重写的std :: ostream flush（）函数未被std :: flush调用

Question

I've overridden std::ostream::flush() function. Below I've removed all other code from the example:

#include <iostream>
#include <ostream>

class CMyStream : public std::streambuf, public std::ostream
{
    public:
        explicit CMyStream() throw() : std::ostream(this)
        {
            // Intentionally empty block
        }

        std::ostream &flush() 
        { 
            std::cout << "Overridden flush called\n"; 
            return (*this);
        }

        int sync()
        {
            std::cout << "Overridden sync called\n";
            return 0;   // Success
        }

};

I try to use it like this:

CMyStream myStream;
myStream << "Test" << std::flush;

, but the overridden CMyStream::flush() or CMyStream::sync() functions are not called by the std::flush manipulator. If I debug I see that the default std::ostream::flush() is called and not my overriden function.

Is there a way around this problem, or do I have to call myStream.flush() directly and not with the manipulator?

Answer 1

The problem is that std::ostream::flush method is not virtual, so you cannot properly override it, and you should not. What you should do instead is to create your own buffer class inherited from std::basic_streambuf or std::basic_filebuf or std::basic_stringbuf and override protected int sync() method in there. Your stream class then should create proper buffer type in it's constructor.