使用str（）和rdbuf（）打印出字符串流

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;
}

Answer 2

Objects of ostringstream class use a string buffer that contains a sequence of characters. This sequence of characters can be accessed directly as a string object, using member str. That explains first part.

std::ostringstream oss("Hello");
std::cout << oss.str(); // works

The rdbuf returns pointer to the associated streambuf object, which is charge of all input/output operations. Thus, you need to use str() again to print the contents as in:

std::cout << oss.rdbuf()->str();

instead of:

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