Creating file names automatically C++

Question

I'm trying to write a program in C++, which creates some files (.txt) and writes down the result in them. The problem is that an amount of these files is not fixed at the beginning and only appears near the end of the program. I would like to name these files as "file_1.txt", "file_2.txt", ..., "file_n.txt", where n is an integer.

I can't use concatenation because the file name requires type "const char*", and I didn't find any way to convert "string" to this type. I haven't found any answer through the Internet and shall be really happy if you help me.

Answer 1

You can get a const char* from an std::string by using the c_str member function.

std::string s = ...;
const char* c = s.c_str();

If you don't want to use std::string (maybe you don't want to do memory allocations) then you can use snprintf to create a formatted string:

#include <cstdio>
...
char buffer[16]; // make sure it's big enough
snprintf(buffer, sizeof(buffer), "file_%d.txt", n);

n here is the number in the filename.

Answer 2

 for(int i=0; i!=n; ++i) {
     //create name
     std::string name="file_" + std::to_string(i) + ".txt"; // C++11 for std::to_string 
     //create file
     std::ofstream file(name);
     //if not C++11 then std::ostream file(name.c_str());
     //then do with file
 }

Answer 3

... another way to bulid the filename

#include <sstream>

int n = 3;
std::ostringstream os;
os << "file_" << n << ".txt";

std::string s = os.str();

Answer 4

Sample code:

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;

string IntToStr(int n) 
{
    stringstream result;
    result << n;
    return result.str();
}

int main () 
{
    ofstream outFile;
    int Number_of_files=20;
    string filename;


  for (int i=0;i<Number_of_files;i++)
  {
        filename="file_" + IntToStr(i) +".txt";
        cout<< filename << "  \n";

        outFile.open(filename.c_str());
        outFile << filename<<" : Writing this to a file.\n";
        outFile.close();
  }


  return 0;
}

Answer 5

I use the following code for this, you may find this useful.

std::ofstream ofile;

for(unsigned int n = 0; ; ++ n)
{
    std::string fname = std::string("log") + std::tostring(n) + std::string(".txt");

    std::ifstream ifile;
    ifile.open(fname.c_str());

    if(ifile.is_open())
    {
    }
    else
    {
        ofile.open(fname.c_str());
        break;
    }

    ifile.close();
}

if(!ofile.is_open())
{
    return -1;
}

ofile.close();