muduo-ThreadLocal实现细节——阻止销毁未定义对象

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_;
};

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;
}