Read a binary file (jpg) to a string using c++

问题:

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?

回答1:

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(); 

回答2:

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; } 

回答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.

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