C++ 嵌套前向声明继承

我的问题是：

定义了类（生成器），在其中我定义了一个前向嵌套结构（主题和it_set）。

我在.cpp文件中声明这个嵌套类。

在此之后，我声明了第二个类（ImageGenerator），它是生成器的继承。

当我尝试在图像生成器的声明文件中尝试时，我遇到了一个问题。

有没有办法使这成为可能？

我的代码是这些：

<i>
 //base.hpp
</i>
 class generator{
 protected:
    struct topics;
    struct it_set;
    NodeHandle _nh;
    cv::Ptr<topics> _topics;
    cv::Ptr<it_set> _set;
    cv::Mat _data;
public:

   generator(ros::NodeHandle&,const std::string&,const std::string&,const std::string&);
    virtual ~generator(void);
    bool ok(void)const;

protected:
    virtual void grab(void) = 0;
};
<i>
// base.cpp
</i>
static void cam_from_sub(const std::string& _subscriber,std::string& _cam){
    std::stringstream str;
    std::vector<std::string> words;
    std::string tmp;

    for(std::string::const_iterator it = _subscriber.begin();it != _subscriber.end();it++)
        (*it != '/')?(str<<*it):(str<<std::endl);
    while(!str.eof()){
        str>>tmp;
        words.push_back(tmp);
        tmp.clear();
    }
    words.pop_back();

    for(std::vector<std::string>::iterator it = words.begin(); it != words.end();it++){
        _cam+=*it+std::string("/");
        it->clear();
    }
    words.clear();
    _cam+= std::string("camera_info");

}

struct generator::topics{
    std::string _publisher;
    std::string _subscriber;
    std::string _camera_info;
    topics(const std::string& _pub,const std::string& _sub,const std::string& _cam):_publisher(_pub),_subscriber(_sub),_camera_info(_cam){}
    topics(const std::string &_pub, const std::string &_sub):_publisher(_pub),_subscriber(_sub){cam_from_sub(_subscriber,_camera_info);}
    ~topics(void){}
};

struct generator::it_set{
    image_transport::ImageTransport _it;
    image_transport::SubscriberFilter _is;
    image_transport::Publisher _pb;
    message_filters::Subscriber<sensor_msgs::CameraInfo> _cam_info;
    it_set(NodeHandle& _nh,cv::Ptr<generator::topics>& _topics):_it(_nh),_is(_it,_topics->_subscriber,1),_cam_info(_nh,_topics->_camera_info,1){ this->_pb = this->_it.advertise(_topics->_publisher,1);}
};
generator::generator(NodeHandle & nh, const std::string & subscribe, const std::string & publish, const std::string & camera_info):_nh(nh),_topics(new topics(publish,subscribe,camera_info)),_set( new it_set(_nh,_topics)){}
generator::~generator(void){ _set.release(); _topics.release();}
bool generator::ok(void)const{ return this->_nh.ok();}
<i>
// image.hpp
</i>
class ImageGenerator : public generator{
private:
    NodeHandle _nh;
    static bool _sht;
    bool _first_sht;
    bool _is_sub;
public:
    typedef void(*function_type)(const cv::Mat&,cv::Mat&);
private:
    function_type _fun;
    virtual void callback(const sensor_msgs::ImageConstPtr&);
    virtual void grab(void);
public:
    ImageGenerator(const NodeHandle&,const std::string&,const std::string&,const std::string&,function_type);
    ~ImageGenerator(void);
    void operator>>(cv::Mat&);
    void operator<<(const cv::Mat&);

};
<i>
// image.cpp
</i>
bool ImageGenerator::_sht = false;

void ImageGenerator::grab(void){
    if(!this->_is_sub)
        this->_set->_is.registerCallback(boost::bind(&ImageGenerator::callback,this,_1));

    ros::CallbackQueue* mloop = ros::getGlobalCallbackQueue();
    while(!this->_sht)
        mloop->callAvailable(ros::WallDuration(0.1f));
    this->_sht = true;
    mloop = NULL;
    this->_is_sub = true;
}
void ImageGenerator::callback(const sensor_msgs::ImageConstPtr &msg){
    cv_bridge::CvImagePtr cv_ptr;
    cv_ptr = cv_bridge::toCvCopy(msg);
    this->_data = cv_ptr->image;
}
ImageGenerator::ImageGenerator(const NodeHandle & nh, const std::string & subscribe, const std::string & publish, const std::string & camera_info, function_type fun):_nh(nh),base::generator(_nh,subscribe,publish,camera_info),_fun(fun){ this->grab();}
ImageGenerator::~ImageGenerator(void){}

我想解决的问题是

void

图像生成器：：抓取（void）

它：

this->_set->_is.registerCallback（boost：：bind（&ImageGenerator：：callback，this，_1））;

编译器的答案：

错误 无效使用不完整的类型生成器：：it_set