C++ 错误 C2011：“XX”：“类”类型重新定义
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Error C2011: 'XX' : 'class' type redefinition
提问by Pacha
I have this compiler error (C2011) with this piece of code. I don't know what is wrong with it.
这段代码有这个编译器错误（C2011）。我不知道这有什么问题。
The namespace (Ogre) doesn't have a definition for PlaneMovement. I also tried a different name and still the same errors.
命名空间 (Ogre) 没有PlaneMovement. 我也尝试了不同的名称，但仍然出现相同的错误。
#include <Ogre.h>
using namespace Ogre;
class PlaneMovement
{
public:
PlaneMovement(Degree startingAngle, Real velocity = 2, Real gravity = 2);
Vector2 updateMovement(const FrameEvent& evt);
private:
Degree currentAngle;
Real currentVelocityX;
Real currentVelocityY;
Real gravity;
bool top;
};
回答by Luchian Grigore
Include guards:
包括守卫：
#ifndef FILE_H
#define FILE_H
//file contents here
#endif
Header files should have include guards for this exact reason - multiple inclusion in the same translation unit can lead to a multiple definition.
出于这个确切原因，头文件应该包含保护——在同一个翻译单元中多次包含会导致多个定义。
The alternative is using
替代方法是使用
#pragma once
but this isn't supported by all compilers.
但这并非所有编译器都支持。
回答by Vladislav Makarov
If someone else faces this situation, it may be when library includes in property of project and header files from this library include in projects file.
如果其他人遇到这种情况，可能是库包含在项目的属性中，而该库中的头文件包含在项目文件中。
回答by sgryzko
Incorrect Forward Declaration
不正确的前向声明
Another possible cause, if you're a goof like me, could be that you used enuminstead of classwhen forward declaring a class.
另一个可能的原因，如果你像我一样是个傻瓜，可能是你在前向声明 a 时使用了enum而不是。classclass
File1.h
文件1.h
namespace MyNamespace { enum NotActuallyAnEnum; }
File2.h
文件2.h
class NotActuallyAnEnum
{
...
}
This will produce something like the following error:
这将产生类似于以下错误的内容：
error C2011: 'enum' type redefinition
Obviously the fix is to correct the forward declaration:
显然，解决方法是纠正前向声明：
namespace MyNamespace { class NotActuallyAnEnum; }
回答by E. van Putten
You might also get this error if you have multiple branches of your project on your development station and use a symlink to point to one of them.
如果您的开发站上有多个项目分支并使用符号链接指向其中之一，您也可能会收到此错误。
Let's suppose you have two different branches of your solution called Project1and Project2and you let a symlink called Projectpoint to either Project1or Project2.
假设您的解决方案有两个不同的分支，称为Project1and，Project2并且您让一个名为的符号链接Project指向Project1or 或Project2。
The idea is that you could switch between branches and the project would always appear to be Projectto your application and possibly some other tools that expect it there.
这个想法是您可以在分支之间切换，并且项目将始终出现Project在您的应用程序中，可能还有其他一些期望它出现在那里的工具。
Disclaimer: yes, version control can switch between branches, but this way you will not have to rebuild the whole application every single time you switch branches. Besides both branches can still be under version control.
免责声明：是的，版本控制可以在分支之间切换，但这样您就不必每次切换分支时都重新构建整个应用程序。此外两个分支仍然可以在版本控制之下。
Okay, so opening Projectwould open either Project1or Project2depending on the symlink. The symlink could be removed/created by some simple mklink_1and mklink_2like script files.
好的，所以打开Project将打开Project1或Project2取决于符号链接。符号链接可以被删除/由一些简单的创建mklink_1和mklink_2类似脚本文件。
Here comes the pitfall:
陷阱来了：
If you don't pay attention and directly open the solution at location 1 or 2 directly (instead of following the directory symlink with Visual Studio), the pre-processor might be tricked into mixing Project1\MyHeader.h(or MyProject2\MyHeader.h) with MyProject\MyHeader.h!
如果你不注意，在位置1或2直接打开该解决方案直接（而不是在与Visual Studio中的目录符号链接），预处理器可能被欺骗混合Project1\MyHeader.h（或MyProject2\MyHeader.h）有MyProject\MyHeader.h！
Even those are technically the same file, the preprocessor doesn't know about the symlink. So here the #pragma oncewould not save you!
即使这些在技术上是相同的文件，预处理器也不知道符号链接。所以这里#pragma once不会救你！
