Question: Why `jwt.ParseWithClaims` needs to pass a pointer of an anonymous struct ?

[niluan304]

jwt/example_test.go

Lines 77 to 99 in bc8bdca

	// Example creating a token using a custom claims type. The RegisteredClaims is embedded
	// in the custom type to allow for easy encoding, parsing and validation of standard claims.
	func ExampleParseWithClaims_customClaimsType() {
	tokenString := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmb28iOiJiYXIiLCJpc3MiOiJ0ZXN0IiwiYXVkIjoic2luZ2xlIn0.QAWg1vGvnqRuCFTMcPkjZljXHh8U3L_qUjszOtQbeaA"
	type MyCustomClaims struct {
	Foo string `json:"foo"`
	jwt.RegisteredClaims
	}
	token, err := jwt.ParseWithClaims(tokenString, &MyCustomClaims{}, func(token *jwt.Token) (interface{}, error) {
	return []byte("AllYourBase"), nil
	})
	if err != nil {
	log.Fatal(err)
	} else if claims, ok := token.Claims.(*MyCustomClaims); ok {
	fmt.Println(claims.Foo, claims.RegisteredClaims.Issuer)
	} else {
	log.Fatal("unknown claims type, cannot proceed")
	}
	// Output: bar test
	}

if use pointer of myCustomClaims, can skip type assertion claims, ok := token.Claims.(*MyCustomClaims)

func ExampleParseWithClaims_customClaimsType() {
	tokenString := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmb28iOiJiYXIiLCJpc3MiOiJ0ZXN0IiwiYXVkIjoic2luZ2xlIn0.QAWg1vGvnqRuCFTMcPkjZljXHh8U3L_qUjszOtQbeaA"

	type MyCustomClaims struct {
		Foo string `json:"foo"`
		jwt.RegisteredClaims
	}

	var myCustomClaims MyCustomClaims
	token, err := jwt.ParseWithClaims(tokenString, &myCustomClaims, func(token *jwt.Token) (interface{}, error) {
		return []byte("AllYourBase"), nil
	})
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(claims.Foo, claims.RegisteredClaims.Issuer)

	// Output: bar test
}

What is the difference?

[godcong]

Don't pay too much attention to that.
The only reason to pass this in is for type reflection, since it supports maps and custom structs.
So it must know what this is.