c++ - 如何避免出现警告C4244:从 'ULONGLONG'转换为 'double'，可能会丢失数据

我正在使用Win32 x64环境

也请转到下面的链接

我从此链接Collecting CPU Usage of a Process找到了答案

在 Windows 条件的第一个答案下，您可以看到如下代码片段:

double getCurrentValue(){
    FILETIME ftime, fsys, fuser;
    ULARGE_INTEGER now, sys, user;
    double percent;

    GetSystemTimeAsFileTime(&ftime);
    memcpy(&now, &ftime, sizeof(FILETIME));

    GetProcessTimes(self, &ftime, &ftime, &fsys, &fuser);
    memcpy(&sys, &fsys, sizeof(FILETIME));
    memcpy(&user, &fuser, sizeof(FILETIME));
    percent = (sys.QuadPart - lastSysCPU.QuadPart) +
        (user.QuadPart - lastUserCPU.QuadPart);
    percent /= (now.QuadPart - lastCPU.QuadPart);
    percent /= numProcessors;
    lastCPU = now;
    lastUserCPU = user;
    lastSysCPU = sys;

    return percent * 100;
}
//--------------------------- FOCUS -----------------------------//
    percent = (sys.QuadPart - lastSysCPU.QuadPart) +(user.QuadPart - lastUserCPU.QuadPart); // ---(1)
    percent /= (now.QuadPart - lastCPU.QuadPart); // ---(2)
    percent /= numProcessors; // --- (3)

如(1)所示，将ULONGLONG(unsigned long long)赋值给double来给编译器warning C4244: conversion from 'ULONGLONG' to 'double', possible loss of data

因此，@ Line＃(1)，(2)＆(3)我们如何才能保证double precent数据的值？

(由于Ulonglong将64位整数数据转换为64位浮点数据)

有时，此precent值会获得大于 100.00 的值作为时间戳，作为CPU使用率

最佳答案

假设您在短时间内调用getCurrentValue函数，则可以安全地将值强制转换为两倍。您还可以检查极端情况，以年为单位分隔 call 。

ULONGLONG used = (sys.QuadPart - lastSysCPU.QuadPart) + (user.QuadPart - lastUserCPU.QuadPart);
ULONGLONG elapsed = (now.QuadPart - lastCPU.QuadPart);
if ( elapsed == 0 )
{
    // Now it is imposible, but it could be problem in future :)
    // Do something sane here. For example restart calculation.
}
else
{
    double percent = ( (double)used / (double)elapsed ) / numProcessors * 100.0;
    // Clamp value for rounding errors in our or system code.
    if ( percent > 100.0 )
        percent = 100.0;

    if ( elapsed >= ( 1ull << 53 ) || used >= ( 1ull << 53 ) )
    {
         // Set some flag that result could be not precise.
         // For 'elapsed' this needs 28 years.
         // 'used' could kick earlier: 28/num_processors years.
         // But if you realy need only 4 or 5 digits you can
         // ignore this case.
    }
}