C++ Iterate an istream

Question

What is the best way to parse or iterate an istream? I need to create a function that takes an istream, parses it and creates an object so was wondering the easiest way to do this. Even something that could convert it to string would be dandy.

Answer 1

You can use an istream_iterator.

typedef std::istream_iterator<std::string> streamiter;
for (streamiter it = streamiter(some_istream); it != streamiter(); it++) {
    // process words
}

This will split the input stream at all whitespaces.

Answer 2

Since C++ doesn't have reflection and persistence built in, you cannot write a function that reads any object and then see what it came up with. You need to know what you are looking for before-hand and read that specifically. (Of course you could always just read tokens and feed those into a parser.)

If you know exactly which type of object to read from the stream, it's often good to give that class a constructor taking a std::istream&. Since usually the class is also where the code to write into the stream is, this puts both of them close together, which is best for maintenance. The parsing code then just creates the object passing the stream to the constructor.

If you don't know which type you will encounter, you will have to write a (probably simple) parsing function. Such formats should start with an identifier that tells which type of objects follow. Your parsing function would first have to read that identifier, and then branch into code that reads the appropriate type from the stream. Since at this point it knows what type of object to read from the stream, reading the actual objects can be implemented in constructors as described above.