What's the point of stringstream?

Question

I'm going to keep this question very simple. I'm learning C++ and I've come across stringstreams. I understand that their main usage is to have variables input into them so they can later output the value they hold using str() as a string. My question is - what's the point of this? This sounds like a very fancy way of just concatenating a bunch of variables in a string object using the + operator. Does it have more to it than that or is it just so it confuses noobs and causes them to fail their exams?

Answer 1

Well, one problem is that you cannot "concatenate a bunch of variables in a string using the + operator" (only other strings or char*s).

So, how are you going to turn all your objects into strings? Unlike Java, C++ does not have any to_string() member convention. On the other hand, every class interested in using iostream will define stream inserters (std::ostream& operator<<(std::ostream& os, const MyClass& foo) and maybe std::istream& operator>>(std::istream& os, MyClass& foo).)

So, you use the stream inserters to convert objects to text. Sometimes you don't want to write to the console or to a file, but instead you want to store it as a string.

Also, using the iostream framework lets you use the manipulators to control precision, width, numerical base, and so on, instead of trying to do all that manually as you construct a string.

Now, that's not to say that the stringstream solution is ideal: in fact, a lot of libraries exist to do the same sort of task better (including at least Boost.Format, Boost.Convert, Boost.Lexical_Cast, and Boost.Spirit just in Boost.)

Answer 2

If you have:

int a = 3;
std::string str = "hello";
MyObject obj;

Then:

std::string concat = a + str + obj;
std::string objstr = obj; 

won't work, while:

std::stringstream stream;
stream << a << str << obj;
std::string concat = stream.str();

std::stringstream stream2;
stream2 << obj;
std::string objstr = stream2.str();

Will work (at least if MyObject defines a operator<<). That's the whole point of std::stringstream: make it easy to redirect "anything" to a string.

Any object that can be redirected to a std::ostream (std::fstream, std::cout...) can also be redirected to a std:::stringstream (as it derives from ̀std::ostream too). Then you just need to declare one std::ostream redirection operator (operator<<) and it can be used to redirect the object everywhere (file, console, but also string...).

The whole point is that you could declare a operator+ and operator+= to make it possible to concatenate your object to a std::string. But then, if you also wish to redirect it to a stream (file, cout), you'll have to declare 3 operators (operator+, operator+= and finally operator<< for streams), all doing almost the same thing. In the end, thanks to std::stringstream, having only one single operator (operator<<) is enough to redirect to file, cout and string.

Answer 3

What's the point of stringstream?

It is a flexible and fast stream, and works as a simulator for other (comparatively) slow streams. I use stringstream for lots of different things.

---

My favorite use is for test. I create a std::istream and fill it with test data. This allows me to create the test data 'file' using the same editor with which I code (and no actual file polluting my work dir). Adding more test cases is remarkably less time consuming.

   //                      012345678901234567890123456789012345678901234567890
   std::istringstream iss("7 ((23, 342), (17, 234), (335, 159), (10, 10))");
   //                      ---|^^^^^^^|^v|vvvvvvv|v^|^^^^^^^^|^v|vvvvvv|^
   //                         1         2          3           4

   // echo to user of test
   std::cout << "Test Data Input: " << iss.str() << std::endl ;

   // code under test starts here ... 
   int    nTowns = 1;
   char  lparen0 = 2;
   iss >> nTowns       // NOTE : formatted input drops whitespace
       >> lparen0;

   // and continues with echo of post conversion
   std::cout << " 0: " << nTowns << ' ' << lparen0 << std::endl;

   // then more of the same - until record is completely read.

---

I have used stringstream to build a screen update, and use that dynamic changing string to 'measure' the banner width, and compute where to start the placement on the screen so that the banner is centered (or left, or right):

static void centerUpdateScreenBanner( uint64_t gen, 
                                      int      pdMS, 
                                      int      changes, 
                                      TC_t&    tc)
  {
     // build contents of screen banner update
     std::stringstream ss;
     ss << std::setw(3) << pdMS << "  "
        << std::setw(4) << gen  << "  "
        << std::setw(4) << changes;

     // compute start column placement for centering
     int rCol = tc.maxCol - 
                static_cast<int>(ss.str().size()) +
                tc.DfltIndnt-1;

     // send banner to terminal device for user info
     tc.termRef << Ansi_t::gotoRC(0, rCol)  // top right
                << ss.str() << std::flush;
  }

---

Inside this link list, I use a stringstream to absorb and recursively concatenate list nodes info. While this builds recursively, other cout or cerr can proceed un-hindered - as if a 3rd channel.

  std::string showR(void) {
     std::stringstream ss;
     ss << m_payload->show();            // this node
     if(m_next)  ss << m_next->showR();  // subsequent nodes
     return (ss.str());
  }

Summary: I have found std::stringstream to be very useful.