std::formatter

#include <algorithm>
#include <format>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string_view>
 
struct QuotableString : std::string_view
{};
 
template<>
struct std::formatter<QuotableString, char>
{
 bool quoted = false;
 
 template<class ParseContext>
 constexpr ParseContext::iterator parse(ParseContext& ctx)
 {
 auto it = ctx.begin();
 if (it == ctx.end())
 return it;
 
 if (*it == '#')
 {
 quoted = true;
 ++it;
 }
 if (it != ctx.end() && *it != '}')
 throw std::format_error ("Invalid format args for QuotableString.");
 
 return it;
 }
 
 template<class FmtContext>
 FmtContext::iterator format(QuotableString s, FmtContext& ctx) const
 {
 std::ostringstream out;
 if (quoted)
 out << std::quoted (s);
 else
 out << s;
 
 return std::ranges::copy (std::move(out).str(), ctx.out()).out;
 }
};
 
int main()
{
 QuotableString a("be"), a2(R"( " be " )");
 QuotableString b("a question");
 std::cout << std::format ("To {0} or not to {0}, that is {1}.\n", a, b);
 std::cout << std::format ("To {0:} or not to {0:}, that is {1:}.\n", a, b);
 std::cout << std::format ("To {0:#} or not to {0:#}, that is {1:#}.\n", a2, b);
}

To be or not to be, that is a question.
To be or not to be, that is a question.
To " \" be \" " or not to " \" be \" ", that is "a question".