C++ 声明如何与自身发生冲突？
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How can a declaration conflict with itself?
提问by eeeeaaii
This is the error I'm getting when trying to compile some code that uses taucs (not my code):
这是我在尝试编译一些使用 taucs 的代码（不是我的代码）时遇到的错误：
.../taucs/src/taucs.h:554: error: conflicting declaration ‘typedef struct taucs_ccs_matrix taucs_ccs_matrix'
.../taucs/src/taucs.h:554: error: ‘taucs_ccs_matrix' has a previous declaration as ‘typedef struct taucs_ccs_matrix taucs_ccs_matrix'
wat? It is conflicting with itself?
哇？和自己有冲突吗？
After I pinched myself, I created a test header and put in a conflicting definition, just to make sure I was right about this:
在我掐了自己之后，我创建了一个测试头并输入了一个相互冲突的定义，只是为了确保我对此是正确的：
In file testit.h:
在文件 testit.h 中：
#include "somethingelse.h"
typedef struct
{
int n;
} foobar;
In file somethingelse.h:
在文件 somethingelse.h 中：
typedef struct
{
int n;
} foobar;
Sure enough, I get:
果然，我得到：
testit.h:6: error: conflicting declaration ‘typedef struct foobar foobar'
somethingelse.h:4: error: ‘foobar' has a previous declaration as ‘typedef struct foobar foobar'
Or if I have this in testit.h:
或者如果我在 testit.h 中有这个：
typedef struct
{
int n;
} foobar;
typedef struct
{
int n;
} foobar;
testit.h:9: error: conflicting declaration ‘typedef struct foobar foobar'
testit.h:4: error: ‘foobar' has a previous declaration as ‘typedef struct foobar foobar'
The line number is always different -- a declaration can't conflict with itself. I don't get it. Anyone ever seen this?
行号总是不同的——声明不能与自身冲突。我不明白。有人见过这个吗？
采纳答案by Timwi
Could it be that your header file (.../taucs/src/taucs.h), which contains the declaration, is (directly or indirectly) included twice by two separate #includedirectives?
是否可能是.../taucs/src/taucs.h包含声明的头文件 ( ) 被两个单独的#include指令（直接或间接）包含两次？
回答by Harper Shelby
Is the single header included in multiple source files? If so, you need to wrap it in "include guards" like so:
单个标头是否包含在多个源文件中？如果是这样，您需要将其包装在“包含守卫”中，如下所示：
#ifndef TAUCS_H
#define TAUCS_H
//Header stuff here
#endif //TAUCS_H
回答by David Feurle
typedef struct
{
int n;
} foobar;
typedef struct
{
int n;
} foobar;
testit.h:9: error: conflicting declaration ‘typedef struct foobar foobar'
testit.h:4: error: ‘foobar' has a previous declaration as ‘typedef struct foobar foobar'
In this example you give 2 declarations of foobar. The compiler does not know which one to choose - so it bails out with conflicting declaration. You can't declare the same thing twice.
在这个例子中，你给出了 2 个 foobar 声明。编译器不知道选择哪一个 - 所以它会因声明冲突而退出。你不能两次声明同样的事情。
回答by R Samuel Klatchko
Don't repeat the definition. C++ allows a definition to only appear one time. What you can do is repeat a declaration.
不要重复定义。C++ 允许一个定义只出现一次。您可以做的是重复声明。
A typedefis always a definition. So the first thing I would recommend is giving the structproper a name (and since this is C++, a typedef does not add any benefit so just drop the typedef):
Atypedef始终是一个定义。所以我建议的第一件事是给一个struct正确的名字（因为这是 C++，typedef 不会增加任何好处，所以只需删除 typedef）：
// file1.h
struct foobar
{
int n;
};
Next, that should be in exactly one file. If you have files that only use pointers to foobar, you can repeat the declaration(just not the definition):
接下来，它应该在一个文件中。如果您的文件只使用指向 foobar 的指针，则可以重复声明（不是定义）：
// file2.h
// This is just a declaration so this can appear as many times as
// you want
struct foobar;
void doit(const foobar *f);
回答by Thomas H?henleitner
I had the same issue linting my code and it was notdouble declaration of a type. PC-Lint complained about the same typedef being used in mixed C and C++ code. I could fix that by avoiding the same declaration in C andC++ files. Hope that helps someone.
我在整理我的代码时遇到了同样的问题，它不是类型的双重声明。PC-Lint 抱怨在混合 C 和 C++ 代码中使用了相同的 typedef。我可以通过避免在 C和C++ 文件中使用相同的声明来解决这个问题。希望能帮助某人。
