从numpunct与numpunct_byname派生自定义facet的差异及选型咨询

Great question! Let's break down the key differences between these two approaches and help you pick the right one for your use case:

1. Default Behavior & Base Initialization #

• Deriving from std::numpunct<>:
  This is the base template class, and its default virtual method implementations all use the C locale (e.g., decimal point is ., boolean names are "true"/"false"). Any method you don't explicitly override will stick to these C locale defaults.

• Deriving from std::numpunct_byname<>:
  This class is a specialization of numpunct that initializes its behavior using a named locale (like "de_DE" for German). When you derive from it, all virtual methods you don't override will automatically use the rules of that target locale. For example, if you use the German locale, the decimal point would be , (instead of the C locale's .) without you having to write any extra code.

In your current example, since you only override do_truename() and do_falsename(), both approaches work the same. But if you needed to match other German numeric formatting rules (like comma as decimal separator), numpunct_byname would handle that out of the box, while numpunct<> would require you to override additional methods manually.

2. System Locale Dependency #

• std::numpunct_byname<>:
  It relies on the target locale being installed on the system where your program runs. If the system doesn't have the locale you specified (e.g., "de_DE" isn't available on a minimal embedded system), constructing this facet may throw an exception or fall back to the C locale behavior (depending on your compiler/standard library implementation).

• std::numpunct<>:
  It has no dependency on external system locales. All behavior is either the C locale default or what you explicitly override, making your code more portable across systems with limited or missing locale support.

3. How to Choose? #

Pick std::numpunct_byname<> if:

• You want to reuse most of an existing locale's numeric formatting rules, and only need to customize a small subset (like boolean names in your case).
• You're targeting systems where you can guarantee the required locale is installed.

Pick std::numpunct<> if:

• You need full control over all numeric formatting behavior, or don't want to rely on system-provided locales.
• You want consistent behavior across all systems, regardless of their installed locale set.
• You only need to override a couple of methods and are fine with the C locale defaults for everything else.

内容的提问来源于stack exchange，提问作者SSteven