SDL_WINDOWPOS_UNDEFINED Explained

The SDL_WINDOWPOS_UNDEFINED macro is used to let SDL determine the position of a new window automatically. When creating a window using SDL_CreateWindow(), you can pass this macro for the x and y parameters to leave the window's placement up to SDL.

How It Works

When you specify SDL_WINDOWPOS_UNDEFINED, SDL doesn’t assign a fixed position to the window. Instead, the operating system decides where the window appears, usually defaulting to the top-left corner of the primary monitor or an arbitrary position chosen by the window manager.

Here’s an example:

1#include <SDL2/SDL.h>
2#include <iostream>
3
4int main() {
5  if (SDL_Init(SDL_INIT_VIDEO) != 0) {
6    std::cerr << "SDL_Init failed: "
7      << SDL_GetError() << '\n';
8    return 1;
9  }
10
11  SDL_Window* window = SDL_CreateWindow(
12    "My Window",
13    SDL_WINDOWPOS_UNDEFINED, 
14    SDL_WINDOWPOS_UNDEFINED, 
15    800, 600, 0);
16
17  if (!window) {
18    std::cerr << "Failed to create window: "
19      << SDL_GetError() << '\n';
20    SDL_Quit();
21    return 1;
22  }
23  
24  // Keep the window open for 2 seconds
25  SDL_Delay(2000);
26  SDL_DestroyWindow(window);
27  SDL_Quit();
28  return 0;
29}

When to Use It

• Quick Prototyping: When you don’t care about the window’s position during development.
• Cross-Platform Simplicity: It lets the OS decide the best default placement, avoiding platform-specific logic.
• Minimizing Visual Disruption: Ensures users’ existing window arrangements are not overridden by fixed positioning.

Alternatives

For specific placement, consider SDL_WINDOWPOS_CENTERED or display-specific macros like SDL_WINDOWPOS_CENTERED_DISPLAY(n). These provide more control over where your window appears.