Question

I have the current code that readings a text file into memory:

std::streampos fsize = 0;
std::ifstream file(fileName, std::ios::binary); // open file

if(!file.good()) {
    std::cout << "Error opening file";
    return 0;
}

// get length of file
file.seekg(0, ios::end);
fsize = file.tellg();

// allocate memory
char *memory = new char[fsize];

// read data as a block
file.seekg (0, ios::beg);
file.read (memory, fsize);

file.close(); // close file

return fsize;

Now i have code that iterates over it. If the line starts with ‘v’ it reads the 3 preceding float values, and same with if it starts with ‘n’, but into different arrays.

char* p = memory;       // pointer to start of memory
char* e = memory + fsize;   // pointer to end of memory

while (p != e) {
    if (memcmp(p, "v", 1) == 0) { 
        sscanf(p, "v %f %f %f", &a[vI], &b[vI], &c[vI]);
        vI++;
    } else if (memcmp(p, "n",  1) == 0) {
        sscanf(p, "v %f %f %f", &d[nI], &e[nI], &f[nI]);
        nI++;           
    while (*p++ != (char) 0x0A);
}

I know there must be a better/safer way to do this.

Answer 1

I assume you have a text-file there. This can be done a lot simpler. First, don’t open the file in binary-mode, and simply read linewise. The following is a possible implementation:

template<class output_iterator>
void read_file(std::istream &input, output_iterator v1, output_iterator v2,
               output_iterator v1) {
    std::string line_buffer;

    while(std::getline(input, line_buffer)) { // read each line of text
        if(line_buffer[0] == 'v') {
            std::stringstream line_stream(line_buffer.substr(1)); // drop the 'v'
            // read three consecutive floats
            line_stream >> *v1++ >> *v2++ >> *v3++; 
        }
    }
}

This code assumes that lines starting with 'v' are well-formed. You can use it like this:

std::vector<float> values1, values2, values3;
std::fstream input_file(fileName);

read_file(input_file, std::back_inserter(values1), std::back_inserter(values2),
          std::back_inserter(values3));