How to print a double with a comma

Question

In C++ I've got a float/double variable.

When I print this with for example cout the resulting string is period-delimited.

cout << 3.1415 << endl
$> 3.1415

Is there an easy way to force the double to be printed with a comma?

cout << 3.1415 << endl
$> 3,1415

Answer 1

imbue() cout with a locale whose numpunct facet's decimal_point() member function returns a comma.

Obtaining such a locale can be done in several ways. You could use a named locale available on your system (std::locale("fr"), perhaps). Alternatively, you could derive your own numpuct, implement the do_decimal_point() member in it.

Example of the second approach:

template<typename CharT>
class DecimalSeparator : public std::numpunct<CharT>
{
public:
    DecimalSeparator(CharT Separator)
    : m_Separator(Separator)
    {}

protected:
    CharT do_decimal_point()const
    {
        return m_Separator;
    }

private:
    CharT m_Separator;
};

Used as:

std::cout.imbue(std::locale(std::cout.getloc(), new DecimalSeparator<char>(',')));

Answer 2

This is controlled by your program's locale.

How you set a program's default locale depends on the platform. On POSIX type platforms, it's with the LANG and LC_* environment variables, for instance.

You can force a particular locale -- different from the default -- within a C++ program by calling ios::imbue. Something like this might work:

#include <locale>
cout.imbue(std::locale("German_germany"));

The idea is to force a locale where comma is the decimal separator. You might need to adjust the "German_germany" string to get the behavior you want on your particular platform.