C++ :: Using Variable As Name For Combined File?

I have been researching how to use the prefix of a user input file name as the prefix of an output file name for a combined file. "Adding two text files into one text file", but the combined file name is entered directly, not from a variable.

std::ifstream file1( "file1.txt" ) ;
std::ifstream file2( "file2.txt" ) ;
std::ifstream file3( "file3.txt" ) ;
std::ofstream combined_file( "combined_file.txt" ) ;
combined_file << file1.rdbuf() << file2.rdbuf() << file3.rdbuf() ;

My code already has the user input file name as a variable. I would like to insert the prefix of that variable in line 4 with a new suffix.

I also found this code in an archived post by Duoas on May 31, 2010, "Changable filename based on user input," but I don't know how to adapt it to my combined file.

#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main() {
ofstream f;
string filename;

[code].....