C++ :: Merging Contents Of Files?

i need to write a c++ code which can merge contents of several .txt files into a single file. i used the following code , it works but after merging the result file contains the contents merged twice.I think it over writes the result.I want to do it without using command line.

#include<iostream>
#include<fstream>
using namespace std;
int main()
{
std::ofstream("merge.txt");
system("type *.txt >> merge.txt");
system("pause");
return 0;
}