两个无符号类型想减结果存储的临时变量还是无符号类型

 1 #include <stdio.h>
 2 int main()
 3 {
 4     unsigned int dwVal = 0;
 5     unsigned __int64 n64Val = 1;
 6     if( dwVal - n64Val >= 20)
 7     {
 8         printf("%I64d\n",dwVal - n64Val);        //执行这行，输出 -1
 9     }
10     return 0;
11 }

个人分析：

在做想减操作(dwVal - n64Val)时，会将dwVal和n64Val都转换为unsigned __int64,
两者想减的结果为-1，存储为0xFFFFFFFFFFFFFFFF,当成unsigned __int64使用
与 后面的数字(20)进行比较，所以计算结果为true，而打印输出的结果是 __int64类型
对应值为-1

进一步验证：

 1 #include <iostream>
 2 int main()
 3 {
 4     unsigned int dwVal = 0;
 5     unsigned __int64 n64Val = 1;
 6     unsigned __int64 n64Val2 = 0xffffffffffffffff;
 7     if( dwVal - n64Val >= n64Val2)
 8     {
 9         printf("%I64d\n",dwVal - n64Val);        //执行这行，输出 -1
10     }
11     n64Val2 = dwVal - n64Val;        //n64Val2 = 0xffffffffffffffff
12     system("pause");
13     return 0;
14 }

 1 #include <iostream>
 2 int main()
 3 {
 4     unsigned int dwVal = 0;
 5     unsigned __int64 n64Val = 2;
 6     unsigned __int64 n64Val2 = 0xffffffffffffffff;
 7     if( dwVal - n64Val >= n64Val2)
 8     {
 9         printf("%I64d\n",dwVal - n64Val);        //执行不到这里
10     }
11     n64Val2 = dwVal - n64Val;        //n64Val2 = 0xfffffffffffffffe,小于n64Val2
12     system("pause");
13     return 0;
14 }

有感于GetTickCount() 49.7天后计数置零

_LogTick = ::GetTickCount64();

    //DoSomething;

    if(::GetTickCount() - _LogTick >= LOG_TICK_MAX)
    {
        printf("%I64d\n",::GetTickCount() - _LogTick);
    }