Printing out a stringstream using str() and rdbuf()

Question

When I have:

std::ostringstream oss("Hello");

Why does this work:

std::cout << oss.str();

but this doesn't print anything:

std::cout << oss.rdbuf();

Reading the definition of operator<<(std::ostream&, std::streambuf*) say that it will print characters from the buffer. Does oss.rdbuf() not contain anything?

Answer 1

This issue is related to the fact that here, oss is ostringstream object (ostringstream is output stream so its destination is to write to it and not to read from it) and to fact how streams manage its internal buffer.

You can change

std::ostringstream oss("Hello");

to

std::istringstream oss("Hello");  // or std::stringstream oss("Hello");

and it will work as expected. Alternatively use

std::cout << oss.rdbuf()->str(); // this will print a copy of all buffer content

Example:

#include <iostream>
#include <sstream>

int main() {
    std::ostringstream oss("Hello");
    std::istringstream oss2("Hello");
    cout << oss.rdbuf()->str() << endl;  // prints "Hello"
    cout << oss2.rdbuf();                // prints "Hello"
    return 0;
}