Using golang constants in struct [SOLVED]

In today’s post, we will discuss how to declare a constant in a Golang struct. For example:

Constants: There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.

A constant value is represented by a rune, integer, floating-point, imaginary, or string literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such as unsafe.Sizeof applied to certain values, cap or len applied to some expressions, real and imag applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constants true and false. The predeclared identifier iota denotes an integer constant.

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Constants struct in Go

Go only supports four types of constants as above listed. It doesn’t support constant struct. So below example would raise an error:

package main

import "fmt"

type Person struct {
    name string
    age  int
}

func main() {
    const per = Person{
        name: "Harry Potter",
        age:  100000,
    }
    fmt.Println(per)
}

Output:

# command-line-arguments
.\structConst.go:11:14: Person{…} (value of type Person) is not constant

A solution is to have a function that returns a struct. In a way, that fulfills the purpose of a constant struct because it always returns the same struct:

package main

import "fmt"

type Person struct {
    name string
    age  int
}

func main() {
    per := constantPerson()
    fmt.Println(per)

    per2 := constantPerson()
    fmt.Println(per2 == per)
}

func constantPerson() Person {
    return Person{
        name: "Harry Potter",
        age:  100000,
    }
}

Output:

{Harry Potter 100000}
true

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Declare golang constants in struct

For example, we want to declare constant in a struct. Let’s assume that constants.CONSTANT_STRING has the value “Hello world”. Then this syntax:

type Person struct {
    name string
    age  int
    constants.CONSTANT_STRING string
}

would evaluate to:

type Person2 struct {
    name string
    age  int
    "Hello world" string
}

which is incorrect syntax and has no semantic meaning. So in Golang, we can not directly declare a constant in a struct. If you want to assign a greeting (a constant string) to each Person variable, you can add an unexported field to the struct along with a method that returns greetingthe value. This way, code outside the package can read the constant string through the method but cannot directly access the variable:

package main

import (
    "fmt"
)

const (
    GREETING1 = "Hello World!"
    GREETING2 = "Bonjour!"
)

type Person struct {
    name     string
    age      int
    greeting string
}

func (per Person) getGreeting() string {
    return per.greeting
}

func main() {
    person1 := Person{
        name:     "Anna",
        age:      21,
        greeting: GREETING1,
    }
    fmt.Println("Greeting:", person1.getGreeting())
}

Output:

Greeting: Hello World!

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Summary

In today’s post, I have given examples of declaring a constant struct or constants in a struct. In general, golang does not allow declaring a constant struct and constant within a struct. But we can use a custom function to perform this purpose. To understand more about using constant in golang, read the declare constant maps article.

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References

https://go.dev/ref/spec#Constants