[android-developers] confused by weakref_impl implementation

I got confused when I read source code about RefBase.cpp

// compile with refcounting debugging enabled
#define DEBUG_REFS 0
#define DEBUG_REFS_ENABLED_BY_DEFAULT 1
#define DEBUG_REFS_CALLSTACK_ENABLED 1

class RefBase::weakref_impl : public RefBase::weakref_type
{
public:
volatile int32_t mStrong;
volatile int32_t mWeak;
RefBase* const mBase;
volatile int32_t mFlags;

#if !DEBUG_REFS

weakref_impl(RefBase* base)
: mStrong(INITIAL_STRONG_VALUE)
, mWeak(0)
, mBase(base)
, mFlags(0)
{
}

void addStrongRef(const void* /*id*/) { }
void removeStrongRef(const void* /*id*/) { }
void addWeakRef(const void* /*id*/) { }
void removeWeakRef(const void* /*id*/) { }
void printRefs() const { }
void trackMe(bool, bool) { }

#else
....