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);
        }
    }
}

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();
    }
}

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).

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();
    }
}