How to dynamically created objects and access their pointer - c++

Question

This might seem like a trivial issue, but I can't seem to find a solution.
What I have:
void VideoHandler::demoBurn(QString fileName) {
// Create a reader for a video
openshot::FFmpegReader r("raw_videos/example0.mp4");
r.Open(); // Open the target reader
// Create a writer
openshot::FFmpegWriter w("edited_videos/NewVideo.mp4");
w.SetAudioOptions(true, "libvorbis", 44100, 2, openshot::ChannelLayout::LAYOUT_STEREO, 128000);
w.SetVideoOptions(true,"libx264" , openshot::Fraction(30,1), r.info.width, r.info.height, openshot::Fraction(1,1), false, false, 300000);
w.Open();
openshot::Timeline t(r.info.width,r.info.height, r.info.fps, 44100,2, openshot::ChannelLayout::LAYOUT_STEREO);
// Clip example
openshot::Clip c1(new openshot::QtImageReader("edited_videos/0.png"));
c1.Layer(1);
c1.Position(5.9);
c1.Start(5.9);
c1.End(10.0);
c1.scale = openshot::SCALE_NONE;
c1.gravity = openshot::GRAVITY_TOP_LEFT;
c1.location_x = 0.0;
c1.location_y = 0.2;
std::list<openshot::Clip> clipList;
for (int i = 0; i < 2; i++) {
QString imageName = QString("edited_videos/%1.png").arg(i);
clipList.push_back(openshot::Clip(new openshot::QtImageReader(imageName.toUtf8().toStdString())));
}
// Add clip values and add to timeline
std::list<openshot::Clip>::iterator it;
int test = 0;
for (it = clipList.begin(); it != clipList.end(); it++) {
it->Layer(1);
if(test == 0) {
it->Position(5.9);
it->Start(5.9);
it->End(10.0);
} else {
it->Position(10.0);
it->Start(10.0);
it->End(15.0);
}
it->scale = openshot::SCALE_NONE;
it->gravity = openshot::GRAVITY_TOP_LEFT;
it->location_x = 0.0;
it->location_y = 0.2;
test++;
t.AddClip(&it);
}
// Add clips to timeline
t.AddClip(&c1);
openshot::Clip c2(new openshot::FFmpegReader("raw_videos/example0.mp4"));
c2.Position(0.0);
c2.Layer(0);
t.AddClip(&c2);
// Open the timeline reader
t.Open();
// Close the timeline reader
w.WriteFrame(&t, 1, r.info.video_length);
// Close the reader & writer
t.Close();
w.Close();
r.Close();
}
I'm using libopenshot to burn some image overlays to the video, and since I don't know how many images there will be, I need to dynamically create a openshot::Clip for each one and give it some values.
I saw some examples on dynamic object allocation and thought this should work, but I'm getting compile errors:
/usr/include/c++/7/ext/new_allocator.h:136: error: use of deleted function ‘openshot::Clip::Clip(const openshot::Clip&)’
{ ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
/home/3rdparty/libopenshot/include/Clip.h:95: error: ‘juce::CriticalSection::CriticalSection(const juce::CriticalSection&)’ is private within this context
And for t.AddClip(&it) error:
cannot initialize a parameter of type 'openshot::Clip *' with an rvalue of type 'std::list<openshot::Clip>::iterator *' (aka '_List_iterator<openshot::Clip> *')
I can somewhat understand this error, but how do I give it the right pointer, I thought the it would hold the pointer to the object?
Since I never done dynamic object allocation, I'm not sure if this is the right way of doing it, am I using a wrong type of list?

Answer 1

std::list<openshot::Clip>::iterator it;
...
t.AddClip(&it);
I thought the it would hold the pointer to the object?
When you use the addressof operator - i.e. the unary & operator - on a value, what you get is pointer to that object. In this case, you use &it, and therefore you get a pointer to it. it is an iterator to a list.
However, AddClip is not expecting a pointer to an iterator. It is expecting a pointer to a openshot::Clip object. That is why you get the error:
cannot initialize a parameter of type [pointer to clip] with an rvalue of type [pointer to iterator]
I don't see how this could be affected by what the iterator "holds".
How might you get a pointer to a clip object? You apply the addressof operator on such object - rather than applying addressof operator on an object of some other type such as an iterator.
How might you get a clip object? You have an iterator to such object; you can indirect through the iterator to get the object.
How might you indirect through an iterator? You use the indirection operator i.e. the unary * operator.
For example:
t.AddClip(&*it);
Since I never done dynamic object allocation, I'm not sure if this is the right way of doing it
I don't know of openshot, so I cannot be certain, but those bare new expressions seem dubious. Unless that API mandates it, you should probably be using a unique pointer assuming dynamic allocation is needed in the first place.

Answer 2

After many trial and error, I managed to get it to work. What I changed is:
created a list of pointers (doesn't matter if it's QList or std:list, tested and both work the same)
and re-referenced all the iteration objects with pointers to the actual object.
Thanks to #eerorika for the info about (&*it)
QList<openshot::Clip*> clipList;
for (int i = 0; i < 2; i++) {
QString imageName = QString("edited_videos/%1.png").arg(i);
clipList.push_back(new openshot::Clip(new openshot::QtImageReader(imageName.toUtf8().toStdString())));
}
// Add clip values and add to timeline
QList<openshot::Clip*>::iterator it;
int test = 0;
for (it = clipList.begin(); it != clipList.end(); it++) {
(*it)->Layer(1);
if(test == 0) {
(*it)->Position(5.9);
(*it)->Start(5.9);
(*it)->End(10.0);
} else {
(*it)->Position(10.0);
(*it)->Start(10.0);
(*it)->End(15.0);
}
(*it)->scale = openshot::SCALE_NONE;
(*it)->gravity = openshot::GRAVITY_TOP_LEFT;
(*it)->location_x = 0.0;
(*it)->location_y = 0.2;
test++;
t.AddClip(&*(*it));
}