Golang Slice index value or reference?

php editor Xigua introduces to you the Slice index problem in Golang. In Golang, Slice is a flexible dynamic array that can easily add, delete, modify and query data. But when using Slice, you sometimes encounter indexing problems, that is, whether the indexed value is a copy of the value or a reference. Understanding the difference between the value of an index and a reference is crucial to using Slice correctly. Next, we will explore this issue in detail to help you better understand and apply Slice.

Question content

I'm learning go and am confused by the "stuff" we get from slice indexing. Suppose we have a structure of type bag:

type bag struct {
    item string
}

Then, let’s say we have a luggage list:

itembag := []bag{
    {item: "item1"},
    {item: "item2"},
    {item: "item3"}
}

Now I try to change the content of the first element of the itembag variable. Naively, I did the following:

item1 := itembag[0]
item1.item = "not item1"

fmt.printf("itembag[0].item == 'not item1'? %v", itembag[0].item == "not item1")

// console print false

The answer I got is wrong because (I guess, please correct me if I'm wrong) when we do item1 := itembag[0] we are copying the itembag [0]The value of and assign it to item1. I believe this is what this blog is about.

To prove this, I tried to get the pointer of itembag[0] and modify its value, and voila, I achieved what I wanted to do:

item1 := &itembag[0]
item1.item = "not item1"

fmt.printf("itembag[0].item == 'not item1'? %v", itembag[0].item == "not item1")

// console print true

This makes sense if we say that when we do a slice index (i.e. slice[index]), we will get a copy of its value instead of the underlying item.

Now when I do the following:

itemBag[0].item = "not item1"

fmt.Printf("itemBag[0].item == 'not item1'? %v", itemBag[0].item == "not item1")

The result is true, which is not what I expected. Because it looks like itembag[0].item is syntactically the same as if I first assigned itembag[0] to a variable and then referenced its underlying item.

I feel like I'm missing an essential chapter in the golang course, please, any redirects/explanations would be greatly appreciated!

Tried googling something like "pass slice index by reference" but couldn't pinpoint the exact keyword on google.

Solution

The assignment operation copies the value. So when you do:

item1 = itembag[0]

You create a copy of the object at itembag[0] with type bag. Now item1 has a copy of it, and any modifications you make to it will be made on that copy.

itemptr = &itembag[0]

The right-hand side of the assignment is a pointer, so this operation creates a copy of that pointer and assigns it to itemper. When itempr.item="x" is executed, it is equivalent to (*itempr).item=x, so the content of the pointer is modified.

itembag[0] is an addressable value, i.e. you can get its address.

https://www.php.cn/link/753a043674f0193523abc1bbce678686

Expression itembag[0].item is also an addressable value, so you can assign something to it and it will be reflected in the slice itself.

However, the following are not addressable values:

m:=map[int]bag{1:bag{item:"str"}}
m[1].item="str1"

This is because m[1] returns a copy of the value in the map, and the value assigned to it will be lost. However, the following problems can be solved:

m:=map[int]*bag{1:&bag{item:"str"}}
m[1].item="str1"

This will set the item field of bag in the map.

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