replace line in a file C++

Question

I\'m trying to find a way to replace a line, containing a string, in a file with a new line.

If the string is not present in the file, then append it to the file.

Answer 1

As mentioned, the code by KarlPhilip is a good starting point (thanks), but did not work correctly according to the ">= 0". Comparing to "0" it is known for MS CString-types and in C#, etc, but it seems not to work properly with the STL. E.g. in VS 2010 (only release), the C++ code behaved wrong with this compare without throwing an error!

Here is a correction to C++ standards: If some wizards have improvements, please post them. I think, it is a very useful and important question/snippet.

#include 
#include 
#include 
#include 

using namespace std;    

int main() 
{   
   string search_string = "cactus";   
   string replace_string = "oranges";   
   string inbuf;
   // Todo: Check, if input_file exists before. 
   // Todo: Try/catch or check, if you have write access to the output file.
   fstream input_file("demo.txt", ios::in);   
   ofstream output_file("result.txt");

   while (!input_file.eof())   
   {
      getline(input_file, inbuf);
      size_t foundpos = inbuf.find(search_string);
      if(foundpos != std::string::npos)
      {
         string tmpstring = inbuf.substr(0,spot);
         tmpstring += replace_string;
         tmpstring += inbuf.substr(spot+search_string.length(), inbuf.length());
         inbuf = tmpstring;
      }

      output_file << inbuf << endl;   
   }

   //TODO: delete demo.txt and rename result.txt to demo.txt      
   // to achieve the REPLACE effect. 
}

I used the following replacement part inside the "if" from another SO question (The code above only replace the first occurrence):

if(foundpos != std::string::npos)
   replaceAll(inbuf, search_string, replace_string); 

//http://stackoverflow.com/questions/3418231/c-replace-part-of-a-string-with-another-string
//
void replaceAll(std::string& str, const std::string& from, const std::string& to) 
{
    size_t start_pos = 0;
    while((start_pos = str.find(from, start_pos)) != std::string::npos) 
    {
        size_t end_pos = start_pos + from.length();
        str.replace(start_pos, end_pos, to);
        start_pos += to.length(); // In case 'to' contains 'from', like replacing 'x' with 'yx'
    }
}