Using Bitwise AND for Flags

Understanding why we use the bitwise AND operator (&) instead of equality (==) for checking flags requires understanding how bit flags work in SDL and many other programming contexts.

How Flags Work

Each flag is represented by a single bit in a larger integer. For example:

1// These are example values - actual SDL
2// values might differ
3// Bit 0
4const Uint32 FLAG_FULLSCREEN = 0b00000001;
5
6// Bit 1
7const Uint32 FLAG_MOUSE_GRABBED = 0b00000010;
8
9// Bit 2
10const Uint32 FLAG_BORDERLESS = 0b00000100;

When multiple flags are set, we combine them using bitwise OR (|):

1SDL_Window* Window{SDL_CreateWindow(
2  "Window Title",
3  100, 100, 800, 600,
4  SDL_WINDOW_FULLSCREEN | SDL_WINDOW_MOUSE_GRABBED
5)};

Why We Use Bitwise AND

When checking flags, we use & because:

1. A window can have multiple flags set simultaneously
2. & checks if specific bits are set, regardless of other bits

Here's an example showing the difference:

1#include <SDL.h>
2#include <iostream>
3
4void CheckFlags(Uint32 flags) {
5  // Incorrect way (using ==)
6  if (flags == SDL_WINDOW_MOUSE_GRABBED) { 
7    std::cout << "This might miss the flag";
8  }
9
10  // Correct way (using &)
11  if (flags & SDL_WINDOW_MOUSE_GRABBED) {  
12    std::cout << "This correctly detects the flag";
13  }
14}
15
16int main(int argc, char** argv) {
17  SDL_Init(SDL_INIT_VIDEO);
18
19  // Create window with multiple flags
20  SDL_Window* Window{SDL_CreateWindow(
21    "Flag Example", 100, 100, 800, 600,
22    SDL_WINDOW_MOUSE_GRABBED
23    | SDL_WINDOW_BORDERLESS
24  )};
25
26  Uint32 flags{SDL_GetWindowFlags(Window)};
27  CheckFlags(flags);
28
29  SDL_DestroyWindow(Window);
30  SDL_Quit();
31  return 0;
32}

1This correctly detects the flag!

The equality check fails because:

• flags == SDL_WINDOW_MOUSE_GRABBED checks if ONLY mouse grab is set
• But our window has both mouse grab AND borderless flags set

The bitwise AND succeeds because:

• flags & SDL_WINDOW_MOUSE_GRABBED checks if the mouse grab bit is set
• It doesn't care about other flags that might also be set