A basic game loop using std::chrono

game_loop.cpp

/*
 * The MIT License (MIT)
 * 
 * Copyright (c) 2016 Mario Badr
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include <chrono>

using namespace std::chrono_literals;

// we use a fixed timestep of 1 / (60 fps) = 16 milliseconds
constexpr std::chrono::nanoseconds timestep(16ms);

struct game_state {
  // this contains the state of your game, such as positions and velocities
};

bool handle_events() {
  // poll for events

  return false; // true if the user wants to quit the game
}

void update(game_state *) {
  // update game logic here
}

void render(game_state const &) {
  // render stuff here
}

game_state interpolate(game_state const & current, game_state const & previous, float alpha) {
  game_state interpolated_state;

  // interpolate between previous and current by alpha here

  return interpolated_state;
}

int main() {
  using clock = std::chrono::high_resolution_clock;

  std::chrono::nanoseconds lag(0ns);
  auto time_start = clock::now();
  bool quit_game = false;

  game_state current_state;
  game_state previous_state;

  while(!quit_game) {
    auto delta_time = clock::now() - time_start;
    time_start = clock::now();
    lag += std::chrono::duration_cast<std::chrono::nanoseconds>(delta_time);

    quit_game = handle_events();

    // update game logic as lag permits
    while(lag >= timestep) {
      lag -= timestep;

      previous_state = current_state;
      update(&current_state); // update at a fixed rate each time
    }

    // calculate how close or far we are from the next timestep
    auto alpha = (float) lag.count() / timestep.count();
    auto interpolated_state = interpolate(current_state, previous_state, alpha);

    render(interpolated_state);
  }
}