Question

While it is easy to write something that does this by myself, I often wondered if there was something like this in iomanip or somewhere. However, I never found something useful. Ideally, it’d be sensitive to locales (e.g. in Germany you’d write 1,234,567.89 as 1.234.567,89) and hence highly superior to building the comma-string by hand.

Answer 1

According to this thread, you can set a locale on your output stream by doing something like:

#include <iostream>
#include <locale>
#include <string>

struct my_facet : public std::numpunct<char> {
    explicit my_facet(size_t refs = 0) : std::numpunct<char>(refs) {}
    virtual char do_thousands_sep() const { return ','; }
    virtual std::string do_grouping() const { return "\003"; }
};

int main() {
    std::locale global;
    std::locale withgroupings(global, new my_facet);
    std::locale was = std::cout.imbue(withgroupings);
    std::cout << 1000000 << std::endl;
    std::cout.imbue(was);

    return 0;
}

Haven’t tried it myself but it certainly sounds like a reasonable approach.