When printing a struct with nested struct which has a String() implemented, the %v format prints an 'unexpected' value per our understanding.
Below is the code snippet.
package main
import (
"fmt"
)
type Inner struct {
}
type A struct {
Inner
FieldA string
}
func (i Inner) String() string {
return "anything"
}
func main() {
myA := A{FieldA: "A"}
fmt.Printf("%v", myA)
}
We expect the output to be
{anything A}
But the actual result is
anything
Why this result? It seems FieldA is ignored? To make it more confusing, if we have two nested structs where both of them have String() implemented, the output is expected.
package main
import (
"fmt"
)
type Inner struct {
}
type InnerAgain struct {
}
type A struct {
Inner
InnerAgain
FieldA string
}
func (i Inner) String() string {
return "anything"
}
func (i InnerAgain) String() string {
return "nothing"
}
func main() {
myA := A{FieldA: "A"}
fmt.Printf("%v", myA)
}
The output is
{anything nothing A}
...
Since you are embedding Inner, you inherent all its properties - including the String() - function. So %v is actually calling Inner.String()
From the docs (https://golang.org/pkg/fmt/) :
- 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).
Because the fmt package states the rules for %v as:
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).
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).
Hence, %v actually uses the String() method for arguments that are stringers.