SideTable

struct weak_table_t {
    weak_entry_t *weak_entries;
    size_t    num_entries;
    uintptr_t mask;
    uintptr_t max_hash_displacement;
};

// Template parameters.
enum HaveOld { DontHaveOld = false, DoHaveOld = true };
enum HaveNew { DontHaveNew = false, DoHaveNew = true };

struct SideTable {
    spinlock_t slock;  //锁
    RefcountMap refcnts; //引用记数表
    weak_table_t weak_table; //弱引用hash表

    //构造方法
    SideTable() {
        memset(&weak_table, 0, sizeof(weak_table));
    }
    //析构方法
    ~SideTable() {
        _objc_fatal("Do not delete SideTable.");
    }
    //加锁
    void lock() { slock.lock(); }
    //解锁
    void unlock() { slock.unlock(); }
    //重置锁
    void forceReset() { slock.forceReset(); }

    // Address-ordered lock discipline for a pair of side tables.
    
    //将两个SideTable加锁
    template<HaveOld, HaveNew>
    static void lockTwo(SideTable *lock1, SideTable *lock2);

    //将两个SideTable解锁
    template<HaveOld, HaveNew>
    static void unlockTwo(SideTable *lock1, SideTable *lock2);
};



template<>
void SideTable::lockTwo<DoHaveOld, DoHaveNew>
    (SideTable *lock1, SideTable *lock2)
{
    spinlock_t::lockTwo(&lock1->slock, &lock2->slock);
}

template<>
void SideTable::lockTwo<DoHaveOld, DontHaveNew>
    (SideTable *lock1, SideTable *)
{
    lock1->lock();
}

template<>
void SideTable::lockTwo<DontHaveOld, DoHaveNew>
    (SideTable *, SideTable *lock2)
{
    lock2->lock();
}

template<>
void SideTable::unlockTwo<DoHaveOld, DoHaveNew>
    (SideTable *lock1, SideTable *lock2)
{
    spinlock_t::unlockTwo(&lock1->slock, &lock2->slock);
}

template<>
void SideTable::unlockTwo<DoHaveOld, DontHaveNew>
    (SideTable *lock1, SideTable *)
{
    lock1->unlock();
}

template<>
void SideTable::unlockTwo<DontHaveOld, DoHaveNew>
    (SideTable *, SideTable *lock2)
{
    lock2->unlock();
}