ifstream :: read即使使用reinterpret_cast也不读取未签名的字符

Question

I am trying to get my code to read a PPM image (P3) and it isn't working as it should. The idea is to get a 3 unsigned chars and store them in RGB. But at the moment it is only resulting in taking the first character and ignoring the rest.

Image Image::readImg(std::string const &filename) {
    std::ifstream ifs;
    ifs.open(filename.c_str(), std::ios::binary);
    Image _in;
    try {
        if (ifs.fail()) {
            throw("Can't open input file");
        }
        std::string header;
        int w, h, max;
        ifs >> header;
        if (strcmp(header.c_str(), "P3") != 0) throw("Can't read input file");
        ifs >> w >> h >> max;
        _in.init(w, h);
        ifs.ignore(256, '\n');
        unsigned char pix[3];
        for (int i = 0; i < h; ++i){
            for (int j = 0; j < w; ++j){
                ifs.read(reinterpret_cast<char *>(pix), 3);
                _in.pixels[i][j].R = pix[0];
                _in.pixels[i][j].G = pix[1];
                _in.pixels[i][j].B = pix[2];
            }
        }
        std::cout << "|" << _in.pixels[0][0].R << " " << _in.pixels[0][0].G << " " << _in.pixels[0][0].B << "|";
        ifs.close();
    }
    catch (const char *err) {
        fprintf(stderr, "%s\n", err);
        ifs.close();
    }
    return _in;
}

Note the std::cout is supposed to output in my scenario |186 0 255|, but instead I get |1 8 6|.

---

EDIT: The file (original.ppm), when opened in Notepad++, looks like this (UNIX / UTF-8):

P3
1024 768
255
186 0 255 186 0 255 186 0 255 186 0 255 186 0 255 186 0 255 186 1 255 
186 1 254 186 1 254 185 2 254 185 2 254 185 1 254 185 2 253 185 3 253 
185 2 252 185 3 252 185 3 252 185 3 252 185 3 251 184 4 251 184 4 251 
184 4 251 184 4 251 184 5 250 184 5 250 183 5 250 183 6 249 183 6 249 
183 6 249 183 6 248 183 7 249 183 7 249 183 7 248 183 7 247 183 8 247 
182 7 247 182 8 246 183 9 247 183 9 246 183 9 246 182 9 246 181 9 246 
182 9 246 181 10 245 182 10 245 181 10 244 181 10 245 181 11 244 181 11 244
...

The result should be: _in.pixels[0][0].R = 186 _in.pixels[0][0].G = 0 _in.pixels[0][0].B = 255 and to continue collecting the RGB of all the pixels in the file.

Answer 1

Using >> operations in streams skip whitespace by default.

If you want to preserve all characters then do (before you read from it):

ifs >> std::noskipws;

you might also need to open the file in binary mode. But I don't think it is necessary.

if you want to read exactly two bytes into a string you can use this instead of getline:

std::string st;
st.resize(2);
ifs.read(&st[0], st.size());

Answer 2

[Updated] This should work:

int pix[3]; // not an unsigned char
...
ifs >> pix[0] >> pix[1] >> pix[2];

instead of:

ifs.read(reinterpret_cast<char *>(pix), 3);

Like you do for width and height?

ifs >> w >> h >> max;