C++ - Don't get result after flush std::cout - c++

Question

using namespace std;
string str{ "text" };
stringstream ss{ str };
cout.rdbuf(ss.rdbuf());
cout.flush(); //cout<<endl;
This code is expected to print text,but showing nothing.
I associate ss to stdout then flush it,but I don't know why it didn't work even if I refer to many illustration.
cout<<rdbuf(ss);
this is OK but where's the different? : (

Answer 1

I think you misunderstood this concept. The line
cout.rdbuf(ss.rdbuf());
sets the pointer to the buffer. It redirects cout to ss and not to console. If you write into cout it will be written to ss.
I hope this clears it for you
#include <iostream>
#include <string>
#include <sstream>
int main() {
std::streambuf *backup;
backup = std::cout.rdbuf();
std::stringstream ss;
std::cout.rdbuf(ss.rdbuf());
std::cout << "Test" << std::endl;
std::cout.rdbuf(backup);
std::cout << ss.rdbuf();
return 0;
}
In the example code I create a copy of the current pointer of cout's buffer. Next I redirect the cout to ss. When I write into cout it is written into ss's buffer. Next I redirect cout back to console and print the content of ss.
If you want to manipulate the buffer you need somethink like
#include <iostream>
#include <string>
#include <sstream>
#include <cstring>
int main() {
std::stringstream ss{ "test" };
std::stringbuf *pbuf = ss.rdbuf();
char buffer[80];
pbuf->sgetn (buffer,80);
std::streambuf *pbuf2 = std::cout.rdbuf();
pbuf2->sputn (buffer, std::strlen(buffer));
return 0;
}

Answer 2

As a complement to #Thomas Sablik answer, here is a code example for printing "text" by redirecting streams:
std::stringstream ss;
ss.basic_ios<char>::rdbuf(std::cout.rdbuf());
ss << "text";
std::cout.flush();
If you want to copy the content of ss's streambuf to std::cout, you can use inserter directly:
std::string str{ "text" };
std::stringstream ss{ str };
std::cout << ss.rdbuf();
std::cout.flush();
http://wordaligned.org/articles/cpp-streambufs