Swift nested structures -- how to access element

Question

In a Swift app, I am attempting to nest structures for greater clarity. Here is the code:

struct ColorStruct {
    var colorname: String = ""
    struct RGB {
        var red:   Int = 0
        var green: Int = 0
        var blue:  Int = 0
    }
}

I can access a ColorStruct element (example: "colorname") as long as it isn't nested.

Q: What am I failing to understanding about how to properly access the "red" variable?

var newColor = ColorStruct()
newColor.colorname = "Red"
newColor.RGB.red   = 255     // Results in error:
// Static member 'RGB' cannot be used on instance of type `ColorStruct`

struct RGB { var red:Int = 0; var green:Int= 0; var blue:Int = 0; }; struct ColorStruct { var colorname:String = ""; var RGB:RGB;} instead? ? — Larme
– Larme, Commented Feb 22, 2018 at 15:06
I've used separate structs as you show, but my point is that I am trying to nest structs for greater clarity. — Vic W.
– Vic W., Commented Feb 22, 2018 at 15:08
The ColorStruct doesn't know about struct RGB without adding the var RGB: RGB to the ColorStruct. — MwcsMac
– MwcsMac, Commented Feb 22, 2018 at 15:20
please note that var RGB: RGB wouldn't compile at all because of name collision between type name and variable name — Max Desiatov
– Max Desiatov, Commented Feb 22, 2018 at 15:21

Max Desiatov · Accepted Answer · 2018-02-22 15:18:50Z

6

A nested struct as given in the question code doesn't automatically create an outer struct member of that type. To achieve that, refine the code this way:

struct ColorStruct {
  var colorname: String = ""
  struct RGB {
    var red:   Int = 0
    var green: Int = 0
    var blue:  Int = 0
  }

  var rgb = RGB()
}

var color = ColorStruct()
color.rgb.red = 255

answered Feb 22, 2018 at 15:18

Max Desiatov

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1 Comment

Vic W. Over a year ago

Max, thank you. Your answer achieves both my goals: understanding why and clarity in code.

J. Doe · Accepted Answer · 2018-02-22 15:22:32Z

2

This line:

struct RGB{ var red: Int = 0}

states that red is an instance variable in a struct RGB (with other words, red does belong to an instance of RGB). When you create a struct of type ColorStruct, you do not create a struct RGB. You can only access instance variabele if you have created an object of it. If you want to access the variabele red, create a struct RGB (RGB()), or make the variabele/struct (is this possible in Swift?) static (don't make it an instance variabele).

answered Feb 22, 2018 at 15:22

J. Doe

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Comments

Sandeep · Accepted Answer · 2018-02-22 15:22:09Z

1

You cannot assign value to struct just like that. Either create a new property of RGB like so,

struct ColorStruct {
  var colorname:String    = ""
  var rgb = RGB()

  struct RGB {
    var red:            Int    = 0
    var green:          Int    = 0
    var blue:           Int    = 0
  }

}

var newColor = ColorStruct()
newColor.colorname = "Red"
newColor.rgb.red   = 255     // Results in error:

Or, make static variable inside ColorStruct,

struct ColorStruct {
  var colorname:String    = ""
  static var rgb = RGB()

  struct RGB {
    var red:            Int    = 0
    var green:          Int    = 0
    var blue:           Int    = 0
  }
}

ColorStruct.rgb.red = 255

answered Feb 22, 2018 at 15:22

Sandeep

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Comments

Agent Smith · Accepted Answer · 2018-02-22 15:16:32Z

0

You can achieve it by this:

struct ColorStruct {
var colorname:String  = ""
var colorValue: RGB!

struct RGB              {
    var red:            Int    = 0
    var green:          Int    = 0
    var blue:           Int    = 0

    mutating func setRed(value: Int) {
        self.red = value
    }
    mutating func setGreen(value: Int) {
        self.green = value
    }
    mutating func setBlue(value: Int) {
        self.blue = value
    }
}
}

And then use the struct as:

var color = ColorStruct()

color.colorname = "blue"
color.colorValue = ColorStruct.RGB.init(red: 0, green: 0, blue: 1)
print(color.colorValue)

And if you want to change a value call this :

color.colorValue.setRed(value: 2)
print(color.colorValue)

Hope it helps

edited Feb 22, 2018 at 15:16

answered Feb 22, 2018 at 15:12

Agent Smith

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3 Comments

Vic W. Over a year ago

Thank you, but while it certainly works it (arguably) does not improve clarity. My goal is twofold: Better clarity in my code -and- understanding why my code does NOT work.

Agent Smith Over a year ago

@VicW. you need to create an object(Passed by value) of the struct if you want to use and to modify any value inside it you will have to call mutating functions.

Agent Smith Over a year ago

@VicW. its explained beautifully here

sebastien FCT · Accepted Answer · 2018-02-22 15:21:36Z

0

You must instantiate your RGB struct instead of ColorStruct:

struct ColorStruct {
    var colorname:String    = ""
    struct RGB              {
        var red:            Int    = 0
        var green:          Int    = 0
        var blue:           Int    = 0
    }
}

var newColor = ColorStruct.RGB()
newColor.red = 255

answered Feb 22, 2018 at 15:21

sebastien FCT

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1 Comment

Vic W. Over a year ago

Sebastien, thank you. My problem with this approach is that when the enclosing ColorStruct has multiple nested structs, the instantiation becomes increasingly complex as additional nested structs are added.

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