Swift variable name with ` (backtick)

Question

I was browsing Alamofire sources and found variable which name is backtick escaped in this source file

open static let `default`: SessionManager = {
    let configuration = URLSessionConfiguration.default
    configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

    return SessionManager(configuration: configuration)
}()

However in places where variable is used there are no backticks. What's the purpose of backticks?

Answer 1

According to the Swift documentation :

To use a reserved word as an identifier, put a backtick before and after it. For example, class is not a valid identifier, but `class` is valid. The backticks are not considered part of the identifier; `x` and x have the same meaning.

In your example, default is a swift reserved keyword, that's why backticks are needed.

Answer 2

Example addendum to the accepted answer, regarding using reserved word identifiers, after they have been correctly declared using backticks.

The backticks are not considered part of the identifier; `x` and x have the same meaning.

Meaning we needn't worry about using the backticks after identifier declaration (however we may):

enum Foo {
    case `var`
    case `let`
    case `class`
    case `try`
}

/* "The backticks are not considered part of the identifier; 
    `x` and x have the same meaning"                          */
let foo = Foo.var
let bar = [Foo.let, .`class`, .try]
print(bar) // [Foo.let, Foo.class, Foo.try]

Answer 3

Simply put, by using backticks you are allowed to use reserved words for variable names etc.

var var = "This will generate an error" 

var `var` = "This will not!"