Encoding nil pointer to struct containing exported function leads to marshalling error

[timoreimann]

I believe to have discovered an inconsistency in behavior between json-iterator and stdlib's encoding/json package.

Specifically, if you try to marshal a pointer to a struct which contains a function field, json-iterator returns an error from Marshal whereas the same method on encoding/json does not. To better illustrate, I have put together an example repository where the main.go file looks like this:

package main

import (
	encjson "encoding/json"
	"fmt"
	"os"

	"github.com/json-iterator/go"
)

type str struct {
	F func()
}

func main() {
	var s *str = nil

	var json = jsoniter.ConfigCompatibleWithStandardLibrary
	if _, err := json.Marshal(s); err != nil {
		fmt.Fprintf(os.Stderr, "json-iterator: marshalling failed: %s\n", err)
	} else {
		fmt.Println("json-iterator: marshalling succeeded.")
	}

	if _, err := encjson.Marshal(s); err != nil {
		fmt.Fprintf(os.Stderr, "encoding/json: marshalling failed: %s\n", err)
	} else {
		fmt.Println("encoding/json: marshalling succeeded.")
	}
}

Executing the application, the output is:

$ go run main.go
json-iterator: marshalling failed: [optional]: [main.str]: unsupported type: func()
encoding/json: marshalling succeeded.

Note that adding tags to ignore the function field might work in this artificial example but not in the general case. In fact, I ran into this issue while trying to marshal a struct holding a pointer to a tls.Config struct. This struct holds a Time func() time.Time field and produces the following error message during Marshal:

[tls.Config]: unsupported type: func() time.Time

Obviously, I cannot add tags to a struct embedded in the stdlib.

[timoreimann]

@taowen thanks for the prompt fix!