How to set maximum read length for a stream in C++?

Question

I'm reading data from a stream into a char array of a given length, and I'd like to make the maximum width of read to be large enough to fit in that char array.

The reason I use a char array is that part of my specification is that the length of any individual token cannot exceed a certain value, so I'm saving myself some constructor calls.

I thought width() did what I wanted, but I was apparently wrong...

EDIT: I'm using the stream extraction operators to perform the extraction, since these are flat text files with values separated by whitespace.

I don't understand. istream::read() lets you specify the amount of characters you want to read into a buffer. Why won't this work for you? — Emile Cormier
– Emile Cormier, Commented Mar 18, 2010 at 16:20
What I mean is that I want the extraction operators to limit themselves to a certain number of characters in length. The more I think about this, though, then more I realize this i probably not standard behavior... — alexgolec
– alexgolec, Commented Mar 18, 2010 at 16:26

Bill · Accepted Answer · 2010-03-18 16:20:55Z

5

If you're processing text, you're looking for the get function: http://cppreference.com/wiki/io/get

const int size = 200;
char myArray[size] = {};

cin.get(myArray, size);

Note: only size - 1 characters are read, which leaves a NULL terminator in myArray.

If it's raw data, you'd probably prefer read: http://cppreference.com/wiki/io/read

const int size = 200;
char myArray[size] = {};

cin.read(myArray, size);

size bytes are read.

answered Mar 18, 2010 at 16:20

Bill

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Brian · Accepted Answer · 2010-03-18 16:33:15Z

4

char x[4];
cin.width(4);
cin >> x;
cout << x;

Input: "abcdef"
Output: "abc" (x[3] is null terminating char)

Width works fine in this case.

Note: Empirical testing indicates that the cin.width call only lasts for one stream operation. It may be more convenient to use cin >> setw(4) >> x; instead, though this requires iomanip.

edited Mar 18, 2010 at 16:33

answered Mar 18, 2010 at 16:26

Brian

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2 Comments

Bill Over a year ago

Note that if the input contains white-space this will fail. Try it for Input: "a b c d e f". Output: "a".

Brian Over a year ago

@Bill: I consider that a good thing. The OP said that the values are separated by whitespace.

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