Handling the Window Close Event in SDL2

Clicking the standard window close button ('X' on Windows/Linux, red circle on macOS) doesn't automatically close your SDL application. Instead, the operating system sends a specific signal to your application indicating the user wants to quit. SDL translates this signal into an event of type SDL_QUIT.

To make your window close properly, you need to modify your main application loop to:

1. Check for pending events.
2. Specifically look for an event of type SDL_QUIT.
3. If SDL_QUIT is detected, signal your main loop to stop running.

Implementing the Event Loop

The standard way to do this is using SDL_PollEvent(). This function checks SDL's internal event queue (which is populated by calls to SDL_PumpEvents(), often implicitly by SDL_PollEvent itself) for the next pending event. If an event exists, SDL_PollEvent() fills an SDL_Event structure with the event details and returns 1 (true). If no events are pending, it returns 0 (false).

We use a while loop with SDL_PollEvent() to process all events that might have occurred since the last frame, and a boolean flag (e.g., running) to control the main application loop.

Here's how you can modify the main.cpp from the lesson to handle the close event:

1// main.cpp
2#include <SDL.h>
3#include "Window.h"
4
5int main(int argc, char** argv) {
6  SDL_Init(SDL_INIT_VIDEO);
7  Window GameWindow; // Creates the window via constructor
8
9  bool running = true;  // Add a flag to control the loop
10  SDL_Event event;      // Structure to hold event info
11
12  // Replace while(true) with while(running)
13  while (running) {  // Loop continues as long as running is true
14
15    // Process all pending events this frame
16    while (SDL_PollEvent(&event)) {  // Check for events
17      // Check if the event is the quit signal
18      if (event.type == SDL_QUIT) { 
19        running = false; // Signal the main loop to stop <h>
20      }
21      // We could check for other events here (keyboard, mouse)
22    }
23
24    // If we were rendering or updating game state,
25    // it would happen here, outside the event polling loop
26    // but inside the main 'while(running)' loop.
27
28    // SDL_PumpEvents() is not strictly needed here, as
29    // SDL_PollEvent() handles event pumping internally.
30  }
31
32  // Destructor of GameWindow runs here, calling SDL_DestroyWindow
33
34  SDL_Quit(); // Clean up SDL subsystems
35  return 0;
36}

With this structure:

• The while(running) loop keeps the application alive.
• SDL_PollEvent() checks for user actions or system events.
• When the 'X' button is clicked, an SDL_QUIT event is generated.
• The if (event.type == SDL_QUIT) condition becomes true.
• running is set to false.
• The while(running) loop condition fails on the next iteration, and the loop terminates.
• The program proceeds to SDL_Quit() and exits gracefully.

Event handling is a fundamental part of SDL applications, and we will cover it much more thoroughly in the upcoming lessons, including handling keyboard and mouse input.