[英]golang type conversion not working as (I) expected
I'm using go-flags to parse command line options. 我正在使用go-flags来解析命令行选项。
Per the go-flags docs: "... [if] either -h or --help was specified in the command line arguments, a help message will be automatically printed. Furthermore, the special error type ErrHelp is returned." 根据go-flags文档: “... [if] -h或--help在命令行参数中指定,将自动打印一条帮助消息。此外,返回特殊错误类型ErrHelp。”
The method I'm calling is: 我打电话的方法是:
func (p *Parser) Parse() ([]string, error) {
I'm calling it with: 我打电话给:
var opts struct {
// ...
}
func main() {
parser := flags.NewParser(&opts, flags.Default)
args, err := parser.Parse()
A snippet from the file that defines ErrHelp looks like this: 定义ErrHelp的文件的片段如下所示:
type ErrorType uint
const (
// Unknown or generic error
ErrUnknown ErrorType = iota
// Expected an argument but got none
ErrExpectedArgument
// ...
// The error contains the builtin help message
ErrHelp
// ...
)
// Error represents a parser error. The error returned from Parse is of this
// type. The error contains both a Type and Message.
type Error struct {
// The type of error
Type ErrorType
// The error message
Message string
}
// Get the errors error message.
func (e *Error) Error() string {
return e.Message
}
func newError(tp ErrorType, message string) *Error {
return &Error{
Type: tp,
Message: message,
}
}
So they have this custom "Error" type. 所以他们有这种自定义的“错误”类型。 And in the Parse() method above, internally, the error is getting created with a block of code like this:
在上面的Parse()方法中,在内部,错误是通过如下代码块创建的:
help.ShowHelp = func() error {
var b bytes.Buffer
p.WriteHelp(&b)
return newError(ErrHelp, b.String())
}
As you can see newError() returns "*Error" as it's type. 正如您所看到的,newError()返回“* Error”作为其类型。 But the anonymous function just above returns type "error" - so those types must be compatible.(?)
但上面的匿名函数返回类型“错误” - 所以这些类型必须兼容。(?)
But now back to the original problem - I'm just trying to see if my "err" is an "Error" and has member "Type" equal to ErrHelp. 但现在回到原来的问题 - 我只是想看看我的“错误”是否是“错误”并且成员“类型”等于ErrHelp。 So I try this:
所以我试试这个:
if err != nil && flags.Error(err).Type == flags.ErrHelp {
Or even just this: 或者就是这样:
fmt.Printf("test:", flags.Error(err))
And either way the compiler gives me: 无论哪种方式编译器给我:
main.go:37: cannot convert err (type error) to type flags.Error
main.go:37:无法将err(类型错误)转换为flags.Error类型
But does not state why that conversion can't be done. 但是没有说明为什么不能进行转换。 Any ideas?
有任何想法吗?
(I don't get how "*Error" successfully converts to "error" in the anonymous function above, and I even more don't get why if that works then I can't convert it back the other way... I must be missing something really dumb here, but I'm not seeing what it is.) (我不知道“* Error”如何在上面的匿名函数中成功转换为“错误”,我甚至更不明白为什么如果有效,那么我无法将其转换回来...我我必须遗漏一些真正愚蠢的东西,但我不知道它是什么。)
An error
is an interface with a single method Error() string
. error
是具有单个方法Error() string
的接口。 See http://golang.org/pkg/builtin/#error 请参阅http://golang.org/pkg/builtin/#error
A flags.Error
has such a method, so it can be used as an error
. flags.Error
有这样的方法,所以它可以用作error
。
Conversely, however, a flags.Error
is a struct, and there's no way to convert an arbitrary value to a struct. 但是,相反,
flags.Error
是一个结构,并且无法将任意值转换为结构。
What you can do is, and I think this is the answer to your question, is that if you've got a flags.Value
inside an error
, then you can cast the error
back to the underlying type. 你可以做的是,我认为这是你的问题的答案,如果你有一个
flags.Value
error
内的值,那么你可以将error
转回基础类型。 The syntax for that is e := err.(*flags.Error)
. 其语法是
e := err.(*flags.Error)
。 This'll give you a value of type *flags.Error
(or panic, if the underlying type isn't *flags.Error
). 这将为您提供类型
*flags.Error
的值(如果底层类型不是*flags.Error
)。 You can avoid the panic in this case by using the comma-ok form, which is e, ok := err.(*flags.Error)
. 在这种情况下,您可以使用逗号表单来避免恐慌,即:
e, ok := err.(*flags.Error)
。
Concretely, you would write: 具体来说,你会写:
args, err := flags.Parse()
if err != nil {
if ferr, ok := err.(*flags.Error); ok {
// ... something using ferr
} else {
// ... deal with non-flags.Error case, if that's possible.
}
}
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