char *result="";
...
result+=ch;

//char *result="";
char result=new char[];

//result+=ch
result[i]=ch;

#include <stdio.h>
#include <string>
#include <fstream>
 
const char * o_file(const char *filename);
const std::string e_file(const char *filename);
const std::string yes_file(const char *filename);

int main() 
{
  const char *p = o_file("c:\\tmp\\q.txt");
  printf( "Value is [%s]\n", p );

  const std::string res = e_file("c:\\tmp\\q.txt");
  printf( "Value is [%s]\n", &res[ 0 ] );

  const std::string res1 = yes_file("c:\\tmp\\q.txt");
  printf( "Value is [%s]\n", &res1[ 0 ] );

  return  0 ;
}

const char * o_file(const char *filename)
{
  char *result = "";
  FILE *f = fopen(filename, "r");
  if( f != NULL )
  {
    char ch;
    fseek(f,  0 , SEEK_SET);
    while( !feof(f) && ch != '\n' &&  ch != '\r' )
    {
      ch = fgetc(f);
      if ( ch != '\n' &&  ch != '\r' && !feof(f)) result+=ch;
    };
    fclose(f);
    return(result);
  }
  else 
    return NULL;
}

const std::string e_file(const char *filename)
{
  FILE *f = fopen(filename, "r");
  if( f != NULL )
  {
    std::string result;
    //fseek(f, 0, SEEK_SET); // ýòî íå íóæíî, ïîñêîëüêó ôàéë òîëüêî ÷òî îòêðûò
    while( !feof(f) )
    {
      int ch = fgetc(f);
      if( ch == '\n' ||  ch == '\r' )
        break;
      result += ch;
    };
    fclose(f);
    return result;
  }
  else 
    return std::string();
}
    
const std::string yes_file(const char *filename)
{
  std::ifstream f( filename );
  if( !f )
    return std::string();
  else 
  {
    std::string res;
    std::getline(f,res);
    return res;
  }
}