Writing to a file at some point

[Void life]

Say I have a file with some information in it and after my program runs I want to 'update' this information. Using seekp function I can set the position where the next character is to be inserted. But this overwrites the already existing information after the position I set to. How do I make it so that it adds information and doesn't overwrite.

[SamuelAdams]

How are you opening the file ?

[Void life]

umm

ofstream dataFile ("Data.txt", ios::out || ios::in)

[AbstractionAnon]

Couple of ways to accomplish that:

ofstream dataFile ("Data.txt", ios::out | ios::ate | ios::in) 
//  ok
dataFile.seekp (ios_base::end);

Note that mode bits to open are combined with the bitwise or operator, not the logical or operator. Using the logical or operator will result in a value of true.

[Void life]

oops that was a type using the logical or operator. But anyway take a look at this.

#include <fstream>      
#include <iostream>
int main () {

  std::fstream outfile ("test.txt", std::ios::out);
if(outfile.is_open())
{
    outfile.write ("This is an apple",16);
    long pos = outfile.tellp();
    outfile.seekp (pos-7);
    outfile.write (" sam",4);
}
else
{
    std::cout<<"failed to open";
}
  outfile.close();

  return 0;
}

This will write "This is a sample" to the file. But i want it to write "This is a samn apple". So nothing is overwritten.

[Chervil]

I don't think inserting into a file like that is possible. Probably the easiest solution is to build the text in memory, then write it out.

        std::string text = "This is an apple";
        text.insert(9, " sam");
        outfile.write (text.c_str(), text.size());

[Void life]

Thanks chervil.
Yeah i was thinking of something like that. But I'd have to load the whole file into memory and then modify it at places I want, I was just trying to find a more efficient solution.
Say i do something like this to load the file into memory

#include <iostream>     
#include <fstream>      
int main () {

  std::ifstream is ("example.txt");
  if (is) {
    // get length of file:
    is.seekg (0, is.end);
    int length = is.tellg();
    is.seekg (0, is.beg);

    char * buffer = new char [length];

    std::cout << "Reading " << length << " characters... ";
    // read data as a block:
    is.read (buffer,length);

    if (is)
      std::cout << "all characters read successfully.";
    else
      std::cout << "error: only " << is.gcount() << " could be read";
    is.close();
    // ...buffer contains the entire file...

    delete[] buffer;
  }
  return 0;
}

How would i add stuff to buffer like i want to. And is there no better ways to do this?

[Chervil]

There are always choices, depending on what it is you want to do.
One way to read the file into a std::string (I'm sure there are other, better ways). This assumes that there is no ASCII zero character in the file.

    std::ifstream is ("example.txt");
    std::string text;
    char delim = 0;
    std::getline(is, text, delim);

[AbstractionAnon]

Read the file into a std::string, then you can user erase/insert/replace to manipulate the data as required.

[Void life]

Ok thanks guys.