C++ Nested forward declaration inheritence - c++

Question

My issue is :
I define class (generator) inside of which I define a forward nested structs (topics and it_set).
I make the declaration of this nested class inside the .cpp file.
After this I declare a second class (ImageGenerator) which is an inheritence of generator.
I get an issue when I try inside of the declaration file of ImageGenerator.
Is there anyway to make that possible ?
My codes are these :
<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){}
The issue which I want to solve is at
void ImageGenerator::grab(void)
It's :
this->_set->_is.registerCallback(boost::bind(&ImageGenerator::callback,this,_1));
the compiler answer :
error invalid use of incomplete type generator::it_set

Answer 1

The type is incomplete because the compiler hasn't seen the definition of that struct.
If you want to use the structs in subclasses of generator, you need to move their definitions inside the definition of generator in "base.hpp".