C++ :: Appended File Path Not Working

I need to append file path for some particular program. But the problem is when I append it like below, it gets error whether filepath seems to be C:/Users/My/Desktop/C++/1.txt

int i=1;
stringstream str;
str<<"C:/Users/My/Desktop/C++/"<<i<<".txt";
string filepath=str.str();
cout<<filepath;
ifstream ipf(filepath);
if(ipf)
{ do some thing...}

But if it was like this no error, program work as desired.

int i=1;
stringstream str;
str<<"C:/Users/My/Desktop/C++/"<<i<<".txt";
string filepath=str.str();
cout<<filepath;
ifstream ipf("C:/Users/My/Desktop/C++/1.txt");
if(ipf)
{ do some thing...}