fmt for interface map Pointer, Address reflect type

Struct

Often in Go programs we have related data—for example, we might have X and Y coordinates. With a struct, we can store these together.

With structs, we create new types that contain different fields and methods. Ints and strings can be stored in structs. We use the struct keyword.

Create `struct`

We specify the type Location struct: this has 3 fields and no methods. The "x" and "y" fields are both ints, and there is a bool that indicates validity.

Info We use the type keyword before the name of the struct. The Location here is not an instance—it is something we must create with new.

Version 1 Here we see the new() syntax for creating an instance of struct type. We assign the "x" field of the struct to a value.

Version 2 We use an initializer to create a new instance of the struct. We can just specify the field names we want to set.

package main

import "fmt"

type Location struct {
    x     int
    y     int
    valid bool
}

func main() {
    // Version 1: create a Location struct instance and set a field.
    loc := new(Location)
    loc.x = 10
    fmt.Println(loc.x)

    // Version 2: use initializer syntax.
    loc2 := Location{x: 10}
    fmt.Println(loc2.x)
}10
10

Slice, `struct` pointers

This program reuses our Location struct. It specifies two fields of the struct on one line. And in main(), it creates a slice of struct pointers.

Start We create an empty slice of structs. These are pointers—which are variables of a specific type.

Next We create two Location structs with the new keyword. We use append to add these to our slice.

Finally We loop over all indexes in our slice. We use fmt.Println to display the struct data to the console.

package main

import "fmt"

type Location struct {
    x, y  int
    valid bool
}

func main() {
    // Create empty slice of struct pointers.
    places := []*Location{}

    // Create struct and append it to the slice.
    loc := new(Location)
    loc.x = 10
    loc.y = 20
    loc.valid = true

    places = append(places, loc)

    // Create another struct.
    loc = new(Location)
    loc.x = 5
    loc.y = 8
    loc.valid = true

    places = append(places, loc)

    // Loop over all indexes in the slice.
    // ... Print all struct data.
    for i := range(places) {
        place := places[i]
        fmt.Println("Location:", place)
    }
}Location: &{10 20 true}
Location: &{5 8 true}

Map keys

A struct can be used as the key of a map. We create struct instances and use their values to assign map data. We access the value of a struct "m" with "*m" to add the key.

Start We create 3 structs and add them all as keys in the "storage" map. The value for each is "true."

Then We create 2 structs and use them to look up values in the map. We get the correct results.

package main

import "fmt"

type Measure struct {
    size int
    unit string
}

func main() {
    // A map with struct keys.
    storage := map[Measure]bool{}

    // Add 3 structs as keys in the map.
    m := new(Measure)
    m.size = 10
    m.unit = "centimeters"   
    storage[*m] = true

    m = new(Measure)
    m.size = 20
    m.unit = "feet"
    storage[*m] = true

    m = new(Measure)
    m.size = 10
    m.unit = "decibels"
    storage[*m] = true

    // There are 3 keys in the map.
    fmt.Println("Map len", len(storage))

    // Create structs to look up values in the map.
    key := new(Measure)
    key.size = 10
    key.unit = "centimeters"
    v := storage[*key]
    fmt.Println("Result", key, v)

    key = new(Measure)
    key.size = 100
    key.unit = "decibels"
    v = storage[*key]
    fmt.Println("Result", key, v)
}Map len 3
Result &{10 centimeters} true
Result &{100 decibels} false

Interface

With interfaces, we create abstractions that can be used to reference many types at once. This is confusing at first, but leads to clearer programs.

Review

With structs, we combine fields and funcs. We can use slices and maps with structs to build more complex collections—this is powerful and intuitive.