add: slice internals backing array and slice header

16-slices/09-slice-internals-2-slice-header/1-theory/main.go

@@ -0,0 +1,46 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+
+	s "github.com/inancgumus/prettyslice"
+)
+
+func main() {
+	//
+	// each int element is 4 bytes (on 64-bit)
+	//
+	// let's say the ages point to 1000th.
+	// ages[1:] will point to 1004th
+	// ages[2:] will point to 1008th and so on.
+	//
+	// they all will be looking at the same
+	// backing array.
+	//
+
+	ages := []int{35, 15, 25}
+	red, green := ages[0:1], ages[1:3]
+
+	s.Show("ages", ages)
+	s.Show("red", red)
+	s.Show("green", green)
+
+	fmt.Println(red[0])
+	// fmt.Println(red[1]) // error
+	// fmt.Println(red[2]) // error
+
+	{
+		var ages []int
+		s.Show("nil slice", ages)
+
+		// or just:
+		s.Show("nil slice", []int(nil))
+	}
+}

16-slices/09-slice-internals-2-slice-header/2-example/main.go

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+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"unsafe"
+
+	s "github.com/inancgumus/prettyslice"
+)
+
+// type collection [4]string // #1
+type collection []string // #2
+
+// go is pass by copy
+// only the slice header is copied: 3 integer fields (24 bytes)
+// think of passing an array with millions of elements instead.
+
+func main() {
+	// SliceHeader lives here:
+	// https://golang.org/src/runtime/slice.go
+
+	s.PrintElementAddr = true
+
+	// #1
+	data := collection{"slices", "are", "awesome", "period", "!!" /* #5 */}
+
+	// data := collection{"slices", "are", "awesome", "period", "!!"}
+
+	change(data) // #1
+
+	s.Show("main's data", data)                           // #1
+	fmt.Printf("main's data slice's header: %p\n", &data) // #3
+
+	// ----------------------------------------------------------------------
+	// #4
+	array := [...]string{"slices", "are", "awesome", "period", "!!" /* #5 */}
+
+	// array's size depends on its elements
+	fmt.Printf("array's size: %d bytes.\n", unsafe.Sizeof(array))
+
+	// slice's size is always fixed: 24 bytes (on a 64-bit system) — slice value = slice header
+	fmt.Printf("slice's size: %d bytes.\n", unsafe.Sizeof(data))
+}
+
+// #1
+// passed value will be copied in the function
+func change(data collection) {
+	// data is a new variable inside the function:
+	// var data collection
+
+	data[2] = "brilliant!"
+
+	s.Show("change's data", data)
+	fmt.Printf("change's data slice's header: %p\n", &data) // #3
+}