Question

如果我有一个介于0到16之间的无符号整数，并且我想将其写入二进制文件而不向其写入整个字节，那么如何移位来实现它？

0-16意味着我只需要4位，所以我应该可以在一个字节中存储2个不同的数字吗？

以下代码将1个数字写入1个字节：

std::ofstream file;
file.open("test.bin", std::ios::binary|std::ios::out);
char oneByteNum = (char)fourByteNum; // Converting from some 4 byte integer to a 1 byte char
file.write(&oneByteNum ,sizeof(char));

使用位移，如何在1字节中实现2个数字？我想从字节中读取数字也是一个类似的反向2步过程吗？

Answer 1

char oneByteWithTwoNums = (oneByteNum1 << 4) + (oneByteNum2 & 0x0f);

Answer 2

试试这个：

  compacted = first_number * 0x10 + second-number;

扩展：

  second_number = compacted & 0x0F;
  first_number = compacted >> 4;

Answer 3

我写了一个简单的例子：

#include <iostream>

typedef unsigned char byte;

byte pack(unsigned int num1, unsigned int num2) {

    // Our byte has the form 0b00000000
    // We want the first four bits to be filled with num1
    byte packed = num1 << 4;

    // We want the last four bits to be filled with num2
    // but, we don't want to overwrite the top four bits, so
    // we mask it with 0x0F (0b0001111)
    packed |= num2 & 0x0F;
    return packed;
}

void unpack(byte b, unsigned int& num1, unsigned int& num2) {
    // Extract the first four bits of our byte
    num1 = b >> 4;

    // Mask off the first four bits of our byte to get only
    // the last four bits
    num2 = b & 0x0F;
}

int main() {

    unsigned int num1 = 5;  // As an example
    unsigned int num2 = 15; // As an example

    byte b = pack(num1, num2);

    unsigned int num3;
    unsigned int num4;
    unpack(b, num3, num4);
    std::cout << num3 << std::endl;
    std::cout << num4 << std::endl;

    return 0;
}

您可能希望在pack / unpack方法中添加检查，以确保有人不会尝试传递[0-15]以外的值。

使用ofstream将4位写入二进制文件

3 个答案: