KwCheng's blog

cout, clog and cerr are pre-defined objects defined in ostream. 

The iostream class inherits from both ostream and istream

cout and clog are buffered , that means all output is temporarily stored in a buffer and then dumps to screen.

cerr is an unbuffered object , that means all output goes immediately to the output device.

In C++ program , we can redirect the cout by the buffer area , but we can't redirect cerr by the same method.

for example:

#include <iostream>

#include <fstream>

using namespace std;

int main()
{
    cout << "the first row" << endl;

    streambuf* cout_buffer = cout.rdbuf();   // save the original cout bufffer

    ofstream file("redirect.txt");

    cout.rdbuf (file.rdbuf());  //redirect the cout buffer to file buffer

    cout << "the last row" << endl;

    cout.rdbuf (cout_buffer);  // recover the cout buffer

    return 0;
}