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) == 0 ? -1 : 1];
            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) == 0 ? -1 : 1];
        //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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