just want to point out, this unjustly uses std::shared_ptr

in main.cpp

  auto flappy = std::make_shared<FlappyBird>();
  flappy->run();

you can do this.

  FlappyBird flappy;
  flappy.run();

in flappy.hpp

  std::shared_ptr<sf::RenderWindow> window;
  std::shared_ptr<sf::Sprite> background, bird, pipeBottom, pipeTop;

just do this.

  sf::RenderWindow window;
  sf::Sprite background, bird, pipeBottom, pipeTop;

in flappy.cpp.

  auto e = std::make_shared<sf::Event>();
  while( window->pollEvent( *e ) ){
    if( e->type == sf::Event::Closed){
      window->close();
    }
  }

just do this

  sf::Event event;
  while (window.pollEvent(event)) {
    if (e.type == sf::Event::Closed) {
      window.close();
    }
  }

It's really that simple.