Hi there fellas,  I'm the new guy in need of help.

I'm writing a simple tangram puzzle using c++ and sdl2. Goal of the game is to fit pieces on the grid. Piece movement is done via mouse drag and drop.

The problem I'm having is when the cursor is over overlapping objects and I click and drag every shape under it gets dragged. I need some kind of check for object on top or something like that (I'm thinking it has to do something with height). Any help would be  greatly appreciated.

I've made Vec2 class that holds two int values for anything that has x  and y values.

Here is GameObject code that I use for puzzle pieces :

void GameObject::render() {
    SDL_SetRenderDrawColor(Game::Instance()->getRenderer(), m_color.r, m_color.g, m_color.b, m_color.a);
    SDL_Rect rects[] = { 
     m_position.getX(), m_position.getY(), m_dimension.getX(), m_dimension.getY(),
     m_position2.getX(), m_position2.getY(), m_dimension2.getX(), m_dimension2.getY() };
    
    SDL_RenderFillRects(Game::Instance()->getRenderer(), rects, 2);
    SDL_RenderDrawRects(Game::Instance()->getRenderer(),rects, 2);
}

void GameObject::update() {
    if (InputHandler::Instance()->getMouseButtonState(0) && 
        intersects(m_position, m_dimension, *InputHandler::Instance()->getMouseButtonPosition()) 
        || intersects(m_position2, m_dimension2, *InputHandler::Instance()->getMouseButtonPosition())) {

            offset = (*InputHandler::Instance()->getMouseButtonPosition() - m_position);
            offset2 = (*InputHandler::Instance()->getMouseButtonPosition() - m_position2);

            dragging = true;
    }
    else if (!InputHandler::Instance()->getMouseButtonState(0) && dragging) {
        offset = Vec2(0,0);
        offset2 = Vec2(0,0);
        dragging = false;
   }
   if(dragging){
        m_position = *InputHandler::Instance()->getMousePosition() - offset;
        m_position2 = *InputHandler::Instance()->getMousePosition() - offset2;
   }
}

bool GameObject::intersects(Vec2 object, Vec2 dimensions, Vec2 mouse) {
    if(mouse.getX() < object.getX() || mouse.getY() < object.getY()) {
        return false;
    }

    if(mouse.getX() > object.getX() + dimensions.getX() || mouse.getY() > object.getY() + dimensions.getY()) {
         return false;
    }
    return true;
}

And here is it being called in the game singleton.

void Game::render() {
    SDL_SetRenderDrawColor(m_pRenderer, 0, 0, 0, 0);
    SDL_RenderClear(m_pRenderer);    
    for (int i = 0; i < m_gameObjects.size(); i++) {
        m_gameObjects[i]->render();

    }
    SDL_RenderPresent(m_pRenderer);
}

void Game::update() {
    for (int i = m_gameObjects.size()-1; i >= 0; i--) {
            m_gameObjects[i]->update();
    }
}

And here is input handler, also a singleton:

void InputHandler::update() {
    SDL_Event event;

    while (SDL_PollEvent(&event)) {
        m_keystates = SDL_GetKeyboardState(0);
        if (event.type == SDL_QUIT) {
            Game::Instance()->quit();
           
        }
        if (event.type == SDL_MOUSEBUTTONDOWN) {
                m_mouseButtonPosition->setX(event.button.x);
                 m_mouseButtonPosition->setY(event.button.y);
            if (event.button.button == SDL_BUTTON_LEFT) {
                 
                 m_mouseButtonStates[LEFT] = true;
            }

            if (event.button.button == SDL_BUTTON_MIDDLE) {
                m_mouseButtonStates[MIDDLE] = true;
            }

            if (event.button.button == SDL_BUTTON_RIGHT) {
                m_mouseButtonStates[RIGHT] = true;
            }
        }

        if (event.type == SDL_MOUSEBUTTONUP) {
            if (event.button.button == SDL_BUTTON_LEFT) {
                m_mouseButtonStates[LEFT] = false;
            }

            if (event.button.button == SDL_BUTTON_MIDDLE) {
                m_mouseButtonStates[MIDDLE] = false;
            }

            if (event.button.button == SDL_BUTTON_RIGHT) {
                m_mouseButtonStates[RIGHT] = false;
            }
        }

        if (event.type == SDL_MOUSEMOTION) {
            m_mousePosition->setX(event.motion.x);
            m_mousePosition->setY(event.motion.y);
        }
    }
}

Maybe you can like search for overlap outside the object and for Release inside of them? (code below)

GameObject* LastDragged = nullptr;
void Game::update() {
	//if released, check it's current overlaps and give it an higer height since it is on top
	if (!LastDragged->IsDragged)
	{
		for (auto& Obj : m_gameObjects)
		{
			if (IsOverlapping(LastDragged,Obj))
			{
				//give higher height to LastDragged
			}
		}
	}
	else
	{
		//Mak a list of who is dragged outside the objects themselves
		vector<GameObject*> Overlaps;
		for (auto& Obj : m_gameObjects)
		{
			if (IsMouseOverlapping(Obj))
			{
				Overlaps.push_back(&Obj);
			}
		}

		//inside of this set the dragged bool to true only for the one on top
		//and return the currently dragged
		LastDragged = FindMaxHeight(Overlaps);
	}

	//update position
	for (int i = m_gameObjects.size() - 1; i >= 0; i--) {
		m_gameObjects[i]->update();
	}
}

And as you said use an height to set the last released as the one on top.

But I am a begginner so take this as a grain of salt and listen to more experienced and efficient suggestions 

My code above seems not the best one since you iterate trough the objects list multiple times, but seems the easiest for sure.

Rather than having each piece move itself based on input logic, create some sort of pointer object that represents the player (who picks up, moves and drops pieces). On mouse button down, have the pointer capture all overlapping puzzle pieces into an array by iterating over them and running the same intersection logic you had placed in the puzzle piece classes. Find out which piece in the array has the highest height (however you're keeping track of draw order should work fine), then set that piece (only that piece) to follow the mouse cursor until a mouse button release event.

EDIT: just read marcus's post, seems like he's getting at the same sort of idea.

Each movable object should have a value, height, heights start at zero. Each movable object should have an Area (or other mechanism) which describes how large or small the object is, so that you do not place large objects over small ones and obscure them. Instead always place larger objects on bottom and smaller ones on top, this modifies the height value. When the mouse clicks an object, get the list of objects that are intersecting, sort through the list and get the top most object, the one with the largest height, and hang onto that object as it is the selected object. When the user stops dragging and releases the object, go through and generate the intersection list, insert the object into it, sort the list based on Area to generate new height values for objects in the list.

So I've been trying to make this work, but I only got it to work for shape that has highest y value. Missing something, can't pin point what >.<

This is modified update method:

std::vector<int> heights;
void Game::update() {
    for(int i = 0; i < pieces.size(); i++) {
        heights.push_back(pieces[i]->getPosition().getY());
    if (InputHandler::Instance()->getMouseButtonState(LEFT)) {
        if (intersects(pieces[i]->getPosition().getX(), pieces[i]->getPosition().getY(), 100,
                        InputHandler::Instance()->getMouseButtonPosition()->getX(),
                        InputHandler::Instance()->getMouseButtonPosition()->getY())) {

            int it = *std::max_element(std::begin(heights), std::end(heights));

            if (pieces[i]->getPosition().getY() == it) {
                pieces[i]->setSelected(true);
            }
            if (!InputHandler::Instance()->getMouseButtonState(LEFT)){
                pieces[i]->setSelected(false);
            }
        }
    } else {
            if(InputHandler::Instance()->getMouseButtonState(LEFT)){
                pieces[i]->setSelected(true);
            }
            if(!InputHandler::Instance()->getMouseButtonState(LEFT)){
                pieces[i]->setSelected(false);
            }
        }

        heights.clear();
        heights.push_back(pieces[i]->getPosition().getY());
        pieces[i]->update();
    }
}

First question: why do you store all the heights of all pieces on every update? If you have 5000 pieces seems expensive, only objects under the mouse should be considered.

Furthermore, on the line:

 heights.push_back(pieces[i]->getPosition().getY());

why you store the Y into heights? Shouldn't X,Y be your coordinate on the grid (top down view) and Z the height?

I have only 7 pieces, but yeah I agree, that should be put elsewhere.

I treat y as height since sdl2 doesn't have z value. I dunno how to invent that(unless it can be anything I want like index in the array or something like that).

22 minutes ago, Dzuvan said:

I have only 7 pieces, but yeah I agree, that should be put elsewhere.

I treat y as height since sdl2 doesn't have z value. I dunno how to invent that(unless it can be anything I want like index in the array or something like that).

You can do it like this:

class Piece{
	SDL_Point position;
	int height;
  public:
  //...
  
  int getZ()const {return height;}
};

SDL has no saying in what you put inside your objects

So, this is the solution I've come up with,  It kinda works but sometimes pieces still go together.

Z values are going from one to n of pieces. So first piece has z value 1, second 2 and so on...

std::vector<GameObject*> overlaps;
std::vector<int> zs; //heights

void Game::update() {
    // Grab pieces under button and push them in vector.
    if (InputHandler::Instance()->getMouseButtonState(LEFT)){
        for(int i = 0; i < pieces.size(); i++) {
            if (intersects(pieces[i]->getPosition().getX(),pieces[i]->getPosition().getY(), 100,
                        InputHandler::Instance()->getMouseButtonPosition()->getX(),
                        InputHandler::Instance()->getMouseButtonPosition()->getY())) {
                overlaps.push_back(pieces[i]);
            }
        }
    }

    if (InputHandler::Instance()->getMouseButtonState(LEFT)) {
        for (GameObject* o : overlaps) {
            zs.push_back(o->getZ());
            int it = *std::max_element(std::begin(zs), std::end(zs));
            if (o->getZ() == it) {
                o->setSelected(true);
            }
        }
    }
    else {
        for (GameObject* o : overlaps) {
            o->setSelected(false);
            overlaps.clear();
            zs.clear();
        }
    }

    for(int i = 0; i < pieces.size(); i++) {
        pieces[i]->update();
    }
}