【swift初学者求助】关于找不到某重载的错误提示

Question

刚开始学swifit，跟着斯坦福大学公开课学的
做一个计算器
代码如下：

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var display: UILabel!


var userIsInTheMiddleOfTypingANumber: Bool = false

var operandStack = [Double]()

var displayValue: Double{
    get {
        return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
    }
    set {
        display.text = "\(newValue)"
        userIsInTheMiddleOfTypingANumber = false
    }
}

func performOperation(operation: (Double, Double) -> Double) {
    if operandStack.count >= 2 {
        displayValue = operation(operandStack.removeLast(), operandStack.removeLast())
        enter()
    }
}

private func performOperation(operation: Double -> Double) {
    if operandStack.count >= 1 {
        displayValue = operation(operandStack.removeLast())
        enter()
    }
}

@IBAction func appendDigit(sender: UIButton) {
    let digit = sender.currentTitle!


    if userIsInTheMiddleOfTypingANumber {
        display.text = display.text! + digit
    }
    else {
        display.text = digit
        userIsInTheMiddleOfTypingANumber = true
    }
}


@IBAction func enter() {
    userIsInTheMiddleOfTypingANumber = false
    operandStack.append(displayValue)
    println("operandStack = \(operandStack)")
}

@IBAction func operate(sender: UIButton) {
    let operation = sender.currentTitle!

    if userIsInTheMiddleOfTypingANumber {
        enter()
    }

    switch operation {
        case "×":

// performOperation {$0 * $1}
performOperation(*)
case "÷":
performOperation {$1 / $0}
case "+":
// performOperation {$0 + $1}
performOperation(+)
case "−":
performOperation {$0 - $1}
case "√":
performOperation(sqrt)
default: break
}
}
}

实现乘法计算的时候
这么写没有错误提示：
performOperation(*)

而实现加法计算的时候
同样的写法提示错误
performOperation(+)

错误内容：
Could not find an overload for '+' that accepts the supplied arguments

弄不明白
乘法和加法都是二元运算
为什么乘法这么写没有错误
加法却有错误呢
望大神指教
谢谢！

Answer 1

http://stackoverflow.com/questions/29457720/compiler-error-method-with-objective-c-selector-conflicts-with-previous-declara
时间那么久了，你应该也解决了吧，上面是类似的问题。不能重载但却重载了的原因。
我也是新手，多交流学习。