Redirecting cout to a callback function
One time or another during a lifetime it’s necessary to redirect cout or another stream to a callback function… I havn’t had the need for it yet, but I was asked if it was possible and here’s my answer:
#include <streambuf>
#include <iostream>
#include <ostream>
template <class Elem = char, class Tr = std::char_traits<Elem> >
class callbackstream : public std::basic_streambuf<Elem, Tr>
{
typedef void (*pfncb)(const Elem *, std::streamsize _Count);
private:
std::basic_ostream<Elem, Tr> &m_stream;
std::streambuf *m_buf;
pfncb m_cb;
public:
callbackstream(std::ostream &stream, pfncb cb)
: m_stream(stream), m_cb(cb)
{
// redirect stream
m_buf = m_stream.rdbuf(this);
};
~callbackstream()
{
// restore stream
m_stream.rdbuf(m_buf);
}
// override xsputn and make it forward data to the callback function
std::streamsize xsputn(const Elem *_Ptr, std::streamsize _Count)
{
m_cb(_Ptr, _Count);
return _Count;
}
// override overflow and make it forward data to the callback function
typename Tr::int_type overflow(typename Tr::int_type v)
{
Elem ch = Tr::to_char_type(v);
m_cb(&ch, 1);
return Tr::not_eof(v);
}
};
Now, to use this class:
void mycallback(const char *ptr, std::streamsize count)
{
// handle data here...
}
int main()
{
//redirect std::cout to the callback function
callbackstream<> redirect(std::cout, mycallback);
// this is sent to the callback function, not the console
std::cout << "Hello" << std::endl;
return 0;
}
