C++ Reading ints as unsigned chars using istringstream

Question

When using std::istringstream to read gray pixel values from a file (0-255), I notice a strange behavior and I can't understand what is happening.

To conserve memory, I want to read these values into unsigned chars (since they have the same 0-255 range). However, there appears to be some sort of casting/truncation going on.

Input file:

Here is my code:

getline(fp, line); 
unsigned char temp;
istringstream iss(line);
iss >> temp;

When the value in the file is 194, for instance, the integer value for temp is 49...

What type of casting is going on?

You may use unformatted input: `std::istream::get` for bytes. — , Nov 18 '15 at 16:51
The question isn't to fix the code, it's what type of casting/truncation is occurring? — user2600959, Nov 18 '15 at 16:57
Please be clearer as to your input data. At the moment we can only _guess_ at its format, because "read values from a file" tells us effectively nothing! My guess is space-delimited numbers in the range 0<=x<=255 in human-readable decimal via ASCII. — Lightness Races in Orbit, Nov 18 '15 at 17:01
/OT @LightnessRacesinOrbit Seems I'm getting old and lame ;-) ... — πάντα ῥεῖ, Nov 18 '15 at 17:06

LogicStuff · Accepted Answer · 2015-11-19T12:28:29.590

3

There's no casting, this behavior is due to overloading.

std::basic_istream::operator>> happens to be overloaded for char types, extracting only one character at a time - 1, ASCII value of which is 49. This is the behavior you want, when you're reading single characters and it's assumed, that you want exactly that, when you're extracting to a character type.

To read the text as whitespace-delimited numbers, you have to use a larger type for the extraction e.g. unsigned short or std::uint16_t and then cast to unsigned char.

edited Nov 19 '15 at 12:28

answered Nov 18 '15 at 16:58

LogicStuff

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Thank you! This is the answer I was looking for! – user2600959 Nov 18 '15 at 16:59
2

You might as well check whether the value is in the range of `unsigned char` before the cast. – LogicStuff Nov 18 '15 at 17:02
Casting to `unsigned char` is well-defined so there's no need to do a check, unless you want to have special handling for unexpected values – M.M Nov 18 '15 at 20:20
I meant you'd want to handle the case that you read `257` for example, and end up with `1`. – LogicStuff Nov 18 '15 at 20:23

C++ Reading ints as unsigned chars using istringstream

1 Answers1