Question

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

Whats the norm for working around this?

Thanks in advance.

Answer 1

Don’t forward declare std:ifstream – just import <iosfwd> instead.

ifstream is a typedef.

See here for further details: http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.2/group__s27__2__iosfwd.html