muduo-ThreadLocal实现细节——阻止销毁未定义对象

来源：cnblogs　　作者：从此寂静无声　　时间：2018/11/2 9:15:57　　对本文有异议

muduo利用pthread_key_t实现ThreadLocal模板类.

具体代码如下所示：

 1     template<typename T>
 2     class ThreadLocal : noncopyable
 3     {
 4     public:
 5         ThreadLocal()
 6         {
 7             MCHECK(pthread_key_create(&pkey_, &ThreadLocal::destructor));
 8         }
 9 
10         ~ThreadLocal()
11         {
12             MCHECK(pthread_key_delete(pkey_));
13         }
14 
15         T &value()
16         {
17             T *perThreadValue = static_cast<T *>(pthread_getspecific(pkey_));
18             if (!perThreadValue)
19             {
20                 T *newObj = new T();
21                 MCHECK(pthread_setspecific(pkey_, newObj));
22                 perThreadValue = newObj;
23             }
24             return *perThreadValue;
25         }
26 
27     private:
28 
29         static void destructor(void *x)
30         {
31             T *obj = static_cast<T *>(x);
32             typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];
33             T_must_be_complete_type dummy;
34             (void) dummy;
35             delete obj;
36         }
37 
38     private:
39         pthread_key_t pkey_;
40     };

代码除destructor均较为容易理解，故对此进行解释：

 1 static void destructor(void *x)
 2 {
 3     T *obj = static_cast<T *>(x);   
 4         //对于只声明，未进行具体定义类型，例如A，sizeof(A)结果为0 
 5         //利用此特性，若T是未定义类型，则T_must_be_complete_type在编译期间会报错
 6     typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];
 7         //消除unused-local-typedefs编译错误提示
 8     T_must_be_complete_type dummy; 
 9         //消除Wno-unused-parameter编译错误提示  
10     (void) dummy;
11     delete obj;
12 }