Can structs be nil?
m
see if a struct is nil or not.
s := GetSiteUser(username, password)
if s == nil {
return "Doesn't exist"
}
When I make, this happens:
cannot use nil as SiteUser
Clark Gaebel
--
Regards,
-Clark
m
out. The variable name or the type. Also, what is "&" mean.
Andrew Gerrand
The zero-value of a struct type is the struct with all its elements
set to their zero values.
Eg,
type T struct {
a int
b string
c *byte
}
var t T
// t == T{0, "", nil}
A value that is not of pointer, channel, map, or slice type cannot be nil.
Andrew
Andrew Gerrand
> I honestly don't get pointers. Never used C. What am I pointing
> out. The variable name or the type. Also, what is "&" mean.
A pointer a memory address of a piece of data. Here is a video that
explains it all:
http://www.youtube.com/watch?v=UvoHwFvAvQE
But in short:
var i int = 1
var p *int = &i
i is a variable of type int with value 1
p is a variable of type pointer-to-int with value [memory address of i]
& means "address of" - it returns the memory address of a value.
* has two meanings, either "pointer to" (in a type definition) or
"dereference" (in an expression).
Dereferencing means "return the value at". Continuing the example above:
var j int = *p
j is a variable of type int with the value at the memory address p. In
this case, 1.
nil is the zero-value of a pointer type. (it doesn't point to anything)
Andrew
m
handle on it. What do I need to do for my code then? Does
GetSiteUser(username, password) need to return a *SiteUser?
On Aug 3, 8:55 pm, Andrew Gerrand <a...@golang.org> wrote:
m
actually run the code things blow up spectacularly.
panic: runtime error: invalid memory address or nil pointer
dereference
✖ chris tighe ✖
Clark Gaebel
points to, literally, nothing. The object at the other end will be
garbage collected and any attempts to dereference will (as you saw) blow up.
--
Regards,
-Clark
m
package dbhelper
import (
mongo "github.com/mikejs/gomongo/mongo"
"io"
"fmt"
"crypto/sha256"
"rand"
"strconv"
)
type SiteUser struct {
ID int64;
Username string;
Passhash string;
Salt int64;
}
func LoginSiteUser(user string, password string) (SiteUser, bool) {
// Get the existing user from the database.
e := GetSiteUserByUsername(user)
// If the site user is empty, return.
if e.Username == "" {
n := &SiteUser{Username:""}
return *n, false
}
// Otherwise, check the password.
hash := createHashedPass(password, e.Salt)
fmt.Println("New hash: " + hash + "\nOld hash: " + e.Passhash)
if hash != e.Passhash {
n := new(SiteUser)
return *n, false
}
// If we've gotten this far, successful login.
return *e, true
}
func GetSiteUserByUsername(user string) *SiteUser {
conn, _ := mongo.Connect("127.0.0.1")
coll := conn.GetDB("mysite_dev").GetCollection("siteuser")
// Query the db.
qFindDoc, _ := mongo.Marshal(&SiteUser{Username:user})
bsonDocOut, _ := coll.FindOne(qFindDoc)
var foundUser *SiteUser
mongo.Unmarshal(bsonDocOut.Bytes(), &foundUser)
return foundUser
}
func CreateSiteUser(user string, password string) {
e := GetSiteUserByUsername(user)
if e.Username != "" { return }
salt := createSalt()
hash := createHashedPass(password, salt)
// Now create a new instance of the SiteUser struct.
s := new(SiteUser)
s.Username = user
s.Passhash = hash
s.Salt = salt
// And save the results to the database.
conn, _ := mongo.Connect("127.0.0.1")
coll := conn.GetDB("mysite_dev").GetCollection("siteuser")
bsonDocIn, _ := mongo.Marshal(s)
coll.Insert(bsonDocIn)
}
func createSalt() int64 {
// This function only creates a salt to user. Nothing more.
return rand.Int63()
}
func createHashedPass(password string, salt int64) string {
// Use sha256 to create the hashed password.
b := sha256.New()
io.WriteString(b, password + strconv.Itob64(salt, 10))
hash := fmt.Sprintf("%x", b.Sum())
return hash
Clark Gaebel
My guess is that...
func GetSiteUserByUsername(user string) *SiteUser {
conn, _ := mongo.Connect("127.0.0.1")
coll := conn.GetDB("mysite_dev").GetCollection("siteuser")
// Query the db.
qFindDoc, _ := mongo.Marshal(&SiteUser{Username:user})
bsonDocOut, _ := coll.FindOne(qFindDoc)
var foundUser *SiteUser
mongo.Unmarshal(bsonDocOut.Bytes(), &foundUser)
return foundUser
}
Needs to be...
func GetSiteUserByUsername(user string) *SiteUser {
conn, _ := mongo.Connect("127.0.0.1")
coll := conn.GetDB("mysite_dev").GetCollection("siteuser")
// Query the db.
qFindDoc, _ := mongo.Marshal(&SiteUser{Username:user})
bsonDocOut, _ := coll.FindOne(qFindDoc)
var foundUser SiteUser
mongo.Unmarshal(bsonDocOut.Bytes(), &foundUser)
return &foundUser
}
--
Regards,
-Clark
✖ chris tighe ✖
Well, you see, you can't actually DEREFERENCE the nil pointer. It
points to, literally, nothing. The object at the other end will be
garbage collected and any attempts to dereference will (as you saw) blow up.
Clark Gaebel
It's all terminology.
On 08/03/10 21:32, ✖ chris tighe ✖ wrote:
--
Regards,
-Clark
m
code. I don't understand what dereferencing means.
Clark Gaebel
or do *pointer, you will dereference it. Since, by definition, it
doesn't point to anything, it will error out on you.
Oh how I miss the days of segfaults.
--
Regards,
-Clark
m
parameter? Either?
Clark Gaebel
var someObject *Object // Initialized to nil
someObject.SomeMethod()
--
Regards,
-Clark
konrad
func GetSiteUserByUsername(user string) *SiteUser {
conn, _ := mongo.Connect("127.0.0.1")
coll := conn.GetDB("mysite_dev").GetCollection("siteuser")
// Query the db.
qFindDoc, _ := mongo.Marshal(&SiteUser{Username:user})
bsonDocOut, _ := coll.FindOne(qFindDoc)
mongo.Unmarshal(bsonDocOut.Bytes(), foundUser)
return foundUser
Kees
a pointer type with *, but when you dereference, or "depointerize",
you use * as well. Would have made more sense to use the same symbols
for declarations and referencing, not declarations and dereferencing.
Or just use 3 different symbols altogether.
Kees
On Aug 4, 12:55 pm, Andrew Gerrand <a...@golang.org> wrote:
Jessta
> I always feel that pointer syntax is the wrong way around. You define
> a pointer type with *, but when you dereference, or "depointerize",
> you use * as well. Would have made more sense to use the same symbols
> for declarations and referencing, not declarations and dereferencing.
> Or just use 3 different symbols altogether.
In C it makes a lot of sense,
int *age; says, if you deference age you'll get a int
this is commonly miswritten as
int* age; with int* being referred to a type of 'pointer to int'
The Go syntax plays on this misunderstanding and changing it would be
be confusing to past C and C++ users without reason.
var age *int
- jessta
--
=====================
http://jessta.id.au
Andrew Gerrand
Rob wrote a good blog post on this topic:
http://blog.golang.org/2010/07/gos-declaration-syntax.html
Andrew
roger peppe
> If you have a pointer to nil, and you try to call any method with it,
> doesn't point to anything, it will error out on you.
actually that's a common misunderstanding.
calling a method on a nil pointer does not necessarily
dereference the pointer. because the pointer has a known
type, the runtime knows which method to call without
looking inside the pointer. the method will get be called with a
receiver value.
one exception is when a value method is called on
a pointer value, in which case the runtime needs to
dereference the value in order to pass it to the method.
interfaces are not pointers, even though they can
be nil - calling a method on a nil interface
*will* panic.
Russ Cox
> I honestly don't get pointers. Never used C. What am I pointing
> out. The variable name or the type. Also, what is "&" mean.
http://www.cs.stanford.edu/cslibrary/PointerFunCBig.avi
(If you can ignore the fact that clay is talking to you,
it does do a good job of explaining what pointers are.)
Russ