C++解析IEEE754中float格式的數據

#include<iostream>
using namespace std;
void FloatToBuf(float num, unsigned char buf[4]);
float BufToFloat(unsigned char buf[4]);
int main()
{
    float f = 639.5f; //待轉換的浮點數
    int *p = reinterpret_cast<int *>(&f);
    for(int i = 31; i >= 0; i--) //原始的二進制位
    {
        cout << (*p >> i & 1);
        if(i % 8 == 0)
        {
            cout << ' ';
        }
    }
    cout << endl;
    unsigned char buf[4];
    FloatToBuf(f, buf);
    for(i = 0; i < 32; i++) //轉換後的大端模式二進制位
    {
        cout << (buf[i / 8] >> (7 - i % 8) & 1);
        if(i % 8 == 7)
        {
            cout << ' ';
        }
    }
    cout << endl;
    cout << BufToFloat(buf) << endl; //再轉回去的浮點數
    return 0;
}
void FloatToBuf(float num, unsigned char buf[4])
{
    float item = 0.5f;
    int index = 0, i;
    if(num == 0.0f)
    {
        buf[0] = buf[1] = buf[2] = buf[3] = 0;
        return;
    }
    if(num > 0.0f)
    {
        buf[0] &= ~(1 << 7);
    }
    else
    {
        buf[0] |= (1 << 7);
        num *= -1.0f;
    }
    while(num >= 2.0f)
    {
        index++;
        num /= 2.0f;
    }
    num -= 1.0f;
    for(i = 9; i < 32; i++, item /= 2.0f)
    {
        if(num >= item)
        {
            buf[i / 8] |= (1 << (7 - i % 8));
            num -= item;
        }
        else
        {
            buf[i / 8] &= ~(1 << (7 - i % 8));
        }
    }
    if(num >= item)
    {
        for(i = 31; i > 8; i--)
        {
            if(buf[i / 8] >> (7 - i % 8) & 1)
            {
                buf[i / 8] &= ~(1 << (7 - i % 8));
            }
            else
            {
                buf[i / 8] |= (1 << (7 - i % 8));
                break;
            }
        }
        if(i == 8)
        {
            index++;
        }
    }
    index += 127;
    for(i = 8; i > 0; i--)
    {
        if(index >> (8 - i) & 1)
        {
            buf[i / 8] |= (1 << (7 - i % 8));
        }
        else
        {
            buf[i / 8] &= ~(1 << (7 - i % 8));
        }
    }
}
float BufToFloat(unsigned char buf[4])
{
    float sum = 1.0f, item = 0.5f;
    int index = 0, base = 1, i;
    for(i = 9; i < 32; i++, item /= 2.0f)
    {
        if(buf[i / 8] >> (7 - i % 8) & 1)
        {
            sum += item;
        }
    }
    for(i = 8; i > 0; i--, base *= 2)
    {
        if(buf[i / 8] >> (7 - i % 8) & 1)
        {
            index += base;
        }
    }
    if(index > 0)
    {
        index -= 127;
        if(index > 0)
        {
            while(index--)
            {
                sum *= 2.0f;
            }
        }
        else
        {
            index *= -1;
            while(index--)
            {
                sum /= 2.0f;
            }
        }
    }
    else
    {
        sum -= 1.0f;
    }
    if(buf[0] >> 7 & 1)
    {
        sum *= -1.0f;
    }
    return sum;
}