问题
Trying to port java code to C++ I've stumbled over some weird behaviour. I can't get double addition to work (even though compiler option /fp:strict which means "correct" floating point math is set in Visual Studio 2008).
double a = 0.4;
/* a: 0.40000000000000002, correct */
double b = 0.0 + 0.4;
/* b: 0.40000000596046448, incorrect
(0 + 0.4 is the same). It's not even close to correct. */
double c = 0;
float f = 0.4f;
c += f;
/* c: 0.40000000596046448 too */
In a different test project I set up it works fine (/fp:strict behaves according to IEEE754).
Using Visual Studio 2008 (standard) with No optimization and FP: strict.
Any ideas? Is it really truncating to floats? This project really needs same behaviour on both java and C++ side. I got all values by reading from debug window in VC++.
Solution: _fpreset(); // Barry Kelly's idea solved it. A library was setting the FP precision to low.
回答1:
The only thing I can think of is perhaps you are linking against a library or DLL which has modified the CPU precision via the control word.
Have you tried calling _fpreset() from float.h before the problematic computation?
回答2:
Yes, it's certainly truncating to floats. I get the same value printing float f = 0.4 as you do in the "inaccurate" case. Try:
double b = 0.0 + (double) 0.4;
The question then is why it's truncating to floats. There's no excuse in the standard for treating 0.0 + 0.4 as a single-precision expression, since floating point literals are double-precision unless they have a suffix to say otherwise.
So something must be interfering with your settings, but I have no idea what.
来源：https://stackoverflow.com/questions/1154221/why-are-doubles-added-incorrectly-in-a-specific-visual-studio-2008-project