What is the correct way to pass a pointer to a function in Go so I can read and/or modify the value-CodePudding

I have a type of struct for which I need to change its values from another function. The function SetPath() takes a pointer to the string I want to change and another pointer to the default value I want to use. When I call the function, I can pass it the address of the string in the struct, but I cannot pass it a pointer or the address of a const that defines the default value.

package main

const DefaultFileName string = "default.config"

type Settings struct {
    FilePath string
}

func SetPath(path, defaultPath *string) {
    if *path == "" {
        *path = *defaultPath
    }
}

func main() {
    settings := new(Settings)

    SetPath(&settings.FilePath, &DefaultFileName)

}

The problem in the code above is the last line of main() - how do I pass a pointer to the constant DefaultFileName so I can use its value in SetPath()? My editors complain about the constant, but not about &settings.FilePath.

I'm sure a solution could be to change the function signature to something like:

func SetPath(path *string, defaultPath string)

But I really want to understand pointers more and I feel like this is a good opportunity to learn more about when, where, and how I should be using them - especially since optimizing memory usage is important in this project.

CodePudding user response：

You can't take the address of constants, for details see Find address of constant in go.

But you don't need to. SetPath() only wants to modify path, but not defaultPath, so defaultPath doesn't need to be a pointer, in fact it's clearer if it's not.

func SetPath(path *string, defaultPath string) {
    if *path == "" {
        *path = defaultPath
    }
}

And calling it:

SetPath(&settings.FilePath, DefaultFileName)