为什么Go不允许使用地图值地址？ [重复]

Question

This question already has an answer here:

• Why do I get a “cannot assign” error when setting value to a struct as a value in a map? [duplicate] 2 answers
• Why does Go forbid taking the address of (&) map member, yet allows (&) slice element? 2 answers

This is the same as Why does go forbid taking the address of (&) map member, yet allows (&) slice element? but I'm not satisfied with the accepted answer: "Slices are backed by a backing array and maps are not."

Note: I have now added my own answer to the referenced question above.

The question Access Struct in Map (without copying) is even better, but its accepted answer says you can't modify a field of a struct value in a map because you cannot take its address (which is my question).

Maps are backed by memory structures (possibly including arrays) just like slices are.

So what is the real reason why I can't take the address of a map value?

I wanted to modify a map struct value in place. Numeric values in maps can be modified in place using operators like ++ or +=

     func icandothis() {
        cmap := make(map[int]complex64)
        cmap[1] += complex(1, 0)
        fmt.Println(cmap[1])
     }

But struct values cannot be modified:

type Complex struct {
    R float32
    I float32
}

func (x *Complex) Add(c Complex) {
    x.R += c.R
    x.I += c.I
}

func but_i_cannot_do_this() {
    cmap := make(map[int]Complex)
    //cmap[1].Add(Complex{1, 0})
    fmt.Println(cmap[1])

}

func so_i_have_to_do_this() {
    cmap := make(map[int]Complex)
    c := cmap[1]
    c.Add(Complex{1, 0})
    cmap[1] = c
    fmt.Println(cmap[1])

}

</div>

Answer 1

Let's start with this false claim:

I wanted to modify a map struct value in place. Numeric values in maps can be modified in place using operators like ++ or +=

 func icandothis() {
    cmap := make(map[int]complex64)
    cmap[1] += complex(1, 0)
    fmt.Println(cmap[1])
 }

---

Let's expand the shorthand form:

package main

import (
    "fmt"
)

func icandothisShort() {
    cmap := make(map[int]complex64)
    cmap[1] += complex(1, 0)
    fmt.Println(cmap[1])
}

func icandothisLong() {
    cmap := make(map[int]complex64)
    // An assignment operation x op= y where op is a binary arithmetic operator
    // is equivalent to x = x op (y) but evaluates x only once.
    // cmap[1] += complex(1, 0)
    v := cmap[1]          // v = zero value = complex(0, 0)
    v = v + complex(1, 0) // v = complex(0, 0) + complex(1, 0) = complex(1, 0)
    cmap[1] = v           // cmap[1] = v = complex(1, 0)
    a := cmap[1]          // a = complex(1, 0)
    fmt.Println(a)        // complex(1, 0)
}

func main() {
    icandothisShort()
    icandothisLong()
}

Playground: https://play.golang.org/p/1OgmI_AD9uN

Output:

(1+0i)
(1+0i)

As you can see in icandothisLong(), the expanded form of icandothisShort(), there is no update in-place.

---

---

The next false claim,

Maps are backed by memory structures (possibly including arrays) just like slices are.

So what is the real reason why I can't take the address of a map value?

---

The real reason is that you don't understand the map data structure.

Maps are backed by bucket memory structures. A map key, through an imperfect, dynamic hash, identifies a current primary bucket. The map keys and values are stored in the primary bucket or an overflow bucket. The map buckets are constantly reorganized as map entries are created, updated, and deleted. A map entry has no fixed location in memory.

Do some basic research. For example,

GopherCon 2016: Keith Randall - Inside the Map Implementation