位字段成员的大小？ [英] Size of a bitfield member?

问题描述

有人知道如何提取位字段成员的大小。下面的代码自然地给了我一个整数的大小，但是如何找出多少位或字节在 mybits.one ？我试过 sizeof（test.one）但是显然不会工作。我知道这是一个量度的位：

Would anyone know how to extract the size of a bit-field member. The below code naturally gives me the size of an integer, but how do I find out how many bits or bytes are in mybits.one? I've tried sizeof(test.one) but which clearly won't work. I realize this is a measure of bits:

#include <iostream>

using namespace std;

int main()
{
    struct mybits {
        unsigned int one:15;
    };

    mybits test;
    test.one = 455;
    cout << test.one << endl;
    cout << "The size of test.one is:  " << sizeof(test) << endl;
}

推荐答案

运行时解决方案，本次讨论： http://social.msdn.microsoft.com/Forums/en-US/7e4f01b6-2e93-4acc-ac6a-b994702e7b66/finding-size-of-bitfield

Runtime solution, the idea from this discussion: http://social.msdn.microsoft.com/Forums/en-US/7e4f01b6-2e93-4acc-ac6a-b994702e7b66/finding-size-of-bitfield

#include <iostream>
using namespace std;

int BitCount(unsigned int value)
{
    int result = 0;

    while(value)
    {
        value &= (value - 1);
        ++result;
    }

    return result;
}

int main()
{
    struct mybits {
        unsigned int one:15;
    };

    mybits test;
    test.one = ~0;

    cout << BitCount(test.one) << endl;

    return 0;
}

列印15。

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