I am converting some code from C to C++ in MS dev studio under win32. In the old code I was doing some high speed timings using QueryPerformanceCounter() and did a few manipulations on the __int64 values obtained, in particular a minus and a divide. But now under C++ I am forced to use LARGE_INTEGER because that's what QueryPerformanceCounter() returns. But now on the lines where I try and do some simple maths on the values I get an error:
error C2676: binary '-' : 'LARGE_INTEGER' does not define this operator or a conversion to a type acceptable to the predefined operator
I tried to cast the variables to __int64 but then get:
error C2440: 'type cast' : cannot convert from 'LARGE_INTEGER' to '__int64'
How do I resolve this?
Thanks,
Answer
LARGE_INTEGER is a union of a 64-bit integer and a pair of 32-bit integers. If you want to perform 64-bit arithmetic on one you need to select the 64-bit int from inside the union.
LARGE_INTEGER a = { 0 };
LARGE_INTEGER b = { 0 };
__int64 c = a.QuadPart - b.QuadPart;