String streams

std::ostringstream is a class whose objects look like an output stream (that is, you can write to them via operator<<), but actually store the writing results, and provide them in the form of a stream.

Consider the following short code:

#include <sstream>
#include <string>                                                                                                                          

using namespace std;

int main()
{
    ostringstream ss;
    ss << "the answer to everything is " << 42;
    const string result = ss.str(); 
}

The line

ostringstream ss;

creates such an object. This object is first manipulated like a regular stream:

ss << "the answer to everything is " << 42;

Following that, though, the resulting stream can be obtained like this:

const string result = ss.str();

(the string result will be equal to "the answer to everything is 42").

This is mainly useful when we have a class for which stream serialization has been defined, and for which we want a string form. For example, suppose we have some class

class foo 
{   
    // All sort of stuff here.
};  

ostream &operator<<(ostream &os, const foo &f);

To get the string representation of a foo object,

foo f;

we could use

ostringstream ss; 
ss << f;
const string result = ss.str();

Then result contains the string representation of the foo object.