M_PI与math.h一起使用，但在Visual Studio中不与cmath一起使用 [英] M_PI works with math.h but not with cmath in Visual Studio

问题描述

我使用Visual Studio 2010。我已经读过，在C + +中，最好使用< cmath> ，而不是< math。 h> 。

但在程序中，如果我写的话，我试图写入（Win32控制台应用程序，空项目）：

  #define _USE_MATH_DEFINES 
 #include< math.h> 
  

它编译，而如果我写

  #define _USE_MATH_DEFINES 
 #include< cmath> 
  

失败，

错误C2065：'M_PI'：未声明的标识符

是否正常？如果我使用cmath或math.h是否重要？

UPDATE ：如果我在GUI中定义_USE_MATH_DEFINES，它可以工作。

解决方案

有趣的是，我在一个应用程序上检查过这个，我得到了同样的错误。 p>

我花了一会儿检查头部，看看是否有任何东西没有定义_USE_MATH_DEFINES，并且没有找到任何东西。

所以我移动了

  #define _USE_MATH_DEFINES 
 #include< cmath> 
  

是我文件中的第一件事（我不使用PCHs，如果你是你必须在#includestdafx.h之后），然后突然编译完成。

尝试将其在页面上移动。完全不确定为什么会导致问题。

编辑：看出来了。 #include发生在cmath的标题保护内。这意味着#include的列表中的更高级别包括没有指定#define的cmath。 math.h是专门设计的，所以你可以包括它的定义现在改变添加M_PI等。这不是cmath的情况。所以你需要确保你#define _USE_MATH_DEFINES之前，你包括任何其他。希望为你清除它：）

如果只是包括math.h你使用非标准的C / C ++已经指出：）

编辑2 ：或者作为大卫在评论中指出，只是使自己成为一个常量，定义的值，你有一个更便携的反正：）

I am using Visual Studio 2010. I have read that in C++ it is better to use <cmath> rather than <math.h>.

But in the program I am trying to write (Win32 console application, empty project) if I write:

#define _USE_MATH_DEFINES
#include <math.h>

it compiles, while if I write

#define _USE_MATH_DEFINES
#include <cmath>

it fails with

error C2065: 'M_PI' : undeclared identifier

Is it normal? Does it matter if I use cmath or math.h? If yes, how can I make it work with cmath?

UPDATE: if I define _USE_MATH_DEFINES in the GUI, it works. Any clue why this is happening?

解决方案

Interestingly I checked this on an app of mine and I got the same error.

I spent a while checking through headers to see if there was anything undef'ing the _USE_MATH_DEFINES and found nothing.

So I moved the

#define _USE_MATH_DEFINES
#include <cmath>

to be the first thing in my file (I don't use PCHs so if you are you will have to have it after the #include "stdafx.h") and suddenly it compile perfectly.

Try moving it higher up the page. Totally unsure as to why this would cause issues though.

Edit: Figured it out. The #include occurs within cmath's header guards. This means that something higher up the list of #includes is including cmath without the #define specified. math.h is specifically designed so that you can include it again with that define now changed to add M_PI etc. This is NOT the case with cmath. So you need to make sure you #define _USE_MATH_DEFINES before you include anything else. Hope that clears it up for you :)

Failing that just include math.h you are using non-standard C/C++ as already pointed out :)

Edit 2: Or as David points out in the comments just make yourself a constant that defines the value and you have something more portable anyway :)

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