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?
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:
#include <SDL.h>
#include <iostream>
void ExplainFlags() {
// Example flags value where multiple
// states are true
Uint32 Flags{
SDL_WINDOW_FULLSCREEN_DESKTOP |
SDL_WINDOW_SHOWN
};
// Check individual flags
bool IsFullscreen{
(Flags & SDL_WINDOW_FULLSCREEN) != 0
};
bool IsShown{
(Flags & SDL_WINDOW_SHOWN) != 0
};
// Wrong way - this won't work!
bool WrongCheck{Flags == SDL_WINDOW_FULLSCREEN};
std::cout << std::boolalpha
<< "Is fullscreen: " << IsFullscreen
<< "\nIs shown: " << IsShown
<< "\nWrong check: " << WrongCheck;
}Is fullscreen: true
Is shown: true
Wrong check: falseUsing == 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:
#include <SDL.h>
#include <bitset>
#include <iostream>
void ShowBitPatterns() {
// For demonstration, using small
// values instead of actual SDL flags
constexpr Uint8 FLAG_A{0b00000001}; // Bit 0
constexpr Uint8 FLAG_B{0b00000010}; // Bit 1
constexpr Uint8 FLAG_C{0b00000100}; // Bit 2
// Set multiple flags
Uint8 Flags{FLAG_A | FLAG_B};
std::cout << "Flag patterns:\n"
<< std::bitset<8>(Flags) << " (All flags)\n"
<< std::bitset<8>(FLAG_A) << " (Flag A)\n"
<< std::bitset<8>(FLAG_B) << " (Flag B)\n"
<< std::bitset<8>(FLAG_C) << " (Flag C)\n\n"
<< "Checking flags:\n"
<< "Has Flag A: " << bool(Flags & FLAG_A)
<< "\nHas Flag B: " << bool(Flags & FLAG_B)
<< "\nHas Flag C: " << bool(Flags & FLAG_C);
}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
Fullscreen Windows
Learn how to create and manage fullscreen windows in SDL, including desktop and exclusive fullscreen modes.