C++ - Incorrect ASCII value ("ë")

Question

First, I apologize for any english mistakes I'll make, but being 15 and french doesn't help...

I'm trying to program a PNG decoder with the help of the file format specification (http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html) but i came across a weird problem.

The specification says that the first eight bytes of a PNG file always contain the following (decimal) values: 137 80 78 71 13 10 26 10.

When I test this simple program :

int main() 
{
    ifstream file("test.png");

    string line;
    getline(file, line);

    cout << line[0] << endl;
}

The output is "ë" which represents 137 in the ascii table. Good, it matches the first byte.

However, when I do int ascii_value = line[0];, the output value is -119, which is not a correct ascii value.

When I try the same thing with another character like "e", it does output the correct ascii value.

Could someone explains what am I doing wrong and what is the solution ? I personally think it's an issue with the extended ascii table, but I'm not sure.

Thank you everybody ! I'll cast my signed char to an unsigned one !

Answer 1

Your system's char type is signed, which is why values thereof can be negative.

You need to be explicit and drop the sign:

const unsigned char value = (unsigned char) line[0];

Note that -119 = 137 in two's complement which your machine seems to be using. So the bits themselves really are correct, it's all about interpreting them properly.