基於結構成員函數的值生成狀態

Question

我目前正試圖想出一個漂亮的解決方案，基於結構生成基於整數的狀態。

struct status{ 
public: 
    status(); 
    /** 
    * @brief busy 
    * true = Currently handling a message in manual mode 
    * false = Not handling 
    */ 
    bool busy; 
    /** 
    * @brief speed 
    * Variable containing the current speed 
    * Speed possibilities [FAST;MEDIUM;SLOW] 
    */ 
    int speed; 
    /** 
    * @brief powered 
    * A boolean determining whether it is powered or not. 
    * true = ON 
    * false = OFF 
    */ 
    bool powered; 
    /** 
    * @brief direction 
    * A boolean determing the direction 
    * true = FORWARD 
    * false = BACKWARDS 
    */ 
    bool direction; 

}; 

該函數需要接受一個struct的實例，並根據成員變量生成一個唯一的狀態。

什麼是一個漂亮的解決方案，不涉及手動檢查，或設置所有的可能性，從而產生狀態？

Answer 1

您可以使用位集（std::bitset或無符號數字類型）來表示您的唯一狀態。

你將需要：

• 1位busy。
• 1位powered。
• 1位direction。
• 2位speed。

總共需要5位來表示所有可能的組合。

例子：

auto status::hash() const noexcept 
{ 
    std::bitset<5> b; 
    b |= speed; // assumes only the last two bits in `speed` are used 
    b.set(4, busy); 
    b.set(3, powered); 
    b.set(2, direction); 
    return b; 
} 

wandbox example

Answer 2

不如std::bitset，但你可以使用位域的整個結構存儲在一個字節：

struct status { 
public: 
    status(Busy busy, Speed speed, Powered powered, Direction direction) 
     : busy{busy}, speed{speed}, powered{powered}, direction{direction}, pad{0} {}; 

    Busy busy : 1; 
    Speed speed : 2; 
    Powered powered : 1; 
    Direction direction : 1; 
    unsigned char pad : 3; // pad to 8 bits 
}; 

全部程序：

#include <bitset> 
#include <iostream> 

#define ENUM_MACRO3(name, v1, v2, v3)\ 
    enum class name : unsigned char { v1, v2, v3};\ 
    std::ostream& operator<<(std::ostream& os, name var) {\ 
     switch (var){\ 
      case name::v1: return os << #v1;\ 
      case name::v2: return os << #v2;\ 
      case name::v3: return os << #v3;\ 
     }\ 
     return os;\ 
    } 

#define ENUM_MACRO2(name, v1, v2)\ 
    enum class name : unsigned char { v1, v2};\ 
    std::ostream& operator<<(std::ostream& os, name var) {\ 
     switch (var){\ 
      case name::v1: return os << #v1;\ 
      case name::v2: return os << #v2;\ 
     }\ 
     return os;\ 
    } 

ENUM_MACRO3(Speed, fast, medium, slow) 
ENUM_MACRO2(Busy, handling, not_handling) 
ENUM_MACRO2(Powered, on, off) 
ENUM_MACRO2(Direction, forwards, backwards) 

struct status { 
public: 
    status(Busy busy, Speed speed, Powered powered, Direction direction) 
     : busy{busy}, speed{speed}, powered{powered}, direction{direction}, pad{0} {}; 

    Busy busy : 1; 
    Speed speed : 2; 
    Powered powered : 1; 
    Direction direction : 1; 
    unsigned char pad : 3; // pad to 8 bits 
}; 

int main() 
{ 
    status s{Busy::not_handling,Speed::slow,Powered::off,Direction::backwards}; 

    std::cout << "Data has size of " << sizeof(status) << '\n'; 
    std::cout << "busy :" << s.busy << '\n'; 
    std::cout << "speed :" << s.speed << '\n'; 
    std::cout << "powered :" << s.powered << '\n'; 
    std::cout << "direction :" << s.direction << '\n'; 

    unsigned char val = reinterpret_cast<unsigned char&>(s); 
    unsigned int num{val}; 
    std::cout << num << '\n'; 
    std::bitset<8> bs{num}; 
    std::cout << bs << '\n'; 
    return 0; 
} 

產地：

Data has size of 1 
busy :not_handling 
speed :slow 
powered :off 
direction :backwards 
29 
00011101 

---

幾點要牢記：

• Bit fields不可移植。另一個實現可能：
  • 使用多個字節。
  • 顛倒位。
  • 引入填充以不同地對齊位。
• 上面的程序正在打破strict aliasing rule。
  • 因此，最好用std::bitset通過直接設置位來安全地生成散列。
• 位域比較慢。