How Can I Determine if a Go Struct Field Has Been Explicitly Set?

Default Values for Go Structs

In Go, structures (structs) have default values for their fields. For instance, integers (int) default to 0. While this is useful in many scenarios, it can pose a challenge when you have a scenario where 0 is a valid value. This makes it difficult to know if a field in a struct has been explicitly set or not.

Distinguishing Between Default and Set Values

Unfortunately, Go does not provide a way to directly determine whether a field has been set or initialized to its default value. However, there are two approaches you can employ:

Using Pointers

You can use pointers for fields that may be unset. Pointers have a nil zero value, which you can use to check if a field has been set:

type Test struct {
    TestIntOne *int
    TestIntTwo *int
}

In this case, TestIntOne and TestIntTwo are pointers to integers. If TestIntOne is nil, it means that it has not been set, while if it is not nil, it has been set to a non-zero value.

Using a Method

Alternatively, you can create a method that sets the field and tracks whether it has been set or not:

type Test struct {
    TestIntOne int
    TestIntTwo int

    isSetOne, isSetTwo bool
}

func (t *Test) SetIntOne(i int) {
    t.TestIntOne = i
    t.isSetOne = true
}

With this method, you can control the setting of TestIntOne, and the isSetOne property will indicate whether it has been set or not.

Conclusion

While Go structs have default values for their fields, there is no direct way to determine if a field has been set manually or initialized to its default value. However, by using pointers or methods, you can work around this limitation and maintain the state of your struct's fields.

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