C++ STD Vector push_back doesn't seem to work

Question

I'm making a game with SDL that used libconfig to read some settings from a file. The problem is that I made a class called ClipList that contains a std::vector<SDL_Rect> to store the settings but when trying to add SDL_Rect objects to the vector, for some reason push_back does nothing and I end up with an empty vector.

This is the class:

class ClipList
{
    public:
        ClipList();
        ClipList(int);
        virtual ~ClipList();
        void addClip(int,int,int,int);
        void getClip(int,SDL_Rect*);
        int getLength();
    protected:
    private:
    std::vector<SDL_Rect> clips;
};
ClipList::ClipList(int l)
{
    clips.reserve(l);
}

void ClipList::addClip(int x,int y,int w,int h){
    SDL_Rect rect;
    rect.x = x;
    rect.y = y;
    rect.w = w;
    rect.h = h;
    clips.push_back(rect);
}

void ClipList::getClip(int i,SDL_Rect* rect){
rect = &(clips.at(i));
}

int ClipList::getLength(){
    return clips.size();
}

And this is the function where I initialize the ClipList object. This function gets called from main.

void set_clips(Config* placlips,ClipList* clips, ClipList* flipclips){
    const Setting& root = placlips->getRoot();
    int x,y,w,h;
    try{
        Setting& clipsett = root["clips"];
        int cliplen = clipsett.getLength();
        clips = new ClipList(cliplen);
        flipclips = new ClipList(cliplen);
        for(int i=0;i<cliplen;i++){
            const Setting& c = clipsett[i];
            if(!(c.lookupValue("x",x)&&c.lookupValue("y",y)&&c.lookupValue("w",w)&&c.lookupValue("h",h))){
                continue;
            }
            clips->addClip(x,y,w,h);
        }
    }catch(const SettingNotFoundException &nfex){
        cerr << "Setting not found at" << nfex.getPath() << endl;
    }
}

Regardless of whether the ClipList objects get initialized in main or set_clips, clips.push_back(rect) doesn't work. The capacity of the vector changes but no object gets stored so I end up with a segfault if I try to do anything else with the vector, even checking if the vector is empty or not.

Answer 1

I am going to guess, the signature of the function

void set_clips(Config* placlips,ClipList* clips, ClipList* flipclips);

is the culprit. You are allocating memory for clips and flipclips in this function but since the pointers are passed by value, the calling function does not see the allocated memory.

If you change the function signature to:

void set_clips(Config* placlips, ClipList*& clips, ClipList*& flipclips);

your problems should go away.