我有以下函数给我“变量声明和未使用”错误:
type Comparison struct {
Left []byte
Right []byte
Name string
}
func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) {
key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil)
side := r.FormValue("side")
comparison := new(Comparison)
err := datastore.Get(c, key, comparison)
check(err)
if( side == "left"){
m, _, err := image.Decode(bytes.NewBuffer(comparison.Left))
} else {
m, _, err := image.Decode(bytes.NewBuffer(comparison.Right))
}
check(err)
w.Header().Set("Content-type", "image/jpeg")
jpeg.Encode(w, m, nil)
}
它给了我以下错误:
dpcompare.go:171: m declared and not used
dpcompare.go:171: err declared and not used
dpcompare.go:173: m declared and not used
dpcompare.go:173: err declared and not used
dpcompare.go:178: undefined: m
dpcompare.go:185: key declared and not used
事情是m
,err
和key
都在使用。我无法理解为什么编译器认为你的不是。
答案 0 :(得分:2)
正如@kostix所说,m
是if
范围的本地。试试这段代码
type Comparison struct {
Left []byte
Right []byte
Name string
}
func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) {
key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil)
side := r.FormValue("side")
comparison := new(Comparison)
err := datastore.Get(c, key, comparison)
check(err)
// NOTE! now m is in the function's scope
var m Image
if( side == "left"){
m, _, err = image.Decode(bytes.NewBuffer(comparison.Left))
} else {
m, _, err = image.Decode(bytes.NewBuffer(comparison.Right))
}
check(err)
w.Header().Set("Content-type", "image/jpeg")
jpeg.Encode(w, m, nil)
}
答案 1 :(得分:1)
我认为你在那些if
分支中声明的变量是这些分支的代码块的本地变量。这不是JavaScript(幸运的是)。因此,只需在if
以上的位置声明您的变量,然后使用=
代替:=
分配给它们。