Like it's written up there, you forget to type #include <sstream>
#include <sstream>
using namespace std;
QString Stats_Manager::convertInt(int num)
{
stringstream ss;
ss << num;
return ss.str();
}
You can also use some other ways to convert int to string, like
char numstr[21]; // enough to hold all numbers up to 64-bits
sprintf(numstr, "%d", age);
result = name + numstr;
check this!