M_PI 适用于 math.h 但不适用于 Visual Studio 中的 cm

问题描述

我使用的是 Visual Studio 2010.我读到在 C++ 中最好使用 而不是 .

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

但是在我尝试编写的程序中(Win32 控制台应用程序，空项目)如果我写:

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

#define _USE_MATH_DEFINES
#include <math.h>

它编译，而如果我写

#define _USE_MATH_DEFINES
#include <cmath>

它失败了

错误 C2065:'M_PI':未声明的标识符

error C2065: 'M_PI' : undeclared identifier

正常吗?我使用 cmath 还是 math.h 有关系吗?如果是，我怎样才能让它与 cmath 一起工作?

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

UPDATE:如果我在 GUI 中定义 _USE_MATH_DEFINES，它就可以工作.知道为什么会这样吗?

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

推荐答案

有趣的是，我在我的一个应用上检查了这个，我得到了同样的错误.

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

我花了一段时间检查标题，看看是否有任何未定义的 _USE_MATH_DEFINES 内容，但一无所获.

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

所以我移动了

#define _USE_MATH_DEFINES
#include <cmath>

成为我文件中的第一件事(我不使用 PCH，所以如果你是，你必须在 #include "stdafx.h" 之后使用它)，突然它完美编译.

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.

尝试将其移到页面更高的位置.完全不确定为什么这会导致问题.

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

编辑:想通了.#include <math.h> 出现在 cmath 的头文件保护中.这意味着在#includes 列表的更高层包含cmath，但没有指定#define.math.h 是专门设计的，因此您可以再次包含它，现在定义更改为添加 M_PI 等.cmath 不是这种情况>.因此，在包含其他任何内容之前，您需要确保 #define _USE_MATH_DEFINES.希望能帮到你:)

Edit: Figured it out. The #include <math.h> 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 :)

如果仅包含 math.h 失败，您将使用非标准 C/C++，正如已经指出的 :)

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

编辑 2:或者正如大卫在评论中指出的那样，让自己成为一个定义值的常量，并且无论如何你都有更便携的东西:)

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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