String Streams in C++

Most C++ programs that print something already rely on a stream. The object cout accepts output, and the object cin supplies input. A natural question follows: if a program wants to write to a file or read from one, does it need a completely different set of habits? The source answers this directly. A file stream can be used the same way as the ordinary stream objects you already know. The question this article follows is simple: what role does a file stream play in a program, and how closely does it match cout and cin?

The source says that ofs can be used as an ordinary ostream, just like cout. In practical terms, a program can treat ofs as a destination for output in the same manner it treats cout. Code that sends values into cout can be understood as sending values into ofs, with the file as the final destination. The behavior of the statements does not need to be relearned. What changes is where the data lands.

The same reasoning applies in the opposite direction. The source says that ifs can be used as an ordinary istream, just like cin. A program that reads values from cin can therefore read values from ifs, with the file supplying the data instead of the keyboard. The input side mirrors the output side: ofs plays the role of an output stream, and ifs plays the role of an input stream.

Taken together, the two directions form one picture. Output goes from the program into a file through ofs. Input comes from a file into the program through ifs. In both cases the program interacts with a stream object in a way it already understands from cout and cin. The diagram above shows the same pairing: each file stream sits beside its screen or keyboard counterpart under the same ordinary stream type.

The statement that ofs and ifs behave like cout and cin begins with a condition: assuming that the tests succeeded. The source does not spell out those tests in this passage, but the condition itself is important. The comparison with ordinary streams holds only when the file stream is in a working state. Treat the claim as conditional rather than as a guarantee that every file stream will behave like cout or cin in every circumstance.

File streams connect a program to a file. A different kind of stream connects a program to a string. The source states that the sstream header of the standard library provides streams to and from a string. The same stream idea that applies to files and to the screen therefore has a counterpart that works with string data held in memory.

The source gives this topic only a brief mention and does not show the operations of string streams. It does establish that streams to and from a string exist and that the sstream header provides them, so a learner knows where to look next. The diagram shows one reasonable way to picture the two directions, with istringstream handling input from a string and ostringstream handling output into one. A third stream type, stringstream, is commonly described as handling both directions; the source itself does not describe its operations.