C++: What is the difference between ostream and ostringstream?

Asked 21/8, 2012 at 12:3 Answered 21/8, 2012 at 14:16

What is the difference between ostream and ostringstream? When would you use one versus the other?

Sholom answered 21/8, 2012 at 12:3 Comment(1)

also ostringstream is a derived class from ostream – Exhibit 21/8, 2012 at 12:6

Put succinctly: ostringstream provides a streambuf, ostream requires the user to provide one.

To understand the implications, it's necessary to understand a little how streams work, and I'm not sure that there's a good explanation of this on the Web. The basic abstraction of ostream is formatting textual output. You give it an int or a double (or a user defined type—more on that later), and it convert it into a stream of characters, of type char. What it does with that stream depends on the streambuf which is attached to it; this is an example of the strategy pattern, where streambuf is an abstract base class of the strategy[1]. The standard provides two implementations of streambuf, filebuf and stringbuf; in practice, in all but the most trivial applications, you'll probably have some that you implement yourself.

When outputting, you always use ostream; it's the class over which the << operators are defined. You're formatting your data into a stream of characters, and you don't really care where the stream ends up.

When creating an instance: if you create an ostream, you must provide it with a streambuf yourself. More often, you'll create an ofstream or an ostringstream. These are both "convenience" classes, which derive from ostream, and provide a streambuf for it (filebuf and stringbuf, as it happens). Practically speaking, all they do is provide the necessary streambuf (which affects the constructor and the destructor, and not very much else); in the case of ofstream, there are also a few extra functions which forward to additional functions in the filebuf interface.

It's usual (but by no means required) when you define your own streambuf to provide convenience overloads of ostream (and istream, if relevant), along the same lines as ofstream or ostringstream.

By the same token, when creating an instance, it's usual to use one of the "convenience" derived classes, rather than to use ostream directly and provide your own streambuf.

And if all of this seems complicated: the iostream classes use just about all of the facilities of C++ (virtual functions, templates and function overloading all play an important role). If you're just learning C++, don't worry too much about it: just use ofstream or ostringstream when you construct an instance, but pass around references to ostream. And as you learn about techniques like virtual functions, templates and operator overloading, return to the iostreams to understand the role they play in making code more flexible.

[1] For various reasons, std::streambuf is not actually abstract. But the implementations of the virtual functions in it are useless; extraction always returns EOF, and insertion always fails.

Amaris answered 21/8, 2012 at 14:16 Comment(0)

Here is nice view of the Inheritance Hierarchy for C++ Stream Classes :) This article at the section 3.1 ofstream and ostringstream has what you need.

In essence : The ofstream class makes it possible to write data to files using stream operations and the ostringstream class makes it possible to write to strings.

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Joeljoela answered 21/8, 2012 at 12:19 Comment(1)

There are some (obvious) errors at the bottom of your hierarchy: the classes which derive from iostream are fstream and stringstream. (In practice, of course, fstream and stringstream are almost never used.) – Amaris 21/8, 2012 at 13:55

ostream is more general (subclasses support writing to different places), ostringstream is a specific one writing to a string

Ghyll answered 21/8, 2012 at 12:5 Comment(0)

ostream has no rdbuf implementation behind it, whereas ostringstream uses a stringbuf. Tried documentation?

Constable answered 21/8, 2012 at 12:5 Comment(6)

Sorry... I did look at the documentation but I am still new to C++. I have seen various uses of ostream and ostringstream but I was not sure of the difference. Seems like my question is too basic for this site. Maybe I'll withdraw it. – Sholom 21/8, 2012 at 12:13

It's not about question being too basic,it's more about the answer being readily available. I'm glad to hear you actually have tried looking for an answer. – Constable 21/8, 2012 at 12:14

Of course I googled... but reading a documentation page like cplusplus.com/reference/iostream/ostringstream isn't straightforward to someone new to C++. Plus, the second question on when one is used over the other has been mostly ignored by the other answers. – Sholom 21/8, 2012 at 12:17

Because the difference in functionality should put you on the right track regarding the use. ostringstream is used when you need stream stuff into string, whereas ostream is mostly used as a type for a parameter (referenced) when the callee is stream implementation agnostic. – Constable 21/8, 2012 at 12:20

Err... the page you link says "ostringstream provides an interface to manipulate strings as if they were output streams". The very first sentence. Is it not straightforward? – Constable 21/8, 2012 at 12:21

And "ostream objects are stream objects used to write and format output as sequences of characters". I didn't understand what an output stream was at the time... but I have read much more about C++ since and got it now. – Sholom 19/12, 2012 at 11:5

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