ios - 避免保留 sysSound

Question

文档建议以下内容，但在使用 ARC 时不允许保留。请问有什么解决方法？

// Create the URL for the source audio file. The URLForResource:withExtension: method is
    //    new in iOS 4.0.
    NSURL *tapSound   = [[NSBundle mainBundle] URLForResource: @"tap"
                                                withExtension: @"aif"];

    // Store the URL as a CFURLRef instance
    self.soundFileURLRef = (CFURLRef) [tapSound retain];