Question

I am writing strings to a text file using:

ofstream ofs("ofs.txt", ios_base::binary);
std::string str = "Hi";
for (int i = 0 ; i < 10000000 ; i++) {
    ofs << str.c_str();
    ofs << "\n" ;
}

However, this takes long time to execute. Can anybody help me how to improve performance of the above. Or any other faster way to writing strings to file.

Thanks.

Answer 1

In several cases I found that C++ I/O streams tend to be slower than C <stdio.h> FILE*.

I had a confirmation also in the following test:

#define _CRT_SECURE_NO_WARNINGS // for stupid fopen_s warning
#include <stdio.h>
#include <exception>
#include <fstream>
#include <iostream>
#include <ostream>
#include <stdexcept>
#include <string>
#include <vector>
#include <windows.h>
using namespace std;

long long Counter() 
{
  LARGE_INTEGER li;
  QueryPerformanceCounter(&li);
  return li.QuadPart;
}

long long Frequency() 
{
  LARGE_INTEGER li;
  QueryPerformanceFrequency(&li);
  return li.QuadPart;
}

void PrintTime(long long start, long long finish, const char * s) 
{
  cout << s << ": " << (finish - start) * 1000.0 / Frequency() << " ms" << endl;
}

// RAII wrapper to FILE*
class File
{
public:
  explicit File(FILE * f)
    : m_file(f)
  {}

  ~File()
  {
    fclose(m_file);
  }

  FILE* Get() const
  {
    return m_file;
  }

  bool IsOpen() const
  {
    return (m_file != nullptr);
  }

private:
  FILE* m_file;

  File(const File&);
  File& operator=(const File&);
};

void TestIoStream(const vector<string>& lines)
{
  ofstream ofs("ofs.txt", ios_base::binary);
  for(auto it = lines.begin(); it != lines.end(); ++it)
  {
    ofs << it->c_str();
    ofs << "\n" ;
  }
}

void TestStdioFile(const vector<string>& lines)
{
  File file( fopen("cfile.txt", "wt") );
  if (! file.IsOpen())
    throw runtime_error("Can't open C FILE*.");

  for(auto it = lines.begin(); it != lines.end(); ++it)
  {
    fputs( it->c_str(), file.Get());
    fputs( "\n", file.Get());
  }
}

int main()
{
  static const int kExitOk = 0;
  static const int kExitError = 1;
  try
  {
    cout << "Building test lines...";
    vector<string> lines;
    for (int i = 0; i < 10000000; i++)
      lines.push_back("Hi");
    cout << "done. ";
    cout << "(Count = " << lines.size() << ")" << endl;

    long long start = 0;
    long long finish = 0;

    start = Counter();  
    TestIoStream(lines);
    finish = Counter();
    PrintTime(start, finish, "C++ I/O stream");

    start = Counter();
    TestStdioFile(lines);
    finish = Counter();
    PrintTime(start, finish, "C FILE*");

    return kExitOk;
  }
  catch(const exception& e)
  {
    cerr << "\n*** ERROR: " << e.what() << endl;
    return kExitError;
  }
}

Compiled with VS2010 SP1 (VC10):

cl /EHsc /W4 /nologo /MT /O2 /GL test.cpp

Test result:

Building test lines...done. (Count = 10000000)
C++ I/O stream: 2892.39 ms
C FILE*: 2131.09 ms