英文:
Why does returning a type that implements error interface call the Error() method automatically?
问题
链接：
https://play.golang.org/p/z50pUnAe4q
package main
import (
"fmt"
"time"
)
type MyError struct {
When time.Time
What string
}
func (e *MyError) Error() string {
return fmt.Sprintf("at %v, %s",
e.When, e.What)
}
func run() error {
return &MyError{
time.Now(),
"it didn't work",
}
}
func main() {
if err := run(); err != nil {
fmt.Println(err)
}
}
我理解内置类型error的定义如下：
type error interface {
Error() string
}
这意味着MyError实现了error接口。
run()方法返回了一个指向MyError类型的指针。
但是，run()方法在方法签名中的返回类型是error类型。
那么，run()方法中的返回语句是如何自动调用Error()方法的呢？
英文:
Link:
https://play.golang.org/p/z50pUnAe4q
package main
import (
"fmt"
"time"
)
type MyError struct {
When time.Time
What string
}
func (e *MyError) Error() string {
return fmt.Sprintf("at %v, %s",
e.When, e.What)
}
func run() error {
return &MyError{
time.Now(),
"it didn't work",
}
}
func main() {
if err := run(); err != nil {
fmt.Println(err)
}
}
I understand that the built in type error looks like this,
type error interface {
Error() string
}
Meaning MyError implements error.
The run() method returns a pointer to MyError type.
But the return type of run() method in the method signature is of type error.
How does the return statement in run() automatically call the Error() method?
答案1
得分: 3
在这里，fmt.Println() 负责调用 Error() 方法，而不是 run()。
引用自文档：
fmt包通过调用错误值的Error()方法来格式化错误值的字符串。错误实现的责任是总结上下文。
os.Open返回的错误格式为 "open /etc/passwd: permission denied"，而不仅仅是 "permission denied"。我们的Sqrt返回的错误缺少关于无效参数的信息。
因此，当你尝试在 main() 中打印 err 时，它会调用其 Error() 方法。
将其替换为其他内容，然后你就不会在输出中看到它：https://play.golang.org/p/IqScH02iGu
英文:
Here fmt.Println() is responsible for calling Error() method not run().
Quoting from docs:
> The fmt package formats an error value by calling its Error() string
> method.
>
> It is the error implementation's responsibility to summarize the
> context. The error returned by os.Open formats as "open /etc/passwd:
> permission denied," not just "permission denied." The error returned
> by our Sqrt is missing information about the invalid argument.
Hence, when you're trying to print err inside main() its Error() method is getting invoked.
Replace it with something else, then you won't see it in output: https://play.golang.org/p/IqScH02iGu
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论