Question

I’ve read a bunch of posts regarding redirecting std::cout to stringstreams, but I’m having problem reading the redirected string.

std::stringstream redirectStream;
std::cout.rdbuf( redirectStream.rdbuf() );

std::cout << "Hello1\n";
std::cout << "Hello2\n";

while(std::getline(redirectStream, str))
{
  // This does not work - as the contents of redirectStream 
  // do not include the '\n' - I only see "Hello1Hello2"
}

I need to pick out the new lines within the initial output – can anyone enlighten me as to how to do that?

Thanks.

Answer 1

Works fine for me:
Note: the std::getline() reads the line (but not the ‘\n’ character, the line terminator is thrown away after each line is read). But the loop will be entered once for each line.

#include <iostream>
#include <sstream>

int main()
{
    std::stringstream   redirectStream;
    std::streambuf*     oldbuf  = std::cout.rdbuf( redirectStream.rdbuf() );

    std::cout << "Hello1\n";
    std::cout << "Hello2\n";

    std::string str;
    while(std::getline(redirectStream, str))
    {
        fprintf(stdout,"Line: %s\n",str.c_str());
        // loop enter once for each line.
        // Note: str does not include the '\n' character.
    }

    // In real life use RAII to do this. Simplified here for code clarity.
    std::cout.rdbuf(oldbuf);
}

Note: you need to put the old stream-buffer back in std::cout. Once the stringstream ‘redirectStream’ goes out of scope its buffer will be destroyed leaving std::cout pointing at an invalid stream-buffer. Since std::cout lives longer than ‘redirectStream’ you need to make sure that std::cout does not access an invalid object. Thus the easiest solution is to put back the old buffer.