自定义 format

标准库 <format> 和 fmt 的定制 format 很像：

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// std::format
template <>
struct std::formatter<MyType> {
	// `parse(auto&& ctx)` to involve other formatters
    constexpr auto parse(std::format_parse_context& ctx) {
        return /* */;
    }

    auto format(const MyType& obj, std::format_context& ctx) const {
        return std::format_to(ctx.out(), /* */);
    }
};

Expression	Return type	Requirement
`g.parse(pc)`	`PC::iterator`	Parses format-spec ([format.string]) for type T in the range [pc.begin(), pc.end()) until the first unmatched character. Throws `format_error` unless the whole range is parsed or the unmatched character is }. Note: This allows formatters to emit meaningful error messages. Stores the parsed format specifiers in `*this` and returns an iterator past the end of the parsed range.
`f.format(t, fc)`	`FC::iterator`	Formats `t` according to the specifiers stored in `*this`, writes the output to `fc.out()` and returns an iterator past the end of the output range. The output shall only depend on `t`, `fc.locale()`, and the range `[pc.begin(), pc.end())` from the last call to `f.parse(pc)`.

Where:

f is a value of type (possibly const) F, (in other words: the format function should be a const member function)
g is an lvalue of type F.

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// fmt::format
template <>
struct fmt::formatter<point> { 
	constexpr auto parse(format_parse_context& ctx); 
	
	template <typename FormatContext>
	auto format(const point& p, FormatContext& ctx) const; 
};

吐槽

妈的真的又比不过 rust 了。人家 rust 直接 #[Derive(Debug)] 就 ok 了😡😡

虽然你这自由度是高，但是基本的 shortcut 也得有吧。。

但是 C++26 static reflection 出来应该就会好了吧！！（确信