# 03.为什么用dispatch-once实现单例

> 面试问到了 自己说到了这点(猜的是线程安全的查了一下确实是这样)

```
+ (XXXX *)sharedInstance
{
    static id sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}
```

`dispatch_once`是线程安全的,可以确保

## 参考

> <http://www.cnblogs.com/hellocby/archive/2012/08/24/2654488.html> <https://blog.csdn.net/qqMCY/article/details/88648000> 单例alloc 、 copy的处理


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