How check if a property was set in a struct

Question

I am trying to find how check if a structure property was set, but i cannot find any way.

I expect something like this but of corse this not works:

type MyStruct struct {
    property    string
}

test := new(MyStruct)
if test.property {
    //do something with this
}

Answer 1

Like dyoo said, you can use nil if your struct properties are pointers. If you want to keep them as strings you can compare with "". Here is a sample:

package main

import "fmt"

type MyStruct struct {
    Property string
}

func main() {
    s1 := MyStruct{
        Property: "hey",
    }

    s2 := MyStruct{}

    if s1.Property != "" {
        fmt.Println("s1.Property has been set")
    }

    if s2.Property == "" {
        fmt.Println("s2.Property has not been set")
    }
}

http://play.golang.org/p/YStKFuekeZ

Answer 2

You can use pointers and their nil value to determine whether something has been set or not. For example, if you change your structure to

type MyStruct struct {
    property *string
}

then property can either be pointing to a string value, in which case it was set, or it can be nil, in which case it hasn't been set yet. This is an approach that the protobuf library uses to determine whether fields are set or not, as you can see in https://code.google.com/p/goprotobuf/source/browse/README#83

Answer 3

In addition to Charlie's answer and to (sort of) answer asafel's question, if the property's type isn't string, you can check the default value assign by Go to that particular type like this:

type MyStruct struct {
    Str  string
    Bool bool
    Int  int
    Uint uint
}

func main() {
    s := MyStruct{}

    fmt.Println(s.Str)
    fmt.Println(s.Bool)
    fmt.Println(s.Int)
    fmt.Println(s.Uint)

}

Output:

""
false
0
0

And you can check whether the value "is set" in a struct or not

if (s.Bool == false) {
    fmt.Println("Bool is either set to false, or wasn't set");
}

Playground

Well as you might have noticed, you can't technically really check whether the property was set or not, but I think this is a worth addition to Charlie's answer