如何为自定义Decimal类实现fmt库formatter特化并复用现有机制

Great question! You don't have to reinvent the wheel here—fmt's design actually makes it easy to reuse existing formatter logic for your custom type, especially if you want to align with floating-point formatting behavior first. Here's how to do it properly:

1. Reuse the Floating-Point Formatter via Inheritance #

The simplest way to get all the common formatting infrastructure (alignment, width, fill characters, plus floating-point-specific specifiers like precision, fixed/scientific) is to have your formatter<Decimal<S,F>> inherit from fmt::formatter<double>. This lets you reuse the existing parse logic entirely, so you don't have to hand-write code to parse format specs.

Here's the modified specialization:

template <int S, int F> 
struct fmt::formatter<Decimal<S,F>> : fmt::formatter<double> { 
  // We don't need to reimplement parse()—the parent class handles all floating-point
  // and generic formatting specifiers automatically!

  template <typename FormatContext> 
  auto format(const Decimal<S,F>& x, FormatContext& ctx) { 
    // Convert your Decimal to a double (adjust this if your type uses an explicit conversion method)
    double converted_val = static_cast<double>(x);
    // Delegate to the parent formatter to handle formatting with the parsed specifiers
    return fmt::formatter<double>::format(converted_val, ctx); 
  } 
};

2. Preserve High Precision (If You Can't Convert to Double) #

If converting to double would lose precision (which is likely for a high-precision Decimal type), you can keep your to_chars logic but still reuse the generic formatting rules for alignment/width. Inherit from fmt::formatter<std::string_view> instead, and let it handle the alignment/width for your generated string:

template <int S, int F> 
struct fmt::formatter<Decimal<S,F>> : fmt::formatter<std::string_view> { 
  // Reuse the parent's parse() for generic specifiers like width and alignment
  template <typename FormatContext> 
  auto format(const Decimal<S,F>& x, FormatContext& ctx) { 
    constexpr size_t bufsize = 64; 
    char buf[bufsize]; 
    const auto end = x.to_chars(buf, buf+bufsize, false); 
    std::string_view sv{buf, static_cast<size_t>(end - buf)};
    // Let the parent formatter handle alignment/width on your pre-generated string
    return fmt::formatter<std::string_view>::format(sv, ctx); 
  } 
};

3. Add Decimal-Specific Features Later #

When you're ready to add custom formatting behavior for Decimal, you can override the parse method to handle your own specifiers while still reusing the parent's logic for everything else:

template <int S, int F> 
struct fmt::formatter<Decimal<S,F>> : fmt::formatter<double> { 
  // A flag for our custom feature, e.g., "R" for custom rounding
  bool use_custom_rounding = false;

  constexpr auto parse(format_parse_context& ctx) { 
    // First let the parent parse all standard floating-point/generic specifiers
    auto it = fmt::formatter<double>::parse(ctx);
    
    // Now handle our custom specifier(s)
    if (it != ctx.end() && *it == 'R') {
      use_custom_rounding = true;
      ++it;
    }
    
    return it;
  }

  template <typename FormatContext> 
  auto format(const Decimal<S,F>& x, FormatContext& ctx) { 
    Decimal<S,F> adjusted_val = x;
    if (use_custom_rounding) {
      // Apply your custom rounding logic here
      adjusted_val = custom_round(adjusted_val);
    }
    
    // Delegate to parent formatter with the adjusted value
    return fmt::formatter<double>::format(static_cast<double>(adjusted_val), ctx); 
  } 
};

Why This Works #

By inheriting from an existing formatter, you get:

• Free generic formatting: Alignment (<, >, ^), width, fill characters all work out of the box.
• Consistent floating-point behavior: All the standard specifiers like .2f, e, g behave exactly like they do for double, so switching from double to Decimal won't break existing format strings.
• Extensibility: You can add custom features later without rewriting all the basic parsing logic.

内容的提问来源于stack exchange，提问作者JDługosz