39 Go interview questions on language fundamentals & types, with detailed answers and code.
Every variable in Go is initialized to its type's zero value: 0 for numbers, false for bools, "" for strings, and nil for pointers, slices, maps, channels, functions and…
new and make?new(T) works for any type. It allocates a zeroed T and returns a *T . make only works for slices, maps and channels. It initializes the runtime data structure behind…
type Size int64 const ( _ = iota // iota = 0, skipped KB Size = 1 << (10 * iota) // iota = 1 MB // iota = 2: repeats the expression GB // iota = 3 ) const ( A, B = iota,…
const huge = 1 << 100 compiles.Untyped constants are exact values that live only at compile time. The spec requires at least 256 bits of precision for integer constants. A constant only gets a…
A typed constant follows the same strict rules as a variable of that type. Go never converts between named types implicitly. An untyped constant can be assigned to any…
var db *sql.DB func initDB() error { db, err := sql.Open("postgres", dsn) // BUG: declares a new local db if err != nil { return err } return db.Ping() } // Later:…
:= redeclaration, and why does this not compile?func parse(s string) (n int, err error) { if s != "" { n, err := strconv.Atoi(s) if err != nil { return // compile error: result parameter err not in scope at return } _…
i := 0 s := []int{10, 20} i, s[i] = 1, 5 fmt.Println(i, s) a, b := 1, 2 a, b = b, a+b fmt.Println(a, b) Output: 1 [5 20] 2 3 A tuple assignment happens in two phases .…
A conversion , T(x) , is checked at compile time: the compiler rejects it unless the types are convertible. It may still do work at run time, such as copying bytes for…
case nil behave?func describe(x any) string { switch v := x.(type) { case nil: return "nil interface" case int, int64: // v has type any here, NOT int return fmt.Sprintf("integer %v",…
s := "héllo" fmt.Println(len(s)) fmt.Println(s[1]) fmt.Println(utf8.RuneCountInString(s)) for i, r := range s { fmt.Printf("%d:%c ", i, r) } Output: 6 195 5 0:h 1:é 3:l…
string(65) produce?s := "a\xffb" fmt.Println(len(s)) // 3 for i, r := range s { fmt.Printf("%d:%U ", i, r) } // 0:U+0061 1:U+FFFD 2:U+0062 fmt.Println(string(rune(65))) // "A"…
string and []byte cost, and how do you build strings efficiently?Converting []byte(s) or string(b) normally allocates and copies the data. This is required because strings are immutable and byte slices are not. The compiler skips the…
func main() { x := 1 defer fmt.Println("deferred arg:", x) defer func() { fmt.Println("closure:", x) }() x = 2 fmt.Println("body:", x) } Output: body: 2 closure: 2…
defer inside a loop?Deferred calls are pushed onto a stack for each function call and run in LIFO order when the function returns, whether it returns normally or by panicking. A defer runs…
f() and g() return?func f() (n int) { defer func() { n *= 2 }() return 3 } func g() int { n := 3 defer func() { n *= 2 }() return n } func safeDiv(a, b int) (q int, err error) { defer…
type T struct{ name string } func (t T) ValPrint() { fmt.Println("val:", t.name) } func (t *T) PtrPrint() { fmt.Println("ptr:", t.name) } func main() { t := T{"a"} defer…
recover? Why doesn't defer recover() stop a panic?recover stops a panic only when it is called directly by a deferred function while the goroutine is panicking. In every other situation it returns nil and does nothing.…
os.Exit, log.Fatal, runtime.Goexit and panic?panic: yes. Deferred calls run while the stack unwinds, and that is where recover can happen. runtime.Goexit: yes. It ends the current goroutine and runs its defers.…
package main import "fmt" var a = c + b var b = f() var c = 1 func f() int { fmt.Println("f called, c =", c) return 2 } func init() { fmt.Println("init: a =", a) } func…
init()?Order of initialization: Imported packages are initialized first, depth-first. Each package is initialized exactly once , no matter how many packages import it. Inside a…
break and continue work? Why does a plain break inside select not leave the loop?An unlabeled break ends the innermost for , switch or select . Inside a select or switch that sits in a loop, a plain break leaves only the select or switch , and the…
goto?Go has goto , but with two rules that keep it safe: It can't jump over a variable declaration if the variable would be in scope at the label. That would create a…
switch differ from C, and what does this fallthrough example print?switch x := 2; x { case 1: fmt.Println("one") case 2: fmt.Println("two") fallthrough case 3: fmt.Println("three") fallthrough case 100: fmt.Println("hundred") default:…
Comparable types ( == and != work): booleans, numbers, strings, pointers, channels, interfaces, arrays of comparable elements, and structs whose fields are all…
Two struct values are equal when every field is equal, compared in the order the fields are declared. Unexported fields are compared too. Blank ( _ ) fields are skipped.…
comparable constraint mean in generics, and what changed in Go 1.20?comparable is a built-in constraint that permits == and != on a type parameter, so it can be used as a map key: func Index[T comparable](s []T, v T) int { for i, x :=…
var s Speaker = Dog{} fail to compile?type Speaker interface{ Speak() string } type Dog struct{ name string } func (d *Dog) Speak() string { return d.name + ": woof" } func main() { // var s Speaker =…
An expression is addressable if you can take its address with & . That includes variables, pointer dereferences, slice elements, and fields or elements of addressable…
Embedding promotes the embedded type's fields and methods to the outer type. For a struct S : If S embeds T : the method set of S gets T 's value methods. The method set…
type MyErr struct{} func (*MyErr) Error() string { return "boom" } func do(fail bool) error { var p *MyErr // nil pointer if fail { p = &MyErr{} } return p // BUG:…
type A B) and an alias (type A = B)?type Celsius float64 creates a new, distinct type . It has the same underlying type, but it doesn't inherit the methods of the original named type, and you need explicit…
arr := [3]int{1, 2, 3} for i, v := range arr { if i == 0 { arr[1] = 100 } fmt.Print(v, " ") } fmt.Println() sl := []int{1, 2, 3} for i, v := range sl { if i == 0 { sl[1]…
var x int8 = 127 x++ fmt.Println(x) // -128: signed overflow wraps (two's complement), no panic var u uint8 = 0 u-- fmt.Println(u) // 255 fmt.Println(-7 / 2, -7 % 2) //…
var fns []func() for i := 0; i < 3; i++ { fns = append(fns, func() { fmt.Print(i, " ") }) } for _, f := range fns { f() } var ptrs []*int for i := range 3 { // range…
func Countdown(n int) iter.Seq[int] { // iter.Seq[V] is func(yield func(V) bool) return func(yield func(int) bool) { defer fmt.Println("iterator cleanup") for i := n; i…
type Bad struct { a bool // offset 0, then 7 bytes padding b int64 // offset 8 c bool // offset 16, then 7 bytes tail padding } type Good struct { b int64 // offset 0 a…
type P struct{ X int } func (p P) Get() int { return p.X } func (p *P) Set(v int) { p.X = v } func main() { p := P{1} get := p.Get // method value: a copy of p is bound…
var s uint = 33 var i = 1 << s // 1 takes type int var j int32 = 1 << s // 1 takes type int32: the bit is shifted out, j == 0 var k = uint64(1 << s) // 1 takes type…