in MainWindow.cpp void MainWindow::InitializeGrid() { //resizes outer vector to grid size gameBoard.resize(gridSize); //resizes inner vectors to grid size and sets to 'false' for (int i = 0; i < gridSize; ++i) { gameBoard[i].resize(gridSize, false); } //passes SetGridSize to m_drawingPanel m_drawingPanel->SetGridSize(gridSize); }