英文:
Is it possible to declare a type with a set bit width in c++ without using a struct?
问题
C++中是否支持不使用struct、class或union位域来指定类型的位宽？使用它们会增加不必要且让人讨厌的间接性级别。
如果C++有类似C#的属性，你可以将Address设置为一个访问器，隐藏掉这种间接性。在Ada中，你可以这样声明Address：
type Address is range 0..2**4 - 1 with Object_Size = 4; -- 这不是很可爱吗！
我尝试了以下声明，理论上应该能工作：
typedef unsigned char Address:4; // 如果我们有这么幸运的话！
C++是否支持这样的构造或解决方法？
英文:
It's been at least a decade since I last touched C++. Supposedly, C++ has had a minor overhaul. Is there a way to specify the bit width of a type without using struct, class, or union bitfields? The problem with them is that it adds an unnecessary and annoying level of indirection:
struct Address{unsigned char val:4 = 0; // C++ in 2020?};struct Device{Address address; // 4-bit addressstring name;};int main(){Device device;device.address.val = 0x8; // Yuckity yuck WTF!return 0;};
If C++ had properties like C#, you could make Address an accessor that hides away the indirection. In Ada, you would simply declare Address like so:
type Address is range 0..2**4 - 1 with Object_Size = 4; -- Isn't this cute and sweet!
I tried the following declaration and there's no reason why it shouldn't work:
typedef unsigned char Address:4; // if we were only so lucky!
Does C++ support such a construct or workaround?
答案1
得分: 1
以下是您提供的代码的翻译部分：
不是开箱即用的，但制作一个小的包装类来进行验证并不难。这里是这种类的草图，甚至可以在编译时进行检查。演示链接：https://onlinegdb.com/RBVQfsAI5#include <iostream>// N是您想要使用的地址位数template<std::size_t N>struct address_t{public:explicit constexpr address_t(std::uintptr_t value) :address{ checked_value(value) }{}// 这是少数情况下隐式转换有用的地方operator std::uintptr_t() const noexcept{return address;}// 赋值运算符检查值。void operator=(std::uintptr_t value){address = checked_value(value);}private:constexpr std::uintptr_t checked_value(std::uintptr_t value){if (value >= (1 << N)) throw std::invalid_argument("地址值过大");return value;}std::uintptr_t address;};int main(){// addres_t<4> 表示最多有 4 位的地址static constexpr address_t<4> compile_time_address{15}; // 如果值过大，将无法编译通过std::cout << compile_time_address << "\n"; // 使用隐式转换到 std::uint_ptr_taddress_t<4> runtime_address{15};std::cout << runtime_address << "\n";runtime_address = 12;std::cout << runtime_address << "\n";try{// 在运行时分配无效值会抛出异常。address_t<4> runtime_address_fail{16};}catch (const std::invalid_argument&){}return 0;}
请注意，翻译只包括代码部分，不包括问题或其他内容。如果您需要进一步的帮助，请随时提问。
英文:
Not out of the box, but it is not so hard to make a small wrapper class that does the validation. Sketch of such a class here, it can even check at compile time.
Demo here : https://onlinegdb.com/RBVQfsAI5
#include <iostream>// N is the number of address bits you want to usetemplate<std::size_t N>struct address_t{public:explicit constexpr address_t(std::uintptr_t value) :address{ checked_value(value) }{}// one of the rare cases implicit conversion is usefuloperator std::uintptr_t() const noexcept{return address;}// assigment operator checks the value.void operator=(std::uintptr_t value){address = checked_value(value);}private:constexpr std::uintptr_t checked_value(std::uintptr_t value){if (value >= (1 << N)) throw std::invalid_argument("address value too large");return value;}std::uintptr_t address;};int main(){//addres_t<4> means an address with max. 4 bitsstatic constexpr address_t<4> compile_time_address{15}; // will not compile if value too bigstd::cout << compile_time_address << "\n"; // uses implicit conversion to std::uint_ptr_taddress_t<4> runtime_address{15};std::cout << runtime_address << "\n";runtime_address = 12;std::cout << runtime_address << "\n";try{// at runtime assigning an invalid value will throw.address_t<4> runtime_address_fail{16};}catch (const std::invalid_argument&){}return 0;}
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