C++ file stream

Question

Could someone please explain how c++ reads in files? I'm not asking the code to read in a file but after the ifstream >> variable, what are the rules to how c++ grabs the data ?

Is the file read like how a cin would read in the user input? meaning it stops after each whitespace? What happens after it reaches the end of a line? does it automatically proceed to the next line or do I have to write code for that? I know that it stops after eof, but I'm unsure of the process of extracting data and I can't write code if I don't understand the process. Thanks

Answer 1

Yes the input operator >> always "tokenizes" (stops at) whitespace. And reading from any input stream is working the same.

For very good information I suggest this reference. Especially the reference for the input operator is very detailed.

Answer 2

Basically when you call ifstream f;, you are creating a variable with access to the library. From there you must declare your intentions with that variable. Using f.open(fileName, ios::in); you can input from fileName using the >> operator, which actually operates like cin. It stops at white spaces like you'd expect. Once it reaches the end of a line, it continues as long as you have code that asks the operator to extract more. You dont have to do anything extra to tell it to move on to the next line.

More info can be found here.

Answer 3

The iostream formatted input and output operators are essentially defined in terms of the C library functions strtol/strtoul/strtod (cf. 22.4.2.1.2) and sprintf (cf. 22.4.2.2.2), respectively.

Answer 4

In C/C++ there is generally no difference in any stream input (besides details eg.: seeking).

Having C++ there are two distinguished ways of input:

• Formatted Input
• Unformatted Input

All formatted input operations involve an operator stream& operator >> (stream&, T). However, not all stream& operator >> (stream&, T) are performing formatted input (eg.: some are involving manipulators or a stream buffer)

Each formatted input starts with skipping white spaces and stops at the first character not being part of the input format (Note: It may be any character, it is not limited to white spaces).

Unformatted input reads all characters (does not ignore any white space) and stops if a requested amount of characters is retrieved or the stream reaches the end (EOF). Specialized functions (like std::getline) might stop early and ignore the delimiting condition character.