Difference between a standard facet and it's _byname version

I have tried deriving a custom numpunct<> facet from both numpunct<> as well as from numpunct_byname<>. Both give the same results and both approaches work.

The example I tried was to override the do_truename() and do_falsename() virtual methods so as to return their German equivalents "wahr" and "falsch".

deriving from numpunct_byname<> :

deriving from numpunct<> :

Both approaches work correctly.

Is there any difference?

How will I know which approach to adopt?

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