Error - Cannot use mutating member on immutable value: function call returns immutable value

Question

So I have this custom struct

public struct Feature {
    var featureID: String = ""
    var featureName: String = ""
    var matchingFieldValue: String = ""

    var polygonCollection =  [MyPolygon]()

    mutating func setFeatureID(featureID: String) {
        self.featureID = featureID
    }

    func getMatchingFieldValue() -> String {
        return matchingFieldValue
    }

    mutating func setMatchingFieldvalue(matchingFieldValue: String) {
        self.matchingFieldValue = matchingFieldValue
    }

    public func getPolygonCollection() -> [MyPolygon] {
         return polygonCollection  
    }  
}

and I am trying to append a polygon to my polygonCollection by calling this function

feature.getPolygonCollection().append(polygon)

but I am getting the error

cannot use mutating member on immutable value: function call returns immutable value

by the way, I am defining the polygon in another class, it is a long class so just put the relevant calling code that gives the error.

All the previously asked ques

I appreciate all the help.

vadian vadian · Accepted Answer · 2016-10-29T18:57:13

Due to value semantics getPolygonCollection() returns an immutable copy of polygonCollection. You cannot change it. That's what the error message says.

Add this function in the struct

mutating func add(polygon: MyPolygon) {
    self.polygonCollection.append(polygon)
}

and call it

feature.add(polygon)

Error - Cannot use mutating member on immutable value: function call returns immutable value

1 Answers