How to store data from a file read in binary mode C++

Question

Hi i am trying to read a file , say for example 'sample.txt', in binary mode-c++ and i need to store the file text (eg."nodeA nodeB") in a vector . eg: "A A B E A B G" if this is what is in the text file , i want to read it in binary form and then store it in some variable and then do some operations to it . Any help would be appreciated. So far i have got is :

int main () {
  streampos begin,end;
  ifstream myfile ("example.bin", ios::binary);
  begin = myfile.tellg();
  myfile.seekg (0, ios::end);
  end = myfile.tellg();
  myfile.close();
  cout << "size is: " << (end-begin) << " bytes.\n";
  return 0;
}

The text part in the file myfile can be got how ?

score 3 · Answer 1 · answered Feb 14 '15 at 18:22

3

The method you are after is ifstream::read(char *,streamsize). Having gotten the size of the file in bytes, you can read the data into a vector<char> of the correct size (after seeking back to the beginning of the file):

streamsize n=end-begin;
vector<char> data((size_t)n);

myfile.seekg(0,ios::beg);
myfile.read(&data[0],n);

The iterator type of vector<char> may not necessarily be a char * pointer, so instead we pass as the first argument to read a pointer to the first element of the vector. The elements of a std::vector are guaranteed to be laid out contiguously, and so we can be assured that writing to &data[0]+k is equivalent to &data[k], for valid indices k.

answered Feb 14 '15 at 18:22

halfflat

1,584
8
11

this is not binary mode , i NEED it in binary mode – user3302936 Feb 20 '15 at 09:06
If the stream is opened in binary mode, which you have done, then you have a stream which bypasses any operating specific text conversions (e.g., translation of newline characters to a native representation.) Separate from that, you have the choice of using formatted (e.g. `operator>>`) or unformatted (e.g. `read` and `seekg`) operations. If you are performing unformatted operations on a stream that has been opened in binary mode, then you are getting the raw character data. – halfflat Feb 20 '15 at 13:17

score 1 · Answer 2 · answered Feb 16 '15 at 20:18

Your file sample.txt, or whatever, is a text file. I believe you want to "read it in binary form" because you think that you have to do that in order to find out the size of the data, so that you can then allocate storage of that size, in some variable, to contain the data.

In that case, really all you want to do is read a text file into a suitable variable, and you can do this very simply, without having to discover the length of the file:

#include <fstream>
#include <iterator>
#include <string>
#include <algorithm>

...

std::istream_iterator<char> eos; // An end-of-stream iterator
// Open your file
std::ifstream in("sample.txt"); 
if (!in) { // It didn't open for some reason.
    // So handle the error somehow and get out of here. 
}
// Your file opened OK
std::noskipws(in);  // You don't want to ignore whitespace when reading it
std::istream_iterator<char> in_iter(in); // An input-stream iterator for `in` 
std::string data;   // A string to store the data
std::copy(in_iter,eos,std::back_inserter(data)); // Copy the file to string

Now, the whole contents of sample.txt are in the string data and you can parse it anyway you want. You could replace std::string with some other standard container type of char, e.g. std::vector<char>, and this would work just the same.

How to store data from a file read in binary mode C++

2 Answers2