I've got a method taking a target interface{} on a type that I use for database access like:
func (c *client) Query(query someType, target interface{}) error {
return c.db.Query(query).Decode(target)
}
This is then called like
result := resultType{}
if err := c.Query(myQuery, &result); err == nil {
// do sth with result
}
Which does what I want it do as I am passing the pointer address of result
The trouble I am now running into is that I do not know how I can mock this kind of behavior (mutating the passed reference) in a test.
In case I wouldn't need to pass interface{} I could imagine it being done like this:
type mockClient struct {
targetValue resultType
}
func (m *mockClient) Query(query someType, target *resultType) error {
*target = m.targetValue
return nil
}
If I try to do the same using my actual signature, I am not able to dereference the value contained in target like this:
type mockClient struct {
targetValue interface{}
}
func (m *mockClient) Query(query someType, target interface{}) error {
target = m.targetValue // this does not mutate the passed target
return nil
}
Can I dereference a pointer value when it is passed in as the empty interface? In case it is not possible, what would be another approach of testing the side effects my method has without having to resort to concrete types as arguments?
You can use 'reflect' package to do it.
package main
import (
"fmt"
"reflect"
)
type mockClient struct {}
func (m *mockClient) Query(query string, target interface{}) error {
a := "changed"
va := reflect.ValueOf(a)
reflect.ValueOf(target).Elem().Set(va)
return nil
}
func main() {
var mc mockClient
target := "initial"
mc.Query("qwe", &target)
fmt.Println(target)
}
The simple example to reference you can find here
Related
A library foo exposes a type A and a function Fn in that library returns a *A.
I have defined a "wrapper" for A called B:
type B foo.A
Can I convert the *A to a *B without dereferencing the A?
In other words, if I have
a := foo.Fn() // a is a *A
b := B(*a)
return &b
How can I convert the *a to a *b without using *a?
The reason that I ask is that in the library that I am using, github.com/coreos/bbolt, the *DB value returned from the Open function includes a sync.Mutex and so the compiler complains when I try to make a copy of the Mutex.
UPDATE TO EXPLAIN HOW I'LL USE THIS
I have a
type Datastore struct {
*bolt.DB
}
I also have a function (one of many) like this:
func (ds *Datastore) ReadOne(bucket, id string, data interface{}) error {
return ds.View(func(tx *bolt.Tx) error {
b, err := tx.CreateBucketIfNotExists([]byte(bucket))
if err != nil {
return fmt.Errorf("opening bucket %s: %v", bucket, err)
}
bytes := b.Get([]byte(id))
if bytes == nil {
return fmt.Errorf("id %s not found", id)
}
if err := json.Unmarshal(bytes, data); err != nil {
return fmt.Errorf("unmarshalling item: %v", err)
}
return nil
})
}
I would like to mock the underlying BoltDB database using a hash map. I ran into a problem mocking this because of the View expecting a function that takes bolt.Tx. That tx is then used to create a new bucket in CreateBucketIfNotExists. I cannot replace that anonymous function argument with one that calls my hash map mock version of CreateBucketIfNotExists.
I came up with this:
package boltdb
import (
"github.com/coreos/bbolt"
)
type (
bucket bolt.Bucket
// Bucket is a wrapper for bolt.Bucket to facilitate mocking.
Bucket interface {
ForEach(fn func([]byte, []byte) error) error
Get(key []byte) []byte
NextSequence() (uint64, error)
Put(key, value []byte) error
}
db bolt.DB
// DB is a wrapper for bolt.DB to facilitate mocking.
DB interface {
Close() error
Update(fn func(*Tx) error) error
View(fn func(*Tx) error) error
}
transaction bolt.Tx
// Tx is a wrapper for bolt.Tx to facilitate mocking.
Tx interface {
CreateBucketIfNotExists(name []byte) (Bucket, error)
}
)
// ForEach executes a function for each key/value pair in a bucket.
func (b *bucket) ForEach(fn func([]byte, []byte) error) error {
return ((*bolt.Bucket)(b)).ForEach(fn)
}
// Get retrieves the value for a key in the bucket.
func (b *bucket) Get(key []byte) []byte {
return ((*bolt.Bucket)(b)).Get(key)
}
// NextSequence returns an autoincrementing integer for the bucket.
func (b *bucket) NextSequence() (uint64, error) {
return ((*bolt.Bucket)(b)).NextSequence()
}
// Put sets the value for a key in the bucket.
func (b *bucket) Put(key, value []byte) error {
return ((*bolt.Bucket)(b)).Put(key, value)
}
// Close releases all database resources.
func (db *db) Close() error {
return ((*bolt.DB)(db)).Close()
}
// Update executes a function within the context of a read-write managed transaction.
func (db *db) Update(fn func(Tx) error) error {
return ((*bolt.DB)(db)).Update(func(tx *bolt.Tx) error {
t := transaction(*tx)
return fn(&t)
})
}
// View executes a function within the context of a managed read-only transaction.
func (db *db) View(fn func(Tx) error) error {
return ((*bolt.DB)(db)).View(func(tx *bolt.Tx) error {
t := transaction(*tx)
return fn(&t)
})
}
// CreateBucketIfNotExists creates a new bucket if it doesn't already exist.
func (tx *transaction) CreateBucketIfNotExists(name []byte) (Bucket, error) {
b, err := ((*bolt.Tx)(tx)).CreateBucketIfNotExists(name)
if err != nil {
return nil, err
}
w := bucket(*b)
return &w, nil
}
So far, in my code, I am only using the functions shown above. I can add more if new code requires.
I will replace each bolt.DB with DB, bolt.Tx with Tx, and bolt.Bucket with Bucket in the real code. The mocker will use replacements for all three types that use the underlying hash map instead of storing to disk. I can then test all of my code, right down to the database calls.
You can simply / directly convert a value of type *A to a value of type *B, you just have to parenthesize *B:
a := foo.Fn() // a is a *A
b := (*B)(a)
return b
You can even convert the return value of the function call:
return (*B)(foo.Fn())
Try it on the Go Playground.
This is possible, because Spec: Conversions:
A non-constant value x can be converted to type T in any of these cases:
x is assignable to T.
...
And Spec: Assignability:
A value x is assignable to a variable of type T ("x is assignable to T") if one of the following conditions applies:
...
x's type V and T have identical underlying types and at least one of V or T is not a defined type.
Both *B and *A types are not defined, and the underlying type of *B is the same as the underlying type of *A (which is the pointer to the underlying type of whatever type there is in the type declaration of A).
I prefer not to dive into the rationale of the situation below. It has to do with unmarshaling an serialized object that can be any of a fixed set of types, but you don't know which type.
I have the following types:
type I interface {
Do()
}
type someI struct {}
func (i *someI) Do() {}
type otherI struct {}
func (i *otherI) Do() {}
So, two structs of which the pointers implement interface I.
Now I have this method that wants to return a value of type I:
func GetSomeI(marshalled []byte) (I, error) {
var obj interface{}
// The following method magically puts an instance
// of either someI or otherI into obj.
magicUnmarshall(marshalled, obj)
// The problem now is that we cannot return obj,
// because the raw structs don't implement I.
// One solution would be to do a type switch like this:
switch obj.(type) {
case someI:
i := obj.(someI)
return &i, nil
case otherI:
i := obj.(otherI)
return &i, nil
default:
return nil, errors.New("marschalled object was not of type I")
}
// But now consider the case that there are quite some
// different implementations of I.
// We would prefer to have a general way of getting
// a reference to obj.
}
To tell if a value wrapped in an interface{} implements some other interface (I), you may simply use a type assertion.
Note that you must pass the address of the variable you want results unmarshaled to.
For demonstration purposes, let's use the following magicUnmarshal() function:
func magicUnmarshal(what int, obj interface{}) {
v := reflect.ValueOf(obj).Elem()
switch what {
case 0:
v.Set(reflect.ValueOf(&someI{}))
case 1:
v.Set(reflect.ValueOf(&otherI{}))
case 2:
v.Set(reflect.ValueOf("just a string"))
case 3:
v.Set(reflect.ValueOf(someI{}))
case 4:
v.Set(reflect.ValueOf(otherI{}))
}
}
Note that case 3 and case 4 are returning non-pointers.
Your GetSomeI() implementation can be:
func GetSomeI(what int) (I, error) {
var obj interface{}
magicUnmarshal(what, &obj)
// Try the value as-is:
if i, ok := obj.(I); ok {
return i, nil
}
// No success. Try a pointer to the value:
v := reflect.Indirect(reflect.New(reflect.TypeOf(obj)))
v.Set(reflect.ValueOf(obj))
pobj := v.Addr().Interface()
if i, ok := pobj.(I); ok {
return i, nil
}
return nil, fmt.Errorf("%T does not implement I!", obj)
}
First GeSomeI() tests if the value got form magicUnmarshal() implements I, and if so, it is used as-is. If not, we construct a new using reflection, and get its address (a pointer to a value), and we test that. If that pointer implements I, we return it.
Testing it:
func main() {
for what := 0; what < 5; what++ {
i, err := GetSomeI(what)
fmt.Printf("%T %v\n", i, err)
}
}
And the output is (try it on the Go Playground):
*main.someI <nil>
*main.otherI <nil>
<nil> string does not implement I!
*main.someI <nil>
*main.otherI <nil>
Short
The following code does not exactly do what expected:
https://play.golang.org/p/sO4w4I_Lle
I assume that I mess up some pointer/reference stuff as usual, however I expect my...
func unmarshalJSON(in []byte, s interface{}) error
... and encoding/jsons...
func Unmarshal(data []byte, v interface{}) error
...to behave the same way (eg. update the referenced passed as second argument).
Long
The example above is a minimal reproducer that does not make much sense. This is in order to make it work on the playground. However, an less minimal example that does make sense is this:
package main
import (
"fmt"
"gopkg.in/yaml.v2"
)
func unmarshalYAML(in []byte, s interface{}) error {
var result map[interface{}]interface{}
err := yaml.Unmarshal(in, &result)
s = cleanUpInterfaceMap(result)
// s is printed as expected
fmt.Println(s) // map[aoeu:[test aoeu] oaeu:[map[mahl:aoec tase:aoeu]]]
return err
}
func cleanUpInterfaceArray(in []interface{}) []interface{} {
out := make([]interface{}, len(in))
for i, v := range in {
out[i] = cleanUpMapValue(v)
}
return out
}
func cleanUpInterfaceMap(in map[interface{}]interface{}) map[string]interface{} {
out := make(map[string]interface{})
for k, v := range in {
out[fmt.Sprintf("%v", k)] = cleanUpMapValue(v)
}
return out
}
func cleanUpMapValue(v interface{}) interface{} {
switch v := v.(type) {
case []interface{}:
return cleanUpInterfaceArray(v)
case map[interface{}]interface{}:
return cleanUpInterfaceMap(v)
case string:
return v
default:
return fmt.Sprintf("%v", v)
}
}
func main() {
s := make(map[string]interface{})
b := []byte(`---
aoeu:
- test
- aoeu
oaeu:
- { tase: aoeu, mahl: aoec}
`)
err := unmarshalYAML(b, &s)
if err != nil {
panic(err)
}
// s is still an empty map
fmt.Println(s) // map[]
}
The idea is to unmarshal YAML to map[string]interface{} (instead of map[interface{}]interface{}) is order to allow to serialize to JSON (where identifiers need to be strings). The unmarshalYAML function should provide the same func signture as yaml.Unmarshal...
Using Type assertion
Inside your unmarshalJSON() function the parameter s behaves like a local variable. When you assign something to it:
s = result
It will only change the value of the local variable.
Since you want it to work with changing the value of a *map[string]interface{} and that is what you pass to it, you could use a simple type assertion to obtain the map pointer from it, and pass this pointer to json.Unmarshal():
func unmarshalJSON(in []byte, s interface{}) error {
if m, ok := s.(*map[string]interface{}); !ok {
return errors.New("Expecting *map[string]interface{}")
} else {
return json.Unmarshal(in, m)
}
}
Try your modified, working example on the Go Playground.
Just passing it along
Also note that however this is completely unnecessary as json.Unmarshal() is also defined to take the destination as a value of type interface{}, the same thing you have. So you don't even have to do anything just pass it along:
func unmarshalJSON(in []byte, s interface{}) error {
return json.Unmarshal(in, s)
}
Try this on the Go Playground.
With a variable of function type
As an interesting thing note that the signature of your unmarshalJSON() and the library function json.Unmarshal() is identical:
// Yours:
func unmarshalJSON(in []byte, s interface{}) error
// json package
func Unmarshal(data []byte, v interface{}) error
This means there is another option, that is you could use a variable named unmarshalJSON of a function type, and just simply assign the function value json.Unmarshal:
var unmarshalJSON func([]byte, interface{}) error = json.Unmarshal
Now you have a variable unmarshalJSON which is of function type, and you can call it as if it would be a function:
err := unmarshalJSON(b, &s)
Try this function value on the Go Playground.
Now on to your unmarshalYAML() function
In your unmarshalYAML() you do the same mistake:
s = cleanUpInterfaceMap(result)
This will only change the value of your local s variable (parameter), and it will not "populate" the map (pointer) passed to unmarshalYAML().
Use the type assertion technique detailed above to obtain the pointer from the s interface{} argument, and once you have that, you can change the pointed object (the "outside" map).
func unmarshalYAML(in []byte, s interface{}) error {
var dest *map[string]interface{}
var ok bool
if dest, ok = s.(*map[string]interface{}); !ok {
return errors.New("Expecting *map[string]interface{}")
}
var result map[interface{}]interface{}
if err := yaml.Unmarshal(in, &result); err != nil {
return err
}
m := cleanUpInterfaceMap(result)
// m holds the results, dest is the pointer that was passed to us,
// we can just set the pointed object (map):
*dest = m
return nil
}
I'm trying to use reflect to call a method on a struct. However, I'm getting a panic: runtime error: invalid memory address or nil pointer dereference even though both the attachMethodValue and the args are non-nil. Any ideas on what it could be?
Go Playground: http://play.golang.org/p/QSVTSkNKam
package main
import "fmt"
import "reflect"
type UserController struct {
UserModel *UserModel
}
type UserModel struct {
Model
}
type Model struct {
transactionService *TransactionService
}
func (m *Model) Attach(transactionService *TransactionService) {
m.transactionService = transactionService
}
type Transactioner interface {
Attach(transactionService *TransactionService)
}
type TransactionService struct {
}
func main() {
c := &UserController{}
transactionService := &TransactionService{}
valueField := reflect.ValueOf(c).Elem().Field(0) // Should be UserController.UserModel
// Trying to call this
attachMethodValue := valueField.MethodByName("Attach")
// Argument
args := []reflect.Value{reflect.ValueOf(transactionService)}
// They're both non-nil
fmt.Printf("%+v\n", attachMethodValue)
fmt.Println(args)
// PANIC!
attachMethodValue.Call(args)
fmt.Println("The end.")
}
It panics because the UserModel pointer is nil. I think you want:
c := &UserController{UserModel: &UserModel{}}
playground example
I have a pair of interfaces defined like so:
type Marshaler interface {
Marshal() ([]byte, error)
}
type Unmarshaler interface {
Unmarshal([]byte) error
}
I have a simple type which implement these:
type Foo struct{}
func (f *Foo) Marshal() ([]byte, error) {
return json.Marshal(f)
}
func (f *Foo) Unmarshal(data []byte) error {
return json.Unmarshal(data, &f)
}
I am using a library which defines a different interface, and implementing it like so:
func FromDb(target interface{}) { ... }
The value being passed for target is a pointer to pointer:
fmt.Println("%T\n", target) // Prints **main.Foo
Typically this function does a type switch and then operates on the type underneath. I would like to have common code for all types that implement my Unmarshaler interface but can't figure out how to get from a pointer-to-pointer of a specific type to my interface.
You cannot define methods on a pointer to a pointer:
func (f **Foo) Unmarshal(data []byte) error {
return json.Unmarshal(data, f)
}
// compile error: invalid receiver type **Foo (*Foo is an unnamed type)
You cannot define receiver methods on pointer types:
type FooPtr *Foo
func (f *FooPtr) Unmarshal(data []byte) error {
return json.Unmarshal(data, f)
}
// compile error: invalid receiver type FooPtr (FooPtr is a pointer type)
Casting to Unmarshaler doesn't work:
x := target.(Unmarshaler)
// panic: interface conversion: **main.Foo is not main.Unmarshaler: missing method Unmarshal
Casting to *Unmarshaler doesn't work either:
x := target.(*Unmarshaler)
// panic: interface conversion: interface is **main.Foo, not *main.Unmarshaler
How can I get from this pointer-to-pointer type to my interface type without needing to switch on every possible implementor type?
It's ugly, but is is possible to have the semantic equivalent of a pointer to pointer receiver. For example:
package main
import "fmt"
type P *int
type W struct{ p P }
func (w *W) foo() {
fmt.Println(*w.p)
}
func main() {
var p P = new(int)
*p = 42
w := W{p}
w.foo()
}
Playground
Output:
42
Note that
fmt.Println(*w.p)
above is actually
fmt.Println(*(*w).p)
where the extra work is done for you by the compiler automagically.
If target is a **Foo and *Foo implements Unmarshaler, you can do:
var x Unmarshaler = *target
If target is a an interface{} containing a **Foo, this would work instead:
var x Unmarshaler = *(target.(**Foo))
It's a similar idea if you have a type switch.
Whenever I have to deal with pointer-to-pointer or pointer-to-reference, the variable life-time is generally pretty short and they quickly go back to being a single pointer (or plain reference) again.
I'd evaluate whether the pointer-to-pointer is indeed required for the use case at hand or whether you can do something like that.