cannot assign to an element of an array which is returned from a function #12650

bigwhite · 2015-09-17T03:16:48Z

I' m doubting about the code below:

package main

func getSlice() []int {
    var a [5]int
    return a[:]
}

func getArray() [5]int {
    var a [5]int
    return a
}

func getPointerToArray() *[5]int {
    var a [5]int
    return &a
}

func main() {
    getSlice()[0] = 1
    getArray()[0] = 1
    getPointerToArray()[0] = 1
}

both "getSlice()[0] = 1" and "getPointerToArray()[0] = 1" are ok for compiler.
but getArray()[0] can not go through.

The go 1.5 compiler give error:
"cannot assign to getArray()[0]"

Is this a problem?if not, Who could tell me why we can't not use the form like above! 3ks!

The text was updated successfully, but these errors were encountered:

mdempsky · 2015-09-17T03:27:38Z

Questions like this are better addressed by the golang-nuts mailing list, but the short summary is the Go programming language specification says the assignment getArray()[0] = 1 is not allowed.

Assignments: "Each left-hand side operand must be addressable, a map index expression, or (for = assignments only) the blank identifier."

Address operators: "The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array."

getSlice()[0] is a slice indexing operation, so it's addressable, so the assignment is valid.

getPointerToArray()[0] is short-hand for (*getPointerToArray())[0], and pointer indirections are addressable, so this is an array indexing of an addressable array.

getArray()[0], however, is an array indexing of a non-addressable array, so the Go compiler is rightfully rejecting the statement as invalid.

bradfitz · 2015-09-17T03:44:18Z

See the language spec's part about addressability.

We can move this discussion to the golang-nuts mailing list.

mdempsky closed this as completed Sep 17, 2015

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cannot assign to an element of an array which is returned from a function #12650

cannot assign to an element of an array which is returned from a function #12650

bigwhite commented Sep 17, 2015

mdempsky commented Sep 17, 2015

bradfitz commented Sep 17, 2015

cannot assign to an element of an array which is returned from a function #12650

cannot assign to an element of an array which is returned from a function #12650

Comments

bigwhite commented Sep 17, 2015

mdempsky commented Sep 17, 2015

bradfitz commented Sep 17, 2015