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代码A：
package main
import "fmt"
func main() {
slice := IntSlice{0,1,2,3,4,5,6,7,8,9}
fmt.Println(slice)
}
type IntSlice []int
输出A：
[0 1 2 3 4 5 6 7 8 9]
代码B：
package main
import "fmt"
func main() {
slice := IntSlice{0,9}
fmt.Println(slice)
}
type IntSlice []int
func (slice IntSlice) Error() string { return "this is called." }
输出B：
this is called.
为什么fmt.Println(切片)的行为对于这两个代码(A和B)是不同的？
或者为什么fmt.Println(slice)会自动调用slice.Error()？
解决方法
这是
documented作为fmt行为的一部分(强调我的)：
Except when printed using the verbs %T and %p,special formatting considerations apply for operands that implement certain interfaces. In order of application:
-
If the operand is a reflect.Value,the operand is replaced by the concrete value that it holds,and printing continues with the next rule.
-
If an operand implements the Formatter interface,it will be invoked. Formatter provides fine control of formatting.
-
If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface,that will be invoked. If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X),the following two rules apply:
-
If an operand implements the error interface,the Error method will be invoked to convert the object to a string,which will then be formatted as required by the verb (if any).
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If an operand implements method String() string,that method will be invoked to convert the object to a string,which will then be formatted as required by the verb (if any).
For compound operands such as slices and structs,the format applies to the elements of each operand,recursively,not to the operand as a whole. Thus %q will quote each element of a slice of strings,and %6.2f will control formatting for each element of a floating-point array.
fmt包看到切片实现错误并打印value.Error(),而不是迭代切片并对每个元素应用格式.
（编辑：李大同）
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