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Custom Types

4:10 with Jay McGavren

We've seen that Go can store integers, floating-point numbers, strings, booleans, and more. But suppose we had one integer that represents a number of minutes, and another that represents a number of hours? How do you tell those two apart? How do you avoid accidentally storing the hours integer in the minutes integer, or vice-versa? Go lets you create custom types that will help you avoid these sort of mix-ups.

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Type declaration
- Defines new named type that has same underlying type as an existing type https://golang.org/ref/spec#Type_declarations

package main

import "fmt"

type Minutes int
type Hours int
type Weight float64
type Title string
type Answer bool

func main() {
    minutes := Minutes(37)
    hours := Hours(2)
    weight := Weight(945.7)
    name := Title("The Matrix")
    answer := Answer(true)
    fmt.Println(minutes, hours, weight, name, answer)
}

Underlying type determines intrinsic operations it supports
- type Minutes int will support all the operations int does: addition, subtraction, etc.
- type Title string will support the operations string does, like concatenation of other strings.

    minutes += 3

Can also be compared to a value of same named type, or value of same underlying type

    if weight > Weight(907.3) {
        fmt.Println("Capacity exceeded")
    }
    if weight > 907.3 {
        fmt.Println("Capacity exceeded")
    }

Cannot be compared to values of different type

    // ERROR
    if minutes > hours {
    }

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[MUSIC] 0:00

We've seen that Go can store integers, floating point numbers, strings, 0:04

booleans and more. 0:08

But suppose we add one integer that represents a number of minutes and 0:09

another that represents a number of hours. 0:13

How do you tell those two apart? 0:16

How do you avoid accidentally storing the hours integer in the minutes integer or 0:18

vice versa? 0:22

Go lets you create custom types that will help you avoid these sort of mix ups. 0:23

To create a new custom type you need to declare it. 0:27

To do that you use the type keyword followed by the name of your new type. 0:30

And then an underlying type that you wanna base your custom type on. 0:35

So for example here we have a minutes type that's base on the int underlying type. 0:38

Here's another type named hour, that's also based on int. 0:43

Here we have a weight that's base on float64. 0:47

Titles that are based on the string type, and 0:51

an answer type that's based on booleans. 0:53

When you define a new custom type that also sets up a conversion 0:57

from the underlying type. 1:01

So we can use the name of our new custom type and 1:03

then do a conversion from a value of the int type. 1:06

Once we've created a value of our new custom type, 1:11

we can then assign it to a variable. 1:13

So here we convert an int to the minutes type. 1:17

Here's a conversion from int to hours, 1:22

here's a conversion from float 64 to the weight type. 1:24

Here's a conversion from a string to the title type, and 1:27

here's a conversion from a boolean to an answer. 1:31

Then down here we simply print out all our new values. 1:36

Let's try running this. 1:39

And you see that it simply prints out the underlying values for 1:43

each of our custom types. 1:47

The underlying type that you base a custom type on will determine what intrinsic 1:49

operations your custom type supports. 1:53

For example, if you based the minutes type on an int, 1:56

it'll support all the same operations that int does, addition, subtraction etc. 1:58

If you base the title type on string underlying type, it'll support all 2:03

the same operations string does like the concatenation of other strings. 2:07

So for example, if we were to take our minutes value. 2:12

We could add three minutes onto it by just adding another integer to it. 2:18

Let's try printing the result out. 2:24

Save that, run it. 2:29

And we see that our number of minutes has been updated from 37 to 40. 2:32

A value of a custom type can also be compared to 2:35

a value of the same custom type. 2:39

Or it can be compared to a value of the same underlying type. 2:44

So for example, here we take a weight of 945.7 and 2:50

we compare it to another weight value. 2:54

And down here we compare it to a float value. 2:56

And the comparison works in both cases. 2:59

It shows that 945.7 is greater than 907.3 and 3:01

in both cases it prints capacity exceeded. 3:05

Values of a custom type cannot be compared to values of a different custom 3:11

type though, even if they have the same underlying type. 3:14

So for example if we took a minutes value and an hours value and 3:18

tried to compare them to each other, if minutes is greater than hours 3:22

If we try running this it wont compile. 3:29

We get an error, invalid operation, mismatch types minutes and hours. 3:32

You may be wondering what good custom types are if they're just glorified 3:38

integers and strings. 3:42

There's one more aggregate data type that's more commonly 3:43

used in conjunction with custom types. 3:46

It's called a struct and it can hold multiple fields of different types. 3:48

We'll be covering structs in a little bit, 3:52

as well as how to base custom types on them. 3:54

But we wanted to show you that a custom type can be based on any underlying type, 3:56

Go Language Overview

Custom Types

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