How to move an object with the mouse in c++? - c++

Question

I have a problem trying to create a script that moves an object with the mouse. The object is supposed to move when I hold the mouse button (which button does not really matter, so let us leave it there. I use SDL, so I have no trouble implementing this). I could of course have a variable moving = true when I start the move and then update position until moving = false, but this seems to be possibly messy. What I want is instead to be able to call a function grab_piece and then let the piece follow the mouse until I call the release function (I am creating a chess game, from where piece comes).
So the ideas, and relevant code this far: The idea is to assign a pointer to the mouse x and y positions and this would of course move the piece at the same time as the mouse. However, this seems tedious since shifting around pointers seems to be highly error prone. Is it possible to do this in a safe way? Otherwise I am open for other proposals.
Example code
SDL_Event e; //Event queue more or less
while (!quit) //Main loop, a little simplified
while (SDL_PollEvent( &e ) != 0){
if (e.type == SDL_QUIT){
quit = true;
}
if(e.type == SDL_MOUSEMOTION){
SDL_GetMouseState(&mouseX, &mouseY);
//do irrelevant things
} else if (e.type == SDL_MOUSEBUTTONDOWN){
//get mouse position
SDL_GetMouseState(&mouseX, &mouseY);
//get the board position
mouseBinPos = misc::screen2idx(mouseX, mouseY);
//location in the piece vector (hard coded)
currentPieceIdx = chess_board.getMappedPiece(mouseBinPos);
if (currentPieceIdx>=0){
chess_board.getPiece(currentPieceIdx)->grab_piece(mouseX, mouseY);
}
} else if ((e.type == SDL_MOUSEBUTTONUP)) {
//will release the piece and snap to nearest bin. Not implemented yet.
snap2bin(in args);
}
}
}
void Piece::grab_piece(int mouseX, int mouseY){
//Set pixel positions
setPixX(mouseX);
setPixY(mouseY);
}
All the variables are integers except the event queue, the board and the pieces which is a struct, user defined class and user defined class.
This is actually more a design issue, so I do not require executable code. More tips on how to do the design, if I should assign a pixX and pixY should be pointers to mouseX and mouseY and so on.
If anyone have an idea I would be happy. I have tried to describe the problem as good as possible, but if you have any questions please comment. I would describe further attempts if I had any, but I do not really know where to start. Also, this is not work, or school and I have been thinking about this for some days now and not found a good way to do it. So I am not just trying to get away easily.

Answer 1

Your ideas about pointers seem ill-defined, and are probably leading you down a path that isn't useful.
You need to use the same event loop as always when you're moving a piece. That means that MOUSEMOTION sometimes has to adjust the location of a piece, and sometimes not. That means that somehow you need to track the "moving a piece" state.
Here's the basic recipe:
On button down, save the idx of the piece that you've clicked on, maybe in a variable called "piece_being_dragged", or something. You also need to save the offset of mouse pointer to object center, so that the object doesn't recenter itself right on the pointer. You'll see what I mean when you get there.
On button up, zero the drag state variables, and handle the "done moving" logic.
On mousemotion, when piece_being_dragged is non-zero, look the piece up from the index, and set the new location based on the mouse pointer, and on the previously saved center offset.
Is this all a little messy? Yes. Is it necessary? Yes.