为什么在此类中复制构造函数，operator =和引用具有编译问题

我试图寻找解决方案，但一无所获。如果这是重复的问题，我事先表示歉意。

我有一个这样的课：

class sync_lock {
    public:
        enum lock_status { free, locked };
        pthread_cond_t cond;
        pthread_mutex_t mtx;
        lock_status status;

        sync_lock() : status(free) {
            pthread_cond_init( &cond, NULL );
            pthread_mutex_init( &mtx, NULL );
        }

        sync_lock(const sync_lock & param) {
            cond = param.cond;
            mtx = param.mtx;
            status = param.status;
        }

        sync_lock & operator=(const sync_lock & param) {
            if (this == &param)
                return *this;
            cond = param.cond;
            mtx = param.mtx;
            status = param.status;
            return *this;
        }

        ~sync_lock() {
            pthread_cond_destroy( &cond );
            pthread_mutex_destroy( &mtx );
        }
};

在驱动程序代码中，我有这个：

line 29: sync_lock lock();
line 30: lock_map[lid] = lock;

另一个地方有这个：

line 33: sync_lock & lock = lock_map[lid];

当我编译时，我得到了：

lock_server.cc: In member function ‘lock_protocol::status lock_server::acquire(int, lock_protocol::lockid_t, int&)’:
lock_server.cc:30:25: error: no match for ‘operator=’ in ‘((lock_server*)this)->lock_server::lock_map.std::map<_Key, _Tp, _Compare, _Alloc>::operator[] [with _Key = long long unsigned int, _Tp = sync_lock, _Compare = std::less<long long unsigned int>, _Alloc = std::allocator<std::pair<const long long unsigned int, sync_lock> >, std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = sync_lock, std::map<_Key, _Tp, _Compare, _Alloc>::key_type = long long unsigned int]((*(const key_type*)(& lid))) = lock’
lock_server.cc:30:25: note: candidate is:
lock_server.h:33:21: note: sync_lock& sync_lock::operator=(const sync_lock&)
lock_server.h:33:21: note:   no known conversion for argument 1 from ‘sync_lock()’ to ‘const sync_lock&’
make: *** [lock_server.o] Error 1

我不知道是什么导致了此错误。有什么想法吗？提前致谢！