对于某些类，我们需要保证系统中只能有一个实例，这种类的设计用到singleton模式模式。
单线程的singleton模式是 straightforward的，下面给出Java和C++11的线程安全singleton实现
Java：
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public class Singleton {
private
static Singleton instance = null;
private
static Object mutex = new
Object();
private
Singleton() { }
public
static Singleton getInstance() {
if(instance == null) {
synchronized(mutex) {
if(instance == null)
instance = new
Singleton();
}
}
return
instance;
}
} |
C++:
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static
Singleton& get(){
static
Singleton instance;
return
instance;
} |
C++11 remove the need for manual locking. Concurrent execution shall wait if a static local variable is already being initialized.