Go by Example: Interfaces

Interfaces are named collections of method signatures.

 
package main
 
import (
    "fmt"
    "math"
)

Here’s a basic interface for geometric shapes.

 
type geometry interface {
    area() float64
    perim() float64
}

For our example we’ll implement this interface on rect and circle types.

 
type rect struct {
    width, height float64
}
type circle struct {
    radius float64
}

To implement an interface in Go, we just need to implement all the methods in the interface. Here we implement geometry on rects.

 
func (r rect) area() float64 {
    return r.width * r.height
}
func (r rect) perim() float64 {
    return 2*r.width + 2*r.height
}

The implementation for circles.

 
func (c circle) area() float64 {
    return math.Pi * c.radius * c.radius
}
func (c circle) perim() float64 {
    return 2 * math.Pi * c.radius
}

If a variable has an interface type, then we can call methods that are in the named interface. Here’s a generic measure function taking advantage of this to work on any geometry.

 
func measure(g geometry) {
    fmt.Println(g)
    fmt.Println(g.area())
    fmt.Println(g.perim())
}

Sometimes it’s useful to know the runtime type of an interface value. One option is using a type assertion as shown here; another is a type switch.

 
func detectCircle(g geometry) {
    if c, ok := g.(circle); ok {
        fmt.Println("circle with radius", c.radius)
    }
}
 
func main() {
    r := rect{width: 3, height: 4}
    c := circle{radius: 5}

The circle and rect struct types both implement the geometry interface so we can use instances of these structs as arguments to measure.

 
    measure(r)
    measure(c)
 
    detectCircle(r)
    detectCircle(c)
}
 
$ go run interfaces.go
{3 4}
12
14
{5}
78.53981633974483
31.41592653589793
circle with radius 5

To understand how Go’s interfaces work under the hood, check out this blog post.

Next example: .

by Mark McGranaghan and Eli Bendersky | source | license