muduo-ThreadLocal实现细节——阻止销毁未定义对象

muduo利用pthread_key_t实现ThreadLocal模板类.

具体代码如下所示：

     template<typename T>
     class ThreadLocal : noncopyable
     {
     public:
         ThreadLocal()
         {
             MCHECK(pthread_key_create(&pkey_, &ThreadLocal::destructor));
         }

         ~ThreadLocal()
         {
             MCHECK(pthread_key_delete(pkey_));
         }

         T &value()
         {
             T *perThreadValue = static_cast<T *>(pthread_getspecific(pkey_));
             if (!perThreadValue)
             {
                 T *newObj = new T();
                 MCHECK(pthread_setspecific(pkey_, newObj));
                 perThreadValue = newObj;
             }
             return *perThreadValue;
         }

     private:

         static void destructor(void *x)
         {
             T *obj = static_cast<T *>(x);
             typedef char T_must_be_complete_type[sizeof(T) ==  ? - : ];
             T_must_be_complete_type dummy;
             (void) dummy;
             delete obj;
         }

     private:
         pthread_key_t pkey_;
     };

代码除destructor均较为容易理解，故对此进行解释：

 static void destructor(void *x)
 {
     T *obj = static_cast<T *>(x);
         //对于只声明，未进行具体定义类型，例如A，sizeof(A)结果为0
         //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
     typedef char T_must_be_complete_type[sizeof(T) ==  ? - : ];
         //消除unused-local-typedefs编译错误提示
     T_must_be_complete_type dummy;
         //消除Wno-unused-parameter编译错误提示
     (void) dummy;
     delete obj;
 }

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