为什么在特定的Visual Studio 2008项目中双击添加不正确？ [英] Why are doubles added incorrectly in a specific Visual Studio 2008 project?

问题描述

尝试将java代码移植到C ++我已经偶然发现了一些奇怪的行为。我不能得到双倍的工作（即使编译器选项/ fp：strict这意味着正确的浮点数学在Visual Studio 2008中设置）。

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 */

在另一个测试项目中工作正常（/ fp：strict根据IEEE754）。

In a different test project I set up it works fine (/fp:strict behaves according to IEEE754).

使用没有优化和FP：strict的Visual Studio 2008（标准）。

Using Visual Studio 2008 (standard) with No optimization and FP: strict.

任何想法？是真的截尾到浮动吗？这个项目在java和C ++端都需要相同的行为。我通过从VC ++中的调试窗口读取所有的值。

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++.

解决方案： _fpreset // Barry Kelly的想法解决了。库已将FP精度设置为低。

Solution: _fpreset(); // Barry Kelly's idea solved it. A library was setting the FP precision to low.

推荐答案

我唯一能想到的就是你链接库或DLL，控制字。

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.

您尝试从 float.h 调用 _fpreset（） code>之前有问题的计算？

Have you tried calling _fpreset() from float.h before the problematic computation?

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