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顺序不会改变。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，这样我就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 future 的读者更清楚。

请参见:Anders K. 和 Michael J 在下面给出了更有说服力的解决方案。

#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

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

取而代之的是有一个内部 union/结构来保存值，这样很容易转换来回(因为你对便携性不感兴趣)

例如

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

编辑:名称“register”足够真实