c++ - 如何使用 C++ 构造函数初始化位域？

Question

首先，我不关心可移植性，可以放心地假设字节序不会改变。假设我读取了一个硬件寄存器值，我想将该寄存器值覆盖在位域上，以便我可以在不使用位掩码的情况下引用寄存器中的各个字段。

编辑：修复了 GMan 指出的问题，并调整了代码，以便未来的读者更清楚。

请参阅： Anders K. & Michael J 在下面的答案，以获得更雄辩的解决方案。

#include <iostream>

/// \class HardwareRegister
/// Abstracts out bitfields in a hardware register.
/// \warning  This is non-portable code.
class HardwareRegister
{
   public:
      /// Constructor.
      /// \param[in]  registerValue - the value of the entire register. The
      ///                             value will be overlayed onto the bitfields
      ///                             defined in this class.
      HardwareRegister(unsigned long registerValue = 0)
      {
         /// Lots of casting to get registerValue to overlay on top of the
         /// bitfields
         *this = *(reinterpret_cast<HardwareRegister*>(&registerValue));
      }


      /// Bitfields of this register.
      /// The data type of this field should be the same size as the register
      /// unsigned short for 16 bit register
      /// unsigned long for 32 bit register.
      ///
      /// \warning Remember endianess! Order of the following bitfields are
      ///          important.
      ///          Big Endian    - Start with the most signifcant bits first.
      ///          Little Endian - Start with the least signifcant bits first.
      unsigned long field1: 8;
      unsigned long field2:16;
      unsigned long field3: 8;
}; //end class Hardware


int main()
{

   unsigned long registerValue = 0xFFFFFF00;
   HardwareRegister  testRegister(registerValue);

   // Prints out for little endianess machine
   // Field 1 = 0
   // Field 2 = 65535
   // Field 3 = 255
   std::cout << "Field 1 = " << testRegister.field1 << std::endl;
   std::cout << "Field 2 = " << testRegister.field2 << std::endl;
   std::cout << "Field 3 = " << testRegister.field3 << std::endl;
}

Answer 1

位域不是那样工作的。您不能将标量值分配给struct完整的位域。看起来您自从使用 以来就已经知道这一点reinterpret_cast，但由于reinterpret_cast不能保证做很多事情，所以它只是掷骰子。

如果要在位域结构和标量之间进行转换，则需要对值进行编码和解码。

    HW_Register(unsigned char value)
      : field1( value & 3 ),
        field2( value >> 2 & 3 ),
        field3( value >> 4 & 7 )
        {}

编辑：您没有得到任何输出的原因是与字段中的数字对应的 ASCII 字符是非打印的。试试这个：

std::cout << "Field 1 = " << (int) testRegister.field1 << std::endl;
std::cout << "Field 2 = " << (int) testRegister.field2 << std::endl;
std::cout << "Field 3 = " << (int) testRegister.field3 << std::endl;

Answer 2

不要这样做

 *this = *(reinterpret_cast<HW_Register*>(&registerValue));

'this' 指针不应该以这种方式摆弄：

HW_Register reg(val)
HW_Register *reg = new HW_Register(val)

这里'this'在内存中的两个不同位置

而是有一个内部联合/结构来保存值，这样它很容易来回转换（因为你对可移植性不感兴趣）

例如

union
{
   struct {
     unsigned short field1:2;
     unsigned short field2:4;
     unsigned short field3:2;
    ...
   } bits;
   unsigned short value;
} reg

编辑：名字“注册”足够真实

Answer 3

试试这个：

class HW_Register
{
public:
    HW_Register(unsigned char nRegisterValue=0)
    {
        Init(nRegisterValue);
    }
    ~HW_Register(void){};

    void Init(unsigned char nRegisterValue)
    {
        nVal = nRegisterValue;
    }

    unsigned Field1() { return nField1; }
    unsigned Field2() { return nField2; }
    unsigned Field3() { return nField3; }

private:
    union
    {
        struct 
        {
            unsigned char nField1:2;
            unsigned char nField2:4;
            unsigned char nField3:2;
        };
        unsigned char nVal;
    };
};


int main()
{
    unsigned char registerValue = 0xFF;
    HW_Register  testRegister(registerValue);

    std::cout << "Field 1 = " << testRegister.Field1() << std::endl;
    std::cout << "Field 2 = " << testRegister.Field2() << std::endl;
    std::cout << "Field 3 = " << testRegister.Field3() << std::endl;

    return 0;
}

Answer 4

HW_Register(unsigned char registerValue) : field1(0), field2(0), field3(0)