As far as I understand, indexing a map returns a copy of the map value. With this in mind, consider the following code:

package main

import "fmt"

func main() {
    m := map[string]int{"xxx": 100}
    m["xxx"]  
    fmt.Println(m["xxx"]) // prints 101
}

The code above prints 101, whereas I expected 100. My reasoning is that m["xxx"] returns a copy of the value assigned to the key "xxx" (i.e., 100), and the operator increments this copy of the value but this doesn't affect the original value stored in the map – only the copy changes.

However, considering that we assign a value to a map key by putting the key within brackets and using = to specify the value (i.e., m[key] = value). Then, if m["xxx"] were translated to m["xxx"] = 1 by the compiler – which is, in turn, equivalent to m["xxx"] = m["xxx"] 1 – this would explain the result of the code above.

My question is whether the increment operator ( ) is syntactic sugar for the by-one addition assignment ( = 1). Otherwise, what am I missing?

CodePudding user response：

The language specification says that the /-- operators should work with a map index expression as the operand.

https://go.dev/ref/spec#IncDec_statements

The " " and "--" statements increment or decrement their operands by the untyped constant 1. As with an assignment, the operand must be addressable or a map index expression.

How that functionality should be implemented is not specified and may therefore be different across different compilers or different versions of the same compiler.

If you search the cmd/compile/internal directory of your Go installation, which is the source code of the Go compiler if I'm not mistaken, you'll find a number of lines that seem to confirm your guess. I have nowhere near enough knowledge about the code to provide a conclusive answer however.

Examples: one and two

And the gccgo implementation of the spec, which I've never looked at, may or may not do the IncDec the same way.

CodePudding user response：

"My reasoning is that m["xxx"] returns a copy of the value" — I don't think this is a fair assumption. The specs under index expressions only say the following:

A primary expression of the form a[x] denotes the element of the [...] map a indexed by x

and

if the map contains an entry with key x, a[x] is the map element with key x

The verbs "denotes" and "is" arguably do not imply a copy. A copy is made only when you assign the result of the index expression to a variable.

The map index expression is simply not addressable, so that you can't memory alias the values stored in the map.

As for whether the increment operator is syntactic sugar for = 1 or not, the specs explicitly state that:

The following assignment statements are semantically equivalent:

IncDec statement    Assignment
x                   x  = 1
x--                 x -= 1

So op has the same meaning of op = 1, and a[x] increments the operand a[x] which "denotes/is" the map element with key x.