如何判断Swift中实例变量的类是什么

Question

Is there any easy way to tell what class an instance variable is in a Swift? In the JVM-based languages that I'm used to, you can do something like println(value.class) to get it's class.

Is there something equivalent in Swift?

The closest thing I can find in the docs is the ability to do "type checking" with the is <Class> keyword but that only helps me guess a little bit.

I've run into a few situations in playing around where I thought I had one type of class, but actually had another and didn't know how to know for sure.

Answer 1

Use type.self to return a type that can be passed into a method that accepts a type-level argument. For example, UILabel.self can be passed to the isKindOfClass method call. The string representation of the class can be found via dynamicType.description() :

var label = UILabel()
println(label.dynamicType.description())
println(label.isKindOfClass(UILabel.self))

Swift-3

var label = UILabel()
println(type(of: label).description())

Output
UILabel
true

Here's a bit more background -- there are two expressions to be aware of: the postfix self expression and the dynamic type expression. From the docs :

Postfix Self
A postfix self expression consists of an expression or the name of a type, immediately followed by .self. It has the following forms:

 expression.self type.self 

The first form evaluates to the value of the expression. For example, x.self evaluates to x.

The second form evaluates to the value of the type . Use this form to access a type as a value. For example, because SomeClass.self evaluates to the SomeClass type itself, you can pass it to a function or method that accepts a type-level argument

Dyamic Type Expression
A dynamicType expression consists of an expression, immediately followed by .dynamicType. It has the following form:

 expression.dynamicType 

The expression can't be the name of a type. The entire dynamicType expression evaluates to the value of the runtime type of the expression.

Answer 2

As of beta 6 _stdlib_getTypeName gets the mangled type name of a variable. Paste this into an empty playground:

import Foundation

class PureSwiftClass {
}

var myvar0 = NSString() // Objective-C class
var myvar1 = PureSwiftClass()
var myvar2 = 42
var myvar3 = "Hans"

println( "TypeName0 = \(_stdlib_getTypeName(myvar0))")
println( "TypeName1 = \(_stdlib_getTypeName(myvar1))")
println( "TypeName2 = \(_stdlib_getTypeName(myvar2))")
println( "TypeName3 = \(_stdlib_getTypeName(myvar3))")

The output is:

TypeName0 = NSString
TypeName1 = _TtC13__lldb_expr_014PureSwiftClass
TypeName2 = _TtSi
TypeName3 = _TtSS

Ewan Swick's blog entry helps to decipher these strings:

eg _TtSi stands for Swift's internal Int type.

Mike Ash has a great blog entry covering the same topic .