Format number with commas in C++

Use std::locale with std::stringstream

#include <iomanip>
#include <locale>

template<class T>
std::string FormatWithCommas(T value)
{
    std::stringstream ss;
    ss.imbue(std::locale(""));
    ss << std::fixed << value;
    return ss.str();
}

Disclaimer: Portability might be an issue and you should probably look at which locale is used when "" is passed