Add a ConstexprString implementation from my notes.

ConstexprString.h

@@ -0,0 +1,145 @@
+static_assert( __cplusplus > 2020'00 );
+
+#pragma once
+
+#include <cstddef>
+
+#include <array>
+#include <type_traits>
+#include <algorithm>
+#include <iostream>
+
+#include <Alepha/Alepha.h>
+
+namespace Alepha::Hydrogen  ::detail::  constexpr_string
+{
+	namespace C
+	{
+		// While a larger size for the maximum might be desirable, this consumes
+		// some constant amount of space in the compiler's symbol table.  To be
+		// mindful of that, we want to not get TOO large.
+		//
+		// As compared to the GCC extension which uses a variadic length NTTP
+		// list, this may use less overall memory, in some ways.  However, the
+		// benefit of the NTTP variadic list way, for compiler memory usage, is that
+		// the a type using the string as a parameter has a symbol size which
+		// is proportional to the number of chars in the string.
+		const std::size_t maxSize= 128;
+	}
+
+	inline namespace exports
+	{
+		struct ConstexprString;
+
+		inline namespace literals
+		{
+			consteval ConstexprString operator ""_cs( const char *s, std::size_t len );
+		}
+	}
+
+	struct exports::ConstexprString
+	{
+		private:
+			class BadConstantStringAllocationError
+				: public std::bad_alloc
+			{
+				public:
+					const char *
+					what() const noexcept final
+					{
+						return "Failure  to allocate enough space for a a compile-time string.";
+					}
+			};
+
+		// Pretend that these are private, despite the fat that they are public.
+		// They have to be public, to work correctly with `template` parameters.
+		public:
+			std::array< char, C::maxSize > storage= {};// Always null terminated
+			std::size_t length= 0;
+
+			friend consteval ConstexprString literals::operator ""_cs( const char *, std::size_t );
+
+		public:
+			constexpr ConstexprString()= default;
+
+			constexpr
+			CosntexprString( const char *const s, std::size_t len )
+			{
+				if( len >= C::maxSize ) throw BadConstantStringAllocationError{};
+
+				std::copy_n( s, len, storage.begin() );
+				length= len;
+			}
+
+			template< std::size_t N >
+			constexpr
+			ConstexprString( const char (&s)[ N ] ) : ConstexprString( s, N ) {}
+
+
+			constexpr bool empty() const noexcept { return length == 0; }
+			constexpr std::size_t size() const noexcept { return length; }
+
+			constexpr auto begin() const noexcept { return storage.begin(); }
+			constexpr auto end() const noexcept { return storage.begin() + length; }
+
+			constexpr const char *c_str() const noexcept { return &storage[ 0 ]; }
+			constexpr const char *data() const noexcept { return &storage[ 0 ]; }
+			constexpr char *data() noexcept { return &storage[ 0 ]; }
+
+			friend constexpr ConstexprString
+			operator + ( const ConstexprString &lhs, const ConstexprString &rhs )
+			{
+				ConstexprString rv;
+
+				rv.length= lhs.size() + rhs.size();
+				if( rv.length >= C::maxSize ) throw BadConstantStringAllocationError{};
+
+				using std::begin, std::end;
+				const auto next= std::copy( begin( lhs ), end( lhs ), begin( rv.storage ) );
+				std::copy( begin( rhs ), end( rhs ), next );
+
+				return rv;
+			}
+
+			// The C++20 rules that build more operators upon these operators are fine:
+
+			friend constexpr bool
+			operator < ( const ConstexprString &lhs, const ConstexprString &rhs ) noexcept
+			{
+				using std::begin, std::end;
+				return std::lexicographical_compare( begin( lhs ), end( lhs ), begin( rhs ), end( rhs ) );
+			}
+
+			friend constexpr bool
+			operator == ( const ConstexprString &lhs, const ConstexprString &rhs ) noexcept
+			{
+				using std::begin, std::end;
+				return lhs.size() == rhs.size() and std::equal( begin( lhs ), end( lhs ), begin( rhs ) );
+			}
+
+			friend std::ostream &
+			operator << ( std::ostream &os, const ConstexprString &rhs )
+			{
+				return os << rhs.c_str();
+			}
+
+			// Total order is not helpful, here.
+			void operator<=>( ConstexprString )= delete;
+	};
+
+	consteval ConstexprString
+	exports::literals::operator ""_cs( const char *const s, const std::size_t len )
+	{
+		return ConstexprString( s, len );
+	}
+}
+
+namespace Alepha::Cavorite::inline exports::inline constexpr_string
+{
+	using namespace detail::constexpr_string::exports;
+}
+
+namespace Alepha::Cavorite::inline exports::inline literals::inline constexpr_string_literals
+{
+	using namespace detail::constexpr_string::exports::literals;
+}