Constness.

Constness.h

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+static_assert( __cplusplus > 2020'00 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <type_traits>
+#include <utility>
+
+#include "meta.h"
+
+namespace Alepha::Hydrogen  ::detail::  Constness_m
+{
+	inline namespace exports
+	{
+		/*!
+		 * Conditional constness primitive.
+		 *
+		 * C++17 and beyond have many conditional attributes on functions.  Conditional `noexcept`,
+		 * conditional `explicit`, and even conditional instantiation.  The `noexcept` and `explicit`
+		 * are conditional on a boolean condition.  This enum permits making templates which
+		 * are conditionally const, depending upon instantiation.
+		 *
+		 * Simple example:
+		 *
+		 * ```
+		 * template< Constness constness >
+		 * struct IntWrapper
+		 * {
+		 *	private:
+		 *		int variable;
+		 *
+		 *  public:
+		 *		maybe_const_t< int &, constness > // This is sort of like if one could write `const( constness ) int &`
+		 *		view() { return variable; }
+		 *
+		 *		const int &
+		 *		view() const { return variable; }
+		 * };
+		 *
+		 * using MutableIntWrapper= IntWrapper< Mutable >;
+		 * using ConstIntWrapper= IntWrapper< Const >;
+		 * ```
+		 *
+		 * Now `Constness< Mutable >` has a member `view` which returns `int &` and `Constness< Const >` has a member
+		 * `view` which returns `const int &`.  This facility can be useful in implementing pairs of `const`/non-`const`
+		 * iterators, and other gadgets.
+		 */
+		enum Constness : bool
+		{
+			Const= true,
+			Mutable= false,
+		};
+
+
+		/*!
+		 * Apply the `Constness` requested to the specified type.
+		 *
+		 * If the type isn't `const`, but `Constness` is set, then
+		 * it evaluates to `const T`.
+		 */
+		template< typename Type, Constness > struct maybe_const;
+
+		template< typename Type >
+		struct maybe_const< Type, Const >
+		{
+			using type= std::add_const_t< Type >;
+		};
+
+		template< typename Type >
+		struct maybe_const< Type, Mutable >
+		{
+			using type= Type;
+		};
+
+		template< typename Type >
+		struct maybe_const< Type &, Const >
+		{
+			using type= std::add_const_t< Type > &;
+		};
+
+		template< typename Type >
+		struct maybe_const< Type &, Mutable >
+		{
+			using type= Type &;
+		};
+
+		/*!
+		 * Conditionally make `Type` `const`.
+		 */
+		template< typename Type, Constness constness >
+		using maybe_const_t= typename maybe_const< Type, constness >::type;
+
+		/*!
+		 * Conditionally make `Type` into `const Type *`.
+		 */
+		template< typename Type, Constness constness >
+		using maybe_const_ptr_t= std::add_pointer_t< maybe_const_t< Type, constness > >;
+
+		/*!
+		 * Conditionally call `std::as_const`.
+		 *
+		 * Sometimes `std::as_const` is appropriate to call, and sometimes it isn't.
+		 * In some cases, one might want to have type deduction driven by an expression
+		 * where, in a `const` "branch" `std::as_const` is called, but in a non-`const`
+		 * branch, it isn't.  This facilitates that:
+		 *
+		 * ```
+		 * template< Constness constness >
+		 * struct IntWrapper
+		 * {
+		 *  private:
+		 *		int variable;
+		 *
+		 *  public:
+		 *		// `std::as_const` will be called when `constness` is true,
+		 *		// and will be skipped otherwise.  This permits the right
+		 * 		// client overload to be called
+		 * 		template< typename Function >
+		 * 		decltype( auto )
+		 * 		apply( Function func )
+		 * 		{
+		 *			return func( maybe_as_const< constness >( variable ) );
+		 * 		}
+		 * };
+		 * ```
+		 */
+		template< Constness constness, typename T >
+		constexpr auto &
+		maybe_as_const( T &t ) noexcept
+		{
+			if constexpr( constness ) return std::as_const( t );
+			else return t;
+		}
+
+		template< Constness constness, typename T >
+		constexpr decltype( auto )
+		maybe_as_const( const T &t ) noexcept
+		{
+			if constexpr( constness ) return std::as_const( t );
+			else static_assert( dependent_value< false, T > );
+		}
+
+		template< Constness constness, typename T >
+		void maybe_as_const( T && )= delete;
+	}
+}
+
+namespace Alepha::Hydrogen::inline exports::inline Constness_m
+{
+	using namespace detail::Constness_m::exports;
+}