What's the Difference Between Type Aliases and Type Definitions in Go?

Understanding the Differences Between Type Alias and Type Definition in Go

In Go, type alias and type definition are two constructs that allow developers to define custom types. While they may seem similar at first glance, there are some subtle differences between them.

Type Alias

A type alias defines a new name for an existing type. The syntax for a type alias is:

type A = string

In this example, the type A becomes an alias for the built-in type string. This means that you can use A anywhere where you would normally use string.

However, it's important to note that a type alias does not create a new type. Instead, it simply provides a different name for an existing one. This has some implications:

• You cannot define methods on a type alias.
• Reflection will not recognize the type alias as a separate type.

Type Definition

A type definition defines a new type with its own unique characteristics and behavior. The syntax for a type definition is:

type A string

In this example, the type A is defined as a new type that has the same underlying representation as the built-in type string. This means that you can convert between A and string without any performance penalty.

However, unlike a type alias, a type definition creates a new type that is distinct from its underlying type. This has the following advantages:

• You can define methods on a type definition.
• Reflection will recognize your type definition as a separate type.

Use Case Example

To illustrate the differences between type alias and type definition, consider the following code:

package main

import (
    "fmt"
)

type A = string
type B string

func main() {
    var a A = "hello"
    var b B = "hello"
    fmt.Printf("a is %T\nb is %T\n", a, b)
}

In this example, A is a type alias for string, while B is a type definition. The output of the program is:

a is string
b is main.B

As you can see, the type alias A is still recognized as a string, while the type definition B is recognized as a separate type.

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