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also have the Clone, PartialEq, and

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Default traits. Let us declare a Student struct next.

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[No Audio]

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The structure will have three fields.

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[No Audio]

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Since the Some_properties

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does not have anything to being implemented,

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so therefore it just serve as a marker.

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Next, we will implement the sum underscore

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properties for Student.

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[No Audio]

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You may note that the compiler is not happy,

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it says the trait bound Student

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call on Default is not satisfied. The

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trait Default is not implemented for Student,

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let us add the Default.

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[No Audio]

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You may note that it is still complaining,

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it will keep on complaining until

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and unless we derive all

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the required traits for the type. So let us

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look at all the required traits.

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[No Audio]

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The code is now compiling. In summary, the marker traits

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are useful for mentioning some constraints on

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a type. Let us now cover the last topic,

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which is the auto traits, but first, I will

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comment out the code.

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[No Audio]

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Auto traits are traits that get

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automatically implemented for a type, if all

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of its members also implement the trait. What

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member means depends on the type, for

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example, fields of a struct or variants of an

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enum. Let us see an example to understand

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auto traits. Consider a simple customer

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struct with two fields.

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[No Audio]

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Let us add the Default

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trait to this type by deriving it.

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[No Audio]

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There were no issues, because the members have

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the type, which in this case are the fields of

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the struct also have the implementation for

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the Default. In fact, all the primitive types

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implements the Default trait. This means that

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the Default trait is an auto trait for the

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customer. Since all of its types also

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implements this trait, however, if we add

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another type to it, which which does not have

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the default implementation, then the compiler

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will not be happy and it will not consider it

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to be an auto trait. Let me add another field

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of relationship to the customer which will be

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an enum with type visit.

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[No Audio]

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Let us define the enum now.

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[No Audio]

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In order to make the Default as an

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auto trait, we will need to implement the

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Default for the Visit type also, so let us add this.

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[No Audio]

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The Default  trait contains a single

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function of Default, so let us add that.

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[No Audio]

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The function will return a variant of new as a

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Default value. Now you may note that the

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compiler has no issues, we can now use this

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function to initialize an instance

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of the struct in the main.

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[No Audio]

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With this we end this tutorial, I hope you

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would have enjoyed this. See you again and

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until next tutorial, enjoy Rust programming