How to distinguish between default value and inputed/changed

I have a struct:

package main

type Test struct {
	FieldInt int
	FieldBool bool
	FieldString string
}

func main() {
    test := Test{}
    ...
}

Default values of this struct is:

• FieldInt: 0
• FieldBool: false
• FieldString: ""

For example, a user can change FieldInt. It can be a positive, negative number and 0.

And I want to check, that the user changed it (but he/she can set 0):

if(test.FieldInt was changed) {
    // do something
}

But if the user inputted 0 then I can't distinguish between default value and inputted/changed one.

How to distinguish between default value and inputted/changed one?

One common way to distinguish between the zero value and an unset value is with pointers:

type Test struct {
    FieldInt    *int
    FieldBool   *bool
    FieldString *string
}

This won't be an appropriate solution for every situation, though, but since you haven't told us your precise situation, I offer this simply as one possible solution.

I would personally tend towards the solution Flimzy provided here for it's simplicity. But just to provide alternative options, you could add a flag for each property to indicate whether or not it's been updated.

type Test struct {
    FieldInt int
    FieldBool bool
    FieldString string

    intUpdated bool
    boolUpdated bool
    stringUpdate bool
    // lets not export these though since it's for managing state
    // internally and consumers of this object shouldn't modify these values
}

To me this is rather a strange thing to see in Go code, however it would be very common place in JS UI apps (almost all modern apps have 'stores' which monitor the state of the UI and typically have flags to indicate whether a value in the UI has changed from what's in the store or not). For back end systems, you usually see even MORE overhead to go with this, for example it would be very common to have fields to store the last time of update and the name of the user who updated it. In fact, that is probably the norm within the data stores for back end services.