go nil 源码
golang nil 代码
文件路径：/test/nil.go
// run
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Test nil.
package main
import (
"fmt"
"time"
)
type T struct {
i int
}
type IN interface{}
func main() {
var i *int
var f *float32
var s *string
var m map[float32]*int
var c chan int
var t *T
var in IN
var ta []IN
i = nil
f = nil
s = nil
m = nil
c = nil
t = nil
i = nil
ta = make([]IN, 1)
ta[0] = nil
_, _, _, _, _, _, _, _ = i, f, s, m, c, t, in, ta
arraytest()
chantest()
maptest()
slicetest()
}
func shouldPanic(f func()) {
defer func() {
if recover() == nil {
panic("not panicking")
}
}()
f()
}
func shouldBlock(f func()) {
go func() {
f()
panic("did not block")
}()
time.Sleep(1e7)
}
// nil array pointer
func arraytest() {
var p *[10]int
// Looping over indices is fine.
s := 0
for i := range p {
s += i
}
if s != 45 {
panic(s)
}
s = 0
for i := 0; i < len(p); i++ {
s += i
}
if s != 45 {
panic(s)
}
// Looping over values is not.
shouldPanic(func() {
for i, v := range p {
s += i + v
}
})
shouldPanic(func() {
for i := 0; i < len(p); i++ {
s += p[i]
}
})
}
// nil channel
// select tests already handle select on nil channel
func chantest() {
var ch chan int
// nil channel is never ready
shouldBlock(func() {
ch <- 1
})
shouldBlock(func() {
<-ch
})
shouldBlock(func() {
x, ok := <-ch
println(x, ok) // unreachable
})
if len(ch) != 0 {
panic(len(ch))
}
if cap(ch) != 0 {
panic(cap(ch))
}
}
// nil map
func maptest() {
var m map[int]int
// nil map appears empty
if len(m) != 0 {
panic(len(m))
}
if m[1] != 0 {
panic(m[1])
}
if x, ok := m[1]; x != 0 || ok {
panic(fmt.Sprint(x, ok))
}
for k, v := range m {
panic(k)
panic(v)
}
// can delete (non-existent) entries
delete(m, 2)
// but cannot be written to
shouldPanic(func() {
m[2] = 3
})
}
// nil slice
func slicetest() {
var x []int
// nil slice is just a 0-element slice.
if len(x) != 0 {
panic(len(x))
}
if cap(x) != 0 {
panic(cap(x))
}
// no 0-element slices can be read from or written to
var s int
shouldPanic(func() {
s += x[1]
})
shouldPanic(func() {
x[2] = s
})
}
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