Preprocessor directive to create file names - c++

Question

I have to open files one by one for reading in C/C++. The name of the files are in0, in1, in2, in3.....
I tried to use preprocessor directive to create file names.
i want something like.
for(int i=0;i<n;i++)
{
string inp_file="/path/"+"in"+APPEND(i); //to generate /path/in1 etc
open(inp_file);
}
where APPEND is a MACRO.
Since
#define APP(i) i
can generate the value
#define APP(i) #i
can convert a token to string.
I am trying to combine them both in many ways but failed.
How to get the desired result or is it even possible to get the such a result with macro?

Answer 1

In your case, the variable i is not a compile-time constant and so it is impossible to use pre-processor or template specialization because the value is simply not known at a time of compilation. What you can do is convert integer into string - boost.lexical_cast is one of the easiest to use solutions:
for (int i = 0; i < n; ++i) {
// Generate /path/in1 etc
std::string inp_file = "/path/in"+ boost::lexical_cast<std::string>(i);
open(inp_file);
}
If you happen to have a compiler with C++11 support, you could use std::to_string(). For example:
for (int i = 0; i < n; ++i) {
// Generate /path/in1 etc
std::string inp_file = "/path/in" + std::to_string(i);
open(inp_file);
}
Hope it helps. Good Luck!

Answer 2

Addendum to Vlad's answer -- if for some reason you're not able/willing to use Boost, you can accomplish what you want using standard C++ with the stringstream class:
#include <sstream>
void Foo() {
for (int i = 0; i < n; ++i) {
std::stringstream converter;
converter << "/path/in" << i;
open(converter.str());
}
}

Answer 3

If you're not using boost, try this:
namespace StringUtils
{
// Converts any type which implements << to string (basic types are alright!)
template<typename T>
std::string StringUtils::toString(const T &t)
{
std::ostringstream oss;
oss << t;
return oss.str();
}
}
Use it this way:
for(int i=0;i<n;i++)
{
string inp_file="/path/"+"in"+ StringUtils::toString(i); //to generate /path/in1 etc
open(inp_file);
}

Answer 4

Just an addition to the existing answers which are all great, if you are using a newer compiler and standard library, c++11 introduces std::to_string(). So you can write code like this:
for(int i = 0; i < n; i++) {
string inp_file = "/path/in"+ std::to_string(i);
open(inp_file);
}

Answer 5

The C solution is this :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
int n =4;
char nstr[12];
sprintf(nstr, "%d", n);
int nlen = strlen( nstr );
const char *fd = "/path/in";
char buff[ strlen(fd) + nlen + 1 ];
sprintf( buff, "%s%d", fd, n );
/* for testing */
printf( "%s\n", buff );
}