How to print struct with String() of fields?

Question

This code:

type A struct {
    t time.Time
}

func main() {
    a := A{time.Now()}
    fmt.Println(a)
    fmt.Println(a.t)
}

prints:

{{63393490800 0 0x206da0}}
2009-11-10 23:00:00 +0000 UTC

A doesn't implement String(), so it's not a fmt.Stringer and prints its native representation. But is very tedious to implement String() for every single struct I want to print. Worse, I have to update the String()s if I add or remove some fields. Is there an easier way to print a struct, with its fields' String()s?

icza · Accepted Answer · 2016-10-02 10:04:22Z

This is how the fmt package is implemented, so you can't change that.

But you can write a helper function which uses reflection (reflect package) to iterate over the fields of a struct, and can call the String() method on the fields if they have such a method.

Example implementation:

func PrintStruct(s interface{}, names bool) string {
    v := reflect.ValueOf(s)
    t := v.Type()
    // To avoid panic if s is not a struct:
    if t.Kind() != reflect.Struct {
        return fmt.Sprint(s)
    }

    b := &bytes.Buffer{}
    b.WriteString("{")
    for i := 0; i < v.NumField(); i++ {
        if i > 0 {
            b.WriteString(" ")
        }
        v2 := v.Field(i)
        if names {
            b.WriteString(t.Field(i).Name)
            b.WriteString(":")
        }
        if v2.CanInterface() {
            if st, ok := v2.Interface().(fmt.Stringer); ok {
                b.WriteString(st.String())
                continue
            }
        }
        fmt.Fprint(b, v2)
    }
    b.WriteString("}")
    return b.String()
}

Now when you want to print a struct, you can do:

fmt.Println(PrintStruct(a, true))

You may also choose to add a String() method to your struct which just has to call our PrintStruct() function:

func (a A) String() string {
    return PrintStruct(a, true)
}

Whenever you change your struct, you don't have to do anything with your String() method as it uses reflection to dynamically walk over all the fields.

Notes:

Since we're using reflection, you have to export the t time.Time field for this to work (also added a few extra fields for testing purposes):

type A struct {
    T          time.Time
    I          int
    unexported string
}

Testing it:

a := A{time.Now(), 2, "hi!"}
fmt.Println(a)
fmt.Println(PrintStruct(a, true))
fmt.Println(PrintStruct(a, false))
fmt.Println(PrintStruct("I'm not a struct", true))

Output (try it on the Go Playground):

{T:2009-11-10 23:00:00 +0000 UTC I:2 unexported:hi!}
{T:2009-11-10 23:00:00 +0000 UTC I:2 unexported:hi!}
{2009-11-10 23:00:00 +0000 UTC 2 hi!}
I'm not a struct

That's exactly what I try to avoid. I want to use time.Time's String(), instead of exposing its internal implementation.

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