bitwise.cpp

// ============================================================
// Ch02 - 位元運算子、constexpr、固定寬度型別、cast 示範
// 編譯：g++ -std=c++17 -Wall -Wextra -o bitwise bitwise.cpp
// 執行：./bitwise
// ============================================================

#include <iostream>
#include <bitset>      // std::bitset 方便以二進位顯示
#include <cstdint>     // 固定寬度整數型別

// 用 bitset 把整數印成 8 位元二進位字串
static std::string toBinary8(unsigned int value) {
    return std::bitset<8>(value).to_string();
}

int main() {
    // ══════════════════════════════════════════
    // 1. 六個位元運算子
    // ══════════════════════════════════════════
    std::cout << "=== 位元運算子 ===" << std::endl;

    unsigned int a = 0b1100;  // 12
    unsigned int b = 0b1010;  // 10

    std::cout << "a      = " << toBinary8(a) << " (" << a << ")" << std::endl;
    std::cout << "b      = " << toBinary8(b) << " (" << b << ")" << std::endl;
    std::cout << "a & b  = " << toBinary8(a & b) << " (" << (a & b) << ")  AND" << std::endl;
    std::cout << "a | b  = " << toBinary8(a | b) << " (" << (a | b) << ")  OR" << std::endl;
    std::cout << "a ^ b  = " << toBinary8(a ^ b) << " (" << (a ^ b) << ")  XOR" << std::endl;
    std::cout << "~a     = " << toBinary8(~a) << "        NOT(低 8 位)" << std::endl;

    // ══════════════════════════════════════════
    // 2. 位移運算子（乘除 2 的次方）
    // ══════════════════════════════════════════
    std::cout << "\n=== 位移運算子 ===" << std::endl;

    int x = 5;
    std::cout << "5 << 1 = " << (x << 1) << "（左移 1 位 = ×2）" << std::endl;   // 10
    std::cout << "5 << 3 = " << (x << 3) << "（左移 3 位 = ×8）" << std::endl;   // 40
    std::cout << "40 >> 2 = " << (40 >> 2) << "（右移 2 位 = ÷4）" << std::endl;  // 10

    // ══════════════════════════════════════════
    // 3. 位元旗標（flags）的實際應用
    // ══════════════════════════════════════════
    std::cout << "\n=== 位元旗標 ===" << std::endl;

    constexpr unsigned int READ    = 1u << 0;  // 0b001
    constexpr unsigned int WRITE   = 1u << 1;  // 0b010
    constexpr unsigned int EXECUTE = 1u << 2;  // 0b100

    unsigned int permission = 0;
    permission |= READ;            // 開啟讀取
    permission |= WRITE;           // 開啟寫入
    std::cout << "權限位元：" << toBinary8(permission) << std::endl;

    std::cout << "可讀取？  " << std::boolalpha << static_cast<bool>(permission & READ) << std::endl;
    std::cout << "可執行？  " << static_cast<bool>(permission & EXECUTE) << std::endl;

    permission &= ~WRITE;          // 關閉寫入
    std::cout << "關閉寫入後：" << toBinary8(permission) << std::endl;

    // ══════════════════════════════════════════
    // 4. constexpr 編譯期常數
    // ══════════════════════════════════════════
    std::cout << "\n=== constexpr ===" << std::endl;

    constexpr int BUFFER_SIZE = 8 * 1024;   // 編譯期就算好
    int buffer[BUFFER_SIZE] = {};           // 陣列大小需要編譯期常數
    std::cout << "BUFFER_SIZE = " << BUFFER_SIZE
              << "，buffer 第一個元素 = " << buffer[0] << std::endl;

    // ══════════════════════════════════════════
    // 5. 固定寬度型別與 static_cast
    // ══════════════════════════════════════════
    std::cout << "\n=== 固定寬度型別 ===" << std::endl;

    uint8_t  byteVal = 200;
    int32_t  big     = 2000000000;

    // 注意：uint8_t 本質是 char，直接輸出會被當字元，需轉成 int
    std::cout << "uint8_t 值：" << static_cast<int>(byteVal) << std::endl;
    std::cout << "int32_t 值：" << big << "（" << sizeof(big) << " bytes）" << std::endl;

    return 0;
}