Question

编辑：例如here的Godbolt链接！

因此，我有一个示例来展示我制作的宏及其用例：

#include <bitset>
#include <iostream>

#define bit_mask(size, offset) (~(~0 << size) << offset)
#define bit_masked_set(dst, src, mask, offset) (dst = (dst & ~mask) | (src << offset))
#define bit_masked_get(src, mask, offset) ((src & mask) >> offset)

constexpr unsigned front_mask = bit_mask(16, 16);
constexpr unsigned back_mask = bit_mask(16, 0);

int main() {
    std::cout << std::bitset<32>(front_mask) << " = front_mask (bit_mask(16, 16))\n";
    std::cout << std::bitset<32>(back_mask) << " = back_mask (bit_mask(16, 0))\n\n";

    unsigned value = 0;
    std::cout << std::bitset<32>(value) << " = value (initial)\n\n";

    bit_masked_set(value, 1, front_mask, 16);
    std::cout << std::bitset<32>(value) << " = value (after bit_masked_set(value, 1, front_mask, 16))\n\n";

    bit_masked_set(value, 1, back_mask, 0);
    std::cout << std::bitset<32>(value) << " = value (after bit_masked_set(value, 1, back_mask, 0))\n\n";

    std::cout << std::bitset<32>(value) << " = value (final)\n";
}

哪个输出：

11111111111111110000000000000000 = front_mask (bit_mask(16, 16))
00000000000000001111111111111111 = back_mask (bit_mask(16, 0))

00000000000000000000000000000000 = value (initial)

00000000000000010000000000000000 = value (after bit_masked_set(value, 1, front_mask, 16))

00000000000000010000000000000001 = value (after bit_masked_set(value, 1, back_mask, 0))

00000000000000010000000000000001 = value (final)

我将Visual Studio 2017与-Wall -c ++ 17一起使用，对此感到满意。但是我注意到，当我将其粘贴到Godbolt网站时，gcc / clang / etc会发出警告或错误提示，例如：

error: constexpr variable 'front_mask' must be initialized by a constant expression
...
error: left operand of shift expression '(-1 << 16)' is negative [-fpermissive]
...
etc

因为这些是预处理程序宏，它们使用分配给已知数据类型的常量值不应该不是问题。

Edit2：

我正在玩这个游戏，发现将我的bit_mask宏更改为：

#define bit_mask(size, offset) (~((unsigned long long)~0 << size) << offset)

似乎可以解决此问题。我不确定100％是否需要额外的long long，但我认为它不会有任何伤害，它会更灵活。

Edit3：

将代码从宏显着更新为简单的结构：

#include <bitset>
#include <iostream>
#include <type_traits>

struct BitMask {
    using Mask_Type = unsigned long long;

    Mask_Type mask;
    unsigned offset;

    constexpr BitMask(unsigned size, unsigned offset) :
        mask{~((Mask_Type)~0 << size) << offset},
        offset{offset} {
    }

    template<typename T, typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>
    constexpr T set_bits(T& dst, T src) const {
        return dst = (dst & ~mask) | (src << offset);
    }

    template<typename T, typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>
    constexpr T get_bits(T src) const {
        return (src & mask) >> offset;
    }
};

constexpr BitMask front_mask = BitMask(16, 16);
constexpr BitMask back_mask = BitMask(16, 0);

int main() {
    std::cout << std::bitset<32>(front_mask.mask) << " = front_mask (bit_mask(16, 16))\n";
    std::cout << std::bitset<32>(back_mask.mask) << " = back_mask (bit_mask(16, 0))\n\n";

    unsigned value = 0;
    std::cout << std::bitset<32>(value) << " = value (initial)\n\n";

    front_mask.set_bits(value, 1U);
    std::cout << std::bitset<32>(value) << " = value (after bit_masked_set(value, 1, front_mask, 16))\n\n";

    back_mask.set_bits(value, 1U);
    std::cout << std::bitset<32>(value) << " = value (after bit_masked_set(value, 1, back_mask, 0))\n\n";

    std::cout << std::bitset<32>(value) << " = value (final)\n";
}

Answer 1

因此，由于#define bit_mask(size, offset) (~((unsigned long long)~0 << size) << offset)似乎可以解决问题，因此我将其标记为已回答。如果有人发布这不是正确的解决方案，而他们提供了其他解决方案，那么我会接受他们的回答，只要它可行。

C ++位操作在我的系统上没有错误地进行编译，但是Godbolt编译器发出警告/错误

1 个答案: