C++: What is std::format with user-defined types?

1#include <format>
2
3struct Color {
4    uint8_t r, g, b;
5};
6
7template<>
8struct std::formatter<Color> {
9    char format_type = 'x';  // Default: hex
10
11    constexpr auto parse(std::format_parse_context& ctx) {
12        auto it = ctx.begin();
13        if (it != ctx.end() && *it != '}') {
14            format_type = *it++;
15        }
16        return it;
17    }
18
19    auto format(const Color& c, auto& ctx) const {
20        if (format_type == 'r') {
21            return std::format_to(ctx.out(), "({},{},{})", c.r, c.g, c.b);
22        }
23        return std::format_to(ctx.out(), "#{:02x}{:02x}{:02x}", c.r, c.g, c.b);
24    }
25};
26
27Color red{255, 0, 0};
28std::cout << std::format("{}", red);    // #ff0000
29std::cout << std::format("{:r}", red);  // (255,0,0)
30
31// Compile-time format string checking
32std::cout << std::format("{} + {} = {}", 1, 2, 3);  // OK
33// std::cout << std::format("{:z}", red);  // Error: invalid format