英文:
Why is "err != nil"?
问题
我有一个情况，变量"err error"的值只能是"nil"，但是一旦被重新赋值，断言"(err == nil) == false"就会出现。
以下是示例代码：
package main
import (
"fmt"
"log"
)
type TestError struct {
Message string
}
func (e *TestError) Error() string {
return e.Message
}
func NewTestError(err error) *TestError {
if err == nil {
return nil
}
log.Printf("NewTestError( ... ): creating new NewTestError err = %s", err)
return &TestError{Message: err.Error()}
}
func main() {
var err error
_, err = fmt.Printf("excuse.\n")
err = NewTestError(err)
log.Printf("main( ... ): err == nil. %v", (err == nil))
log.Printf("main( ... ): err = %#v", err)
}
我从上面的代码中得到以下输出：
excuse.
2015/07/30 08:28:28 main( ... ): err == nil. false
2015/07/30 08:28:28 main( ... ): err = (*main.TestError)(nil)
这两行输出是如何产生的呢？
英文:
I have a situation where the value of a variable "err error" value can only be "nil" but asserts "( err == nil ) == false" once it has been reassigned.
Example code below:
package main
import (
"fmt"
"log"
)
type TestError struct {
Message string
}
func (e *TestError) Error() string {
return e.Message
}
func NewTestError(err error) *TestError {
if err == nil {
return nil
}
log.Printf("NewTestError( ... ): creating new NewTestError err = %s", err)
return &TestError{Message: err.Error()}
}
func main() {
var err error
_, err = fmt.Printf("excuse.\n")
err = NewTestError(err)
log.Printf("main( ... ): err == nil. %v", (err == nil))
log.Printf("main( ... ): err = %#v", err)
}
I get the following output from the above code:
excuse.
2015/07/30 08:28:28 main( ... ): err == nil. false
2015/07/30 08:28:28 main( ... ): err = (*main.TestError)(nil)
How can those last two lines be output?
答案1
得分: 2
对于一个接口值（比如一个error值）与nil相等，实际包含的值和实际包含的类型都必须是nil。在你的情况下，你有一个接口值，它持有一个nil指针，但是类型是*TestError（即非nil）。
PS. 以防不清楚这些“接口值”是什么，你可以查看这个链接：http://research.swtch.com/interfaces
英文:
For an interface value (like an error value) to compare equal to nil, both the actual contained value and the actual contained type have to be nil. In your case you have an interface value, which holds a nil pointer, but a *TestError (i.e. non-nil) type.
PS. Just in case it's not clear what are these "interface values", you can check this http://research.swtch.com/interfaces
答案2
得分: 0
在第二种情况下，你打印的是接口实例的信息，包括类型和值，类型是(*main.TestError)，值是(nil)。而在第一种情况下，你实际上比较的不是nil，因为它也是接口。
英文:
In the second case you're printing the interface instances information which is the type and the value, type being (*main.TestError) value being (nil). In the first case what you're actually comparing isn't nil because it's also the interface.
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