How do I change the formatting of standard types in C++ std::format?

Question

How can I customize the behaviour of std::format so I can get e.g. thousand separators for integer and float types?

Answer 1

The standard format used for standard types, like integers and floating-point types, has an L option to enable locale-specific formatting:

L (locale-specific formatting)

The L option causes the locale-specific form to be used. This option is only valid for arithmetic types.

• For integral types, the locale-specific form inserts the appropriate digit group separator characters according to the context's locale.
• For floating-point types, the locale-specific form inserts the appropriate digit group and radix separator characters according to the context's locale.
• For the textual representation of bool, the locale-specific form uses the appropriate string as if obtained with std::numpunct::truename or std::numpunct::falsename.

For example:

int intValue = 1000000;
double dblValue = 1000000.0;
std::format("{:L} {:L}", intValue, dblValue);

If the user's locale does not define a thousands separator, you can provide your own std::locale that does, eg:

#include <locale>

class MyNumPunct : public std::numpunct<char>
{
protected:
    char do_thousands_sep() const override { return ','; }
    std::string do_grouping() const override { return "\03"; }
};

...

std::format(
    std::locale( std::locale(""), new MyNumPunct ),
    "{:L} {:L}", intValue, dblValue
);

Demo