iostream的C ++编译错误

Question

I am using fstream and got some error.

Here is what I have:

class CLog
{
    void printOn(std::ostream& dbg) const;
}

void operator>>(const CLog& s, std::ofstream& dbg)
{
    s.printOn(dbg);
}

But when I compile I got the following error:

error C2664: 'printOn' : cannot convert parameter 1 from 
'class std::basic_ofstream<char,struct std::char_traits<char> >' to 
'class std::basic_ostream<char,struct std::char_traits<char>  > &' 
A reference that is not to 'const' cannot be bound to a non-lvalue

I thought that ofstream inherit from ostream so why it is not possible?

Thanks

Answer 1

More correct declaration of output operator is following:

std::ostream& operator << (std::ostream& dbg, const CLog& s)
{
    s.printOn(dbg);
    return dbg;
}

Answer 2

Make printOn public and include fstream header :).

#include <fstream>
class CLog
{
public:
    void printOn(std::ostream& dbg) const
    {

    }
};

std::ofstream & operator<<( std::ofstream& dbg, const CLog& s)
{
    s.printOn(dbg);
}

Answer 3

I'd suggest posting complete code allowing to reproduce the problem. I've none with:

#include <fstream>

class CLog
{
public:
    void printOn(std::ostream& dbg) const;
};

void operator>>(const CLog& s, std::ofstream& dbg)
{
    s.printOn(dbg);
}