如何用C ++构造函数初始化位域？(How to initialize bitfields with a C++ Constructor?)

首先，我不关心可移植性，并且可以安全地假设字节序不会改变。假设我读取了一个硬件寄存器值，我想将这个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：修正GMan指出的问题，并调整代码，以便未来的读者更清楚。

请参阅： Anders K.＆Michael J的答案，以获得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; }

First off, I’m not concerned with portability, and can safely assume that the endianness will not change. Assuming I read a hardware register value, I would like to overlay that register value over bitfields so that I can refer to the individual fields in the register without using bit masks.

EDIT: Fixed problems pointed out by GMan, and adjusted the code so it's clearer for future readers.

SEE: Anders K. & Michael J's answers below for a more eloquent solution.

#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; }