从内部函数安全引用外部函数中的值？

Question

I am moving to Go from Node.js and I am concerned whether a construct that I would use in Node is safe to do in Go and whether there is a more idiomatic way to accomplish the same thing. I am using the Echo framework and want to set a route specific struct that will be available within the context object. I could generate the struct for every call within middleware, but it's expensive to do so. Instead, I set the struct once in an outer func that then returns an inner func that refers to the struct in the outer func. My hope is that I then only incur the generation cost once and then have the correct struct associated with my route for every call.

e.POST(path, POST.GenericPostHandler, func(next echo.HandlerFunc)  echo.HandlerFunc {
    operation := getOperationMap(path)
    return func(c echo.Context) error {
        c.Set("op", operation)
        return next(c)
    }
})

Are there any concerns with this code? Will it cause problems with GC? Is there a more efficient way to accomplish the same thing? I assume a copy of the struct is made every time the middleware is called.

Answer 1

If operationMap never changes after initialization, You can declare operationMap as a singleton instance like following:

package main

import (
    "fmt"
    "sync"
)

var (
    operationMapInst map[string]string // I don't know the exact type of map, so you should change the type.
    operationMapOnce sync.Once
)

func getOperationMap() map[string]string {
    // operationMapOnce.Do() runs only once
    // when the first time getOperationMap() is called.
    operationMapOnce.Do(func() {
        // Initialize operationMapInst.
        operationMapInst = map[string]string{"/": "root", "/ver": "version"}
        fmt.Println("operaionMap has initialized!")
    })

    return operationMapInst
}

func main() {
    // The initialization logic runs only once.
    // Because getOperationMap() returns map,
    // syntax for the value for a path should be getOperationMap()[path],
    // not getOperationMap(path).
    rootOp, ok := getOperationMap()["/"]
    fmt.Println(rootOp, ok)

    // repetition
    rootOp, ok = getOperationMap()["/"]
    fmt.Println(rootOp, ok)
    verOp, ok := getOperationMap()["/ver"]
    fmt.Println(verOp, ok)
    verOp, ok = getOperationMap()["/ver"]
    fmt.Println(verOp, ok)
}

You can run this code here.

I recommend http://marcio.io/2015/07/singleton-pattern-in-go/ for understanding singleton pattern in Go.