c++ ifstream undeclared identifier

You need to reference the standard namespace (std). Try this:

#include <iostream>
#include <iomanip>
#include <fstream>
void main()
{
    std::ifstream infile("file.txt");
    std::ofstream outfile("out.txt");
}

You need to scope it. Use using namespace std; or preface ifstream and ostream with std::

For example, std::ifstream

Currently, the compiler does not know where these structures are defined (since they are declared/defined within the std namespace). This is why you need to scope your structures/functions in this case.