How can I set the decimal separator to be a comma?

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You should use basic_ios::imbue to set the preferred locale.

Take a look here: http://www.cplusplus.com/reference/ios/ios_base/imbue/

Locales allow you to use the preferred way by the user, so if a computer in Italy uses comma to separate decimal digits, in the US the dot is still used. Using locales is a Good Practice.

But if you want to explicitly force the use of the comma, take a look here:
http://www.cplusplus.com/reference/locale/numpunct/decimal_point/

Here a small example I just made with g++ which enforces the char ‘,’ (passing the separator as template argument is just for fun, not really necessary)

#include <iostream>
#include <locale>

template <class charT, charT sep>
class punct_facet: public std::numpunct<charT> {
protected:
    charT do_decimal_point() const { return sep; }
};

int main(int argc, char **argv) {
    std::cout.imbue(std::locale(std::cout.getloc(), new punct_facet<char, ','>));
    std::cout << "My age is " << 3.1415 << " lightyears.\n";
}

Note that using cout.getloc() I’m overriding just a single facet in the currently set locale, that is, in the current locale settings of cout, I’m changing only how the punctuation is done.

do_decimal_point is a virtual function of std::numpunct that you can redefine to provide your custom separator. This virtual function will be used by numpunct::decimal_point when printing your number.

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