Map of methods in Go

Question

I have several methods that I'm calling for some cases (like Add, Delete, etc..). However over time the number of cases is increasing and my switch-case is getting longer. So I thought I'd create a map of methods, like Go map of functions; here the mapping of functions is trivial. However, is it possible to create a map of methods in Go?

When we have a method:

func (f *Foo) Add(a string, b int) { }

The syntax below create compile-time error:

actions := map[string]func(a, b){
        "add": f.Add(a,b),
}

Is it possible to create a map of methods in Go?

Do you want to store just the function or both the function and a specific receiver ? — Denys Séguret, Feb 26 '13 at 13:16
Without a receiver a method is basically useless, thus I guess I need definitely the receiver. Otherwise how can I call f.add(a,b)? — Fatih Arslan, Feb 26 '13 at 13:19
Well, you could pass the receiver dynamically when using the map, as in http://stackoverflow.com/questions/8103617/call-a-struct-and-its-method-by-name-in-go — Denys Séguret, Feb 26 '13 at 13:20
map[string]func/method seems like a poor variant of an interface in some sources floating around the intertubes. Hope this is not the same case. — zzzz, Feb 26 '13 at 13:32
@dystroy thanks. I guess MethodByName is what I really want. I can wrap it around a function and call every method directly with a string. Than I don't need any map of methods anymore. — Fatih Arslan, Feb 26 '13 at 13:42
@FatihArslan If you want it to be fast, you'd better use it only for initialization. You have other faster answers below and it's possible an interface would solve your problem more elegantly. — Denys Séguret, Feb 26 '13 at 13:53
Oh, why is the answers below more faster than the MethodByName way? Is using `reflect` slow? — Fatih Arslan, Feb 26 '13 at 14:06

zzzz · Accepted Answer · 2013-02-28T07:26:58.800

11

Yes. Currently:

actions := map[string]func(a string, b int){
        "add": func(a string, b int) { f.Add(a, b) },
}

Later: see the go11func document guelfi mentioned.

edited Feb 28 '13 at 07:26

answered Feb 26 '13 at 13:35

zzzz

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Great! I really like this (creating a func which calls the method). Thank you very much. Btw, I'm calling this like: `actions["add](a, b)` . – Fatih Arslan Feb 26 '13 at 14:05
It should be `actions["add"](a, b)` – Fatih Arslan Feb 26 '13 at 22:18

score 3 · Answer 2 · answered Feb 26 '13 at 13:25

There is currently no way to store both receiver and method in a single value (unless you store it in a struct). This is currently worked on and it may change with Go 1.1 (see http://golang.org/s/go11func).

You may, however, assign a method to a function value (without a receiver) and pass the receiver to the value later:

package main

import "fmt"

type Foo struct {
    n int
}

func (f *Foo) Bar(m int) int {
    return f.n + m
}

func main() {
    foo := &Foo{2}
    f := (*Foo).Bar
    fmt.Printf("%T\n", f)
    fmt.Println(f(foo, 42))
}

This value can be stored in a map like anything else.

This works BTW because `func (f *Foo) Bar(m int)` is the same as `func Bar(f *Foo, m int)`. — harm, Feb 26 '13 at 13:39

score 1 · Answer 3 · answered Aug 26 '22 at 06:43

I met with a similar question.

How can this be done today, 9 years later:

the thing is that the receiver must be passed to the method map as the first argument. Which is pretty unusual.

package main

import (
    "fmt"
    "log"
)

type mType struct {
    str string
}

func (m *mType) getStr(s string) {
    fmt.Println(s)
    fmt.Println(m.str)
}

var (
    testmap = make(map[string]func(m *mType, s string))
)

func main() {
    test := &mType{
        str: "Internal string",
    }
    testmap["GetSTR"] = (*mType).getStr
    method, ok := testmap["GetSTR"]
    if !ok {
        log.Fatal("something goes wrong")
    }
    method(test, "External string")
}

https://go.dev/play/p/yy3aR_kMzHP

score 0 · Answer 4 · answered Feb 05 '20 at 01:04

You can do this using Method Expressions:

https://golang.org/ref/spec#Method_expressions

However, this makes the function take the receiver as a first argument:

actions := map[string]func(Foo, string, int){
  "add": Foo.Add
}

Similarly, you can get a function with the signature func(*Foo, string, int) using (*Foo).Add

score 0 · Answer 5 · answered Jan 06 '21 at 01:45

If you want to use pointer to type Foo as receiver, like in:

func (f *Foo) Add(a string, b int) { }

then you can map string to function of (*Foo, string, int), like this:

var operations = map[string]func(*Foo, string, int){
    "add": (*Foo).Add,
    "delete": (*Foo).Delete,
}

Then you would use it as:

var foo Foo = ...
var op string = GetOp() // "add", "delete", ...
operations[op](&foo, a, b)

where GetOp() returns an operation as string, for example from a user input. a and b are your string and int arguments to methods.

This assumes that all methods have the same signatures. They can also have return value(s), again of the same type(s).

It is also possible to do this with Foo as receiver instead of *Foo. In that case we don't have to de-reference it in the map, and we pass foo instead of &foo.

Map of methods in Go

5 Answers5