C ++向前声明问题 [英] C++ forward declaration problem

问题描述

我有一个头文件有一些转发声明，但是当我将头文件包含在实现文件中时，它包含在以前的转发声明的包含之后，会导致这样的错误。

I have a header file that has some forward declarations but when I include the header file in the implementation file it gets included after the includes for the previous forward declarations and this results in an error like this.

error: using typedef-name ‘std::ifstream’ after ‘class’
/usr/include/c++/4.2.1/iosfwd:145: error: ‘std::ifstream’ has a previous declaration.

class ifstream;

class A
{
    ifstream *inStream;
}
// End of A.h

#include <ifstream>
using std::ifstream;

#include "A.h"

// etc

提前感谢。

推荐答案

不要转发declare std：ifstream - 改为只导入< iosfwd> 。

ifstream是一个typedef。

ifstream is a typedef.

详情请参阅： http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.2/group_ s27 _2__iosfwd.html

See here for further details: http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.2/group_s27_2__iosfwd.html

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