Testing whether swift captures by value or by reference.

capturing.swift

func testCapturing() -> () {
    
    var x = 3
    
    let c1 = { () -> () in
        println( "unmodified x: \(x)")
    }
    
    let c2 = { () -> () in
        x += 1
        println( "modified x: \(x)")
    }
    
    c1()  // "unmodified x: 3"
    c2()  // "modified x: 4"
    c1()  // "unmodified x: 4"
}

testCapturing()

The guide suggested:
“As an optimization, Swift may instead capture and store a copy of a value if that value is not mutated by a closure, and if the value is not mutated after the closure is created. Swift also handles all memory management involved in disposing of variables when they are no longer needed.”

That means without any side effect, the compiler can store a copy of value in capturing. But since c2 may mutate the value of x, the optimization won't be effective. Programmers do not need to concern the underlying memory management, we could always think it is capture-by-reference during programming. Language features never change, but implementations of compiler may be different.