compile visual studio c++ online

Language: Layout:

#include <iostream>
#include <string> // <cstring> <string.h> // **** changed
#include <fstream>

const int NUMBER_OF_FILES = 21;

void cleanData()
{
    //file names
    const std::string outFileName = "output.txt";
    const std::string inFileName = "input_";
    const std::string inFileExt = ".txt";

// create/open output file, should be in directory
    // note: std::ios::_Nocreate would be ignored; std::ios::app dominates the non-standard std::ios::_Nocreate
    // to verify that the file exists:
    // if( !std::ifstream(outFileName).is_open() ) { /* error, file does not exist */ }
    std::clog << "*** info: trying to open output file '" << outFileName << "'\n" ;
    std::ofstream output( outFileName, std::ios::out | std::ios::app );

if( !output.is_open() )
    {
        std::cerr << "*** error: could not open output file\n" ;
    }

else
    {
        std::clog << "*** info: opened file '" << outFileName << "' for output\n" ;
        
        for( int i = 1; i <= NUMBER_OF_FILES; i++ )
        {
            const std::string inputPath = inFileName + std::to_string(i) + inFileExt ;
            std::ifstream f(inputPath) ;
            
            if( !f.is_open() )
            {
                std::clog << "*** info: skipping file '" << inputPath << "' (could not open file)\n" ;
            }
            
            else
            {
                std::string buff ;
                while( std::getline( f, buff ) )
                {
                    const auto sz = buff.size() ;
                    if( sz > 2 ) output << buff.substr( 0, sz - 2 ) ;
                    output << '\n' ;
                }
            }
        }
    }
}

int main()
{
    cleanData() ;
}

Show compiler warnings [ + ] Compiler args [ + ] Show input
Compilation time: 1.7 sec, absolute running time: 0.09 sec, absolute service time: 1.8 sec

Error(s):

*** info: trying to open output file 'output.txt'
*** info: opened file 'output.txt' for output
*** info: skipping file 'input_1.txt' (could not open file)
*** info: skipping file 'input_2.txt' (could not open file)
*** info: skipping file 'input_3.txt' (could not open file)
*** info: skipping file 'input_4.txt' (could not open file)
*** info: skipping file 'input_5.txt' (could not open file)
*** info: skipping file 'input_6.txt' (could not open file)
*** info: skipping file 'input_7.txt' (could not open file)
*** info: skipping file 'input_8.txt' (could not open file)
*** info: skipping file 'input_9.txt' (could not open file)
*** info: skipping file 'input_10.txt' (could not open file)
*** info: skipping file 'input_11.txt' (could not open file)
*** info: skipping file 'input_12.txt' (could not open file)
*** info: skipping file 'input_13.txt' (could not open file)
*** info: skipping file 'input_14.txt' (could not open file)
*** info: skipping file 'input_15.txt' (could not open file)
*** info: skipping file 'input_16.txt' (could not open file)
*** info: skipping file 'input_17.txt' (could not open file)
*** info: skipping file 'input_18.txt' (could not open file)
*** info: skipping file 'input_19.txt' (could not open file)
*** info: skipping file 'input_20.txt' (could not open file)
*** info: skipping file 'input_21.txt' (could not open file)