(C++) File input/output

The reading from and writing to a file. Aften abbreviated to File I/O.

Example #1

include <fstream>

include <string>

include <cassert>


void writeToFile(const std::string& fileName, const double& value)
{
  std::ofstream myFile(fileName.c_str());
  assert(myFile.is_open()==true);
  myFile << "bilderbikkelWasHere" << '\n'
    << value;
  myFile.close();
}
double readFromFile(const std::string& fileName)
{
  std::ifstream myFile(fileName.c_str());
  std::string myData = "Nothing yet"; 
  double myValue = 0.0;
  myFile >> myData >> myValue;
  assert(myData=="bilderbikkelWasHere");
  return myValue;
} 
int main()
{
  writeToFile("temp.tmp",69.69);
  const double myValue = readFromFile("temp.tmp");
  assert(myValue==69.69);
}

Example #2

include <cassert>

include <vector>

include <string>

include <fstream>


//A helper function
bool fileExists(const std::string& fileName)
{
  std::fstream file;
  file.open(fileName.c_str(),std::ios::in);
  if( file.is_open() == true)
  {
    file.close();
    return true;
  }
  file.close();
  return false;
}
std::vector<std::string> ReadFile(const std::string& fileName)
{
  assert(fileExists(fileName)==true);
  std::fstream file(fileName.c_str(),std::ios::in);
  assert( file.is_open() == true);
  std::vector<std::string> v;
  while (file.eof() == false)
  {
    std::string s; file >> s;
    v.push_back(s);
  }
  file.close();
  return v;
}

You could also use the Boost's serialization library.

Questions

Windows XP issues

Windows XP automatically defragments the harddisk. If you open a file for longer then 13 minutes without using it, the opened file might get moved. Therefore, do not open your files without using them [1].

Testing this, with the code below, for 100 minutes, however, does not result in an error.

include <fstream>

include <string>

include <iostream>

include <vector>

include <cassert>

include <boost/timer.hpp>

include <boost/lexical_cast.hpp>

include <boost/shared_ptr.hpp>


int main()
{
  //Start the timer measuring total program execution time
  boost::timer mainTimer;

  //Create 100 files
  const int nFiles = 100;
  std::vector<boost::shared_ptr<std::ofstream> > files;
  for (int i=0; i!=nFiles; ++i)
  {
    //Create a file
    const std::string fileName = 
      "test" 
      + boost::lexical_cast<std::string>(i) 
      + ".txt";
    files.push_back(
      boost::shared_ptr<std::ofstream>(
        new std::ofstream(fileName.c_str())
        )
      );
  }
  //Write to and remove one file per minute
  for (int i=0; i!=nFiles; ++i)
  {
    //Create a timer
    boost::timer myTimer;
    //Wait one minute
    while (myTimer.elapsed() < 60.0) { /* wait */ }
    //Write to file
    (*files.back()) << "Test" << i << std::endl;
    //Close the file
    files.pop_back();
    //Output to screen
    std::cout << "Did " << i << "th file" << std::endl;
  }
  //Finished. But will XP make use get at this point in the program?
  std::cout << "Finished without problems after "
    << mainTimer.elapsed() << " seconds" << std::endl;
}

'File I/O' links

C
Prolog

Code links

References

1) http://windowsxp.devx.com/articles/perfen/default.asp

last edited (May 29, 2007) by bilderbikkel, Number of views: 28750, Current Rev: 22 (Diff)

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