Fullscreen Windows

Using Bitwise AND with Window Flags

What's the purpose of the bitwise AND operator (`&`) when checking window flags? Why can't we use `==` instead?

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The bitwise AND operator (&) is used with window flags because SDL uses a technique called bit flags to store multiple boolean values in a single integer. Each bit in the flags value represents a different window state.

How Bit Flags Work

Here’s an example where we set and check bit flags:

1#include <SDL.h>
2#include <iostream>
3
4void ExplainFlags() {
5  // Example flags value where multiple
6  // states are true
7  Uint32 Flags{
8    SDL_WINDOW_FULLSCREEN_DESKTOP |
9    SDL_WINDOW_SHOWN
10  };
11
12  // Check individual flags
13  bool IsFullscreen{
14    (Flags & SDL_WINDOW_FULLSCREEN) != 0
15  };
16  bool IsShown{
17    (Flags & SDL_WINDOW_SHOWN) != 0
18  };
19
20  // Wrong way - this won't work!
21  bool WrongCheck{Flags == SDL_WINDOW_FULLSCREEN};
22
23  std::cout << std::boolalpha
24    << "Is fullscreen: " << IsFullscreen
25    << "\nIs shown: " << IsShown
26    << "\nWrong check: " << WrongCheck;
27}

1Is fullscreen: true
2Is shown: true
3Wrong check: false

Using == wouldn't work because it checks if the entire flags value exactly matches a single flag. When multiple flags are set (which is common), == would return false even when the flag we're interested in is set.

Visualizing Bit Patterns

Here's a simplified example showing how bits work:

1#include <SDL.h>
2#include <bitset>
3#include <iostream>
4
5void ShowBitPatterns() {
6  // For demonstration, using small
7  // values instead of actual SDL flags
8  constexpr Uint8 FLAG_A{0b00000001}; // Bit 0
9  constexpr Uint8 FLAG_B{0b00000010}; // Bit 1
10  constexpr Uint8 FLAG_C{0b00000100}; // Bit 2
11
12  // Set multiple flags
13  Uint8 Flags{FLAG_A | FLAG_B};
14
15  std::cout << "Flag patterns:\n"
16    << std::bitset<8>(Flags) << " (All flags)\n"
17    << std::bitset<8>(FLAG_A) << " (Flag A)\n"
18    << std::bitset<8>(FLAG_B) << " (Flag B)\n"
19    << std::bitset<8>(FLAG_C) << " (Flag C)\n\n"
20    << "Checking flags:\n"
21    << "Has Flag A: " << bool(Flags & FLAG_A)
22    << "\nHas Flag B: " << bool(Flags & FLAG_B)
23    << "\nHas Flag C: " << bool(Flags & FLAG_C);
24}

This pattern allows SDL to efficiently store and check multiple window states using a single integer value.

We cover bit flags and bitwise operators in a dedicated lesson:

Bitwise Operators and Bit Flags

Unravel the fundamentals of bitwise operators and bit flags in this practical lesson

This Question is from the Lesson:

Fullscreen Windows

Learn how to create and manage fullscreen windows in SDL, including desktop and exclusive fullscreen modes.

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5 months ago

This Question is from the Lesson:

Fullscreen Windows

Learn how to create and manage fullscreen windows in SDL, including desktop and exclusive fullscreen modes.

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Using Bitwise AND with Window Flags

What's the purpose of the bitwise AND operator (`&`) when checking window flags? Why can't we use `==` instead?

How Bit Flags Work

Visualizing Bit Patterns

Bitwise Operators and Bit Flags

Fullscreen Windows

Why would we want to use desktop fullscreen mode if exclusive fullscreen has better performance?

What happens if a player has a monitor with a different aspect ratio than our game's resolution?

How can I smoothly transition between windowed and fullscreen modes without the screen going black?

When using exclusive fullscreen, how can I ensure the display settings are properly restored if my game crashes?

How can I save the player's preferred fullscreen mode and restore it the next time they launch the game?

Fullscreen Windows

Game Dev with SDL2

This course includes:

Professional C++

Using Bitwise AND with Window Flags

What's the purpose of the bitwise AND operator (&) when checking window flags? Why can't we use == instead?

How Bit Flags Work

Visualizing Bit Patterns

Bitwise Operators and Bit Flags

Fullscreen Windows

Why would we want to use desktop fullscreen mode if exclusive fullscreen has better performance?

What happens if a player has a monitor with a different aspect ratio than our game's resolution?

How can I smoothly transition between windowed and fullscreen modes without the screen going black?

When using exclusive fullscreen, how can I ensure the display settings are properly restored if my game crashes?

How can I save the player's preferred fullscreen mode and restore it the next time they launch the game?

Fullscreen Windows

Game Dev with SDL2

This course includes:

Professional C++

What's the purpose of the bitwise AND operator (`&`) when checking window flags? Why can't we use `==` instead?