Why assign a reference to a struct in go?

Question

I'm having a look at the code at this page:

And there's one thing I don't understand - at some point, a new structure is created and initialized like this:

client := &http.Client{
    CheckRedirect: redirectPolicyFunc,
}

Why use & when creating this structure?

I've also read this blog post and structs are initialized like this:

r := Rectangle{}

What is the difference between both and how should I know which one to use?

the word reference is confusing in regard to its usage within the spec. golang.org/ref/spec#Package_initialization A reference to a variable or function is an identifier denoting that variable or function.. Go does have a concept of pointers, although. — user4466350
– user4466350, Commented Sep 28, 2021 at 21:40

Denys Séguret · Accepted Answer · 2013-02-02 16:58:17Z

11

The difference is in the type of your variable.

client := &http.Client{

makes client of type *http.Client

while

client := http.Client{

builds a http.Client.

answered Feb 2, 2013 at 16:58

Denys Séguret

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2 Comments

laurent Over a year ago

I understand the difference between pointers and values but in this particular example, why did they create a pointer? Why not call client := http.Client? Are there benefit from building it using client := &http.Client?

Denys Séguret Over a year ago

A pointer is light. When you don't have a pointer, you copy the structure each time you assign it to a variable. Which means you might do operations on copies of your structure instead of your original structure and which might be heavy. You should use a pointer for most complex structure. See also this related question.

masebase · Accepted Answer · 2013-02-02 16:18:48Z

3

The top one is returning a pointer. It is a Go idiom instead of using new. The second one is just a value object. If you need a pointer use the top.

Check the effective go doc for more about this

http://golang.org/doc/effective_go.html#allocation_new

answered Feb 2, 2013 at 16:18

masebase

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Comments

newacct · Accepted Answer · 2013-02-03 02:51:25Z

In object-oriented programming, in order for an object to have dynamic lifetime (i.e. not tied to the current function call), it needs to be dynamically allocated in a place other than the current stack frame, thus you manipulate the object through a pointer. This is such a common pattern that in many object-oriented languages, including Java, Python, Ruby, Objective-C, Smalltalk, JavaScript, and others, you can only deal with pointers to objects, never with an "object as a value" itself. (Some languages though, like C++, do allow you to have "objects as values"; it comes with the RAII idiom which adds some complexity.)

Go is not an object-oriented language, but its ability to define custom types and define methods that operates on that custom type, can be made to work very much like classes and methods. Returning a pointer to the type from the "constructor" function allows the "object" to have a dynamic lifetime.

Sajidur Rahman · Accepted Answer · 2021-09-28 22:14:01Z

1

When we use reference, we use a single item throughout the program runtime. Even if we assign that to a new variable or pass through a function. But when we use value, we make new copies of individual items.

( Reference is not right word according to golang convention. "Address of value" would be more appropriate here https://golang.org/ref/spec#Package_initialization )

An example will make it much clear I hope.

type Employee struct {
    ID      int
    Name    string
    Address string
}

func main() {

    andy := &Employee{}

    andy.Name = "Andy"

    brad := andy

    brad.Name = "Brad"

    fmt.Println(andy.Name)
}

The result of this code block would be:

Brad

As we made new variable from it but still referring to same data. But if we use value instead of reference and keep the rest of the code same.

// from 
andy := &Employee{}
// to
andy := Employee{}

This time the result would be:

Andy

As this time they both are individual items and not referring to same data anymore.

edited Sep 28, 2021 at 22:14

answered Sep 28, 2021 at 19:13

Sajidur Rahman

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2 Comments

user4466350 Over a year ago

the word reference is confusing in regard to its usage within the spec. golang.org/ref/spec#Package_initialization A reference to a variable or function is an identifier denoting that variable or function.. Go does have a concept of pointers, although.

Sajidur Rahman Over a year ago

You are right. Thanks for pointing out. I'll update accordingly. Though current one would make sense to people understands c/c++, but it's not correct according to Go.

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