Read a binary file (jpg) to a string using

Question

I need to read a jpg file to a string. I want to upload this file to our server, I just find out that the API requires a string as the data of this pic. I followed the suggestions in a former question I've asked Upload pics to a server using c++ .

int main() {
    ifstream fin("cloud.jpg");
    ofstream fout("test.jpg");//for testing purpose, to see if the string is a right copy
    ostringstream ostrm;

    unsigned char tmp;
    int count = 0;
    while ( fin >> tmp ) {
        ++count;//for testing purpose
        ostrm << tmp;
    }
    string data( ostrm.str() );
    cout << count << endl;//ouput 60! Definitely not the right size
    fout << string;//only 60 bytes
    return 0;
}

Why it stops at 60? It's a strange character at 60, and what should I do to read the jpg to a string?

UPDATE

Almost there, but after using the suggested method, when I rewrite the string to the output file, it distorted. Find out that I should also specify that the ofstream is in binary mode by ofstream::binary. Done!

By the way what's the difference between ifstream::binary & ios::binary, is there any abbreviation for ofstream::binary?

Answer 1

You shouldn't read the file to a string because it is legal for a jpg to contain values that are 0. However in a string, the value 0 has a special meaning (it's the end of string indicator aka \0). You should instead read the file into a vector. You can do this easily like so:

#include <algorithm>
#include <iostream>
#include <fstream>
#include <vector>

int main(int argc, char* argv[])
{
    std::ifstream ifs("C:\\Users\\Borgleader\\Documents\\Rapptz.h");

    if(!ifs)
    {
        return -1;
    }

    std::vector<char> data = std::vector<char>(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());

    //If you really need it in a string you can initialize it the same way as the vector
    std::string data2 = std::string(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());

    std::for_each(data.begin(), data.end(), [](char c) { std::cout << c; });

    std::cin.get();
    return 0;
}

Answer 2

Open the file in binary mode, otherwise it will have funny behavior, and it will handle certain non-text characters in inappropriate ways, at least on Windows.

ifstream fin("cloud.jpg", ios::binary);

Also, instead of a while loop, you can just read the whole file in one shot:

ostrm << fin.rdbuf();

Answer 3

Try opening the file in binary mode:

ifstream fin("cloud.jpg", std::ios::binary);

At a guess, you were probably trying to read the file on Windows and the 61^st character was probably 0x26 -- a control-Z, which (on Windows) will be treated as marking the end of the file.

As far as how to best do the reading, you end up with a choice between simplicity and speed, as demonstrated in a previous answer.