Method pop

src/bthread/id.cpp:55–70

        }
    }
    bool pop(T* val) {
        if (_size > 0) {
            *val = _c[_begin];
            ++_begin;
            if (_begin >= N) {
                _begin -= N;
            }
            --_size;
            return true;
        } else if (_full && !_full->empty()) {
            *val = _full->front();
            _full->pop_front();
            return true;
        }
        return false;
    }
    bool empty() const {
        return _size == 0 && (_full == NULL || _full->empty());
    }