How to Customize the Printing of Go Structs Without Explicitly Defining a String() Method for Each Struct?

Printing Structs with Custom Field Formatting

While fmt.Println conveniently displays built-in types, its behavior for structs can be verbose and uninformative. Consider a struct representing a time value with additional fields:

type A struct {
    t time.Time
}

Printing this struct using fmt.Println results in:

{{63393490800 0 0x206da0}}

which is not readily interpretable. Specifically, the struct lacks a String() method, which prevents its fields from being formatted as desired.

Problem:

How can we print a struct with custom string representations for its fields, without explicitly defining a String() method for each struct?

Solution:

Reflection can be used to iterate over the fields of a struct and call their String() methods dynamically. Here's a helper function that accomplishes this:

func PrintStruct(s interface{}, names bool) string {
    v := reflect.ValueOf(s)
    t := v.Type()
    // Handle non-struct input
    if t.Kind() != reflect.Struct {
        return fmt.Sprint(s)
    }
    // Initialize buffer
    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(":")
        }
        // Handle Stringer fields
        if v2.CanInterface() {
            if st, ok := v2.Interface().(fmt.Stringer); ok {
                b.WriteString(st.String())
                continue
            }
        }
        // Print non-Stringer fields
        fmt.Fprint(b, v2)
    }
    b.WriteString("}")
    // Return formatted string
    return b.String()
}

Usage:

This function can be used to print a struct with custom field formatting:

a := A{time.Now()}
fmt.Println(PrintStruct(a, true)) // Display field names
fmt.Println(PrintStruct(a, false)) // Omit field names

Notes:

• The PrintStruct function assumes exported struct fields.

• For added convenience, you can define a String() method for your struct that simply calls PrintStruct:

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

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