Make allocator and string_base compat easier to read and remove unneccessary concepts subdirectory

include/rexy/compat/cpp20/allocator.hpp

@@ -0,0 +1,139 @@
+/**
+	This file is a part of rexy's general purpose library
+	Copyright (C) 2022 rexy712
+
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef REXY_COMPAT_CPP20_DEFAULT_ALLOCATOR_HPP
+#define REXY_COMPAT_CPP20_DEFAULT_ALLOCATOR_HPP
+
+#include <cstddef> //ptrdiff_t, size_t
+#include <type_traits> //true_type, false_type
+#include <new>
+#include <limits> //numeric_limits
+#include <memory> //allocator
+
+#ifdef __cpp_concepts
+
+	#include <concepts> //convertible_to
+	template<class T>
+	concept Allocator = requires(T a, typename T::size_type sz, typename T::pointer pa){
+		typename T::value_type;
+		typename T::size_type;
+		typename T::pointer;
+		typename T::const_pointer;
+		{a.allocate(sz)} -> std::convertible_to<typename T::pointer>;
+		{a.deallocate(pa, sz)};
+	};
+	#define REXY_ALLOCATOR_CONCEPT Allocator
+
+#else //__cpp_concepts
+
+	#define REXY_ALLOCATOR_CONCEPT class
+
+#endif // __cpp_concepts
+
+#include "../../rexy.hpp"
+#include "../if_consteval.hpp"
+
+namespace rexy{
+
+	template<class T>
+	struct allocator
+	{
+		using pointer = T*;
+		using const_pointer = const T*;
+		using void_pointer = void*;
+		using const_void_pointer = const void*;
+		using value_type = T;
+		using size_type = std::size_t;
+		using difference_type = std::ptrdiff_t;
+		using is_always_equal = std::true_type;
+		using propagate_on_container_copy_assignment = std::false_type;
+		using propagate_on_container_move_assignment = std::false_type;
+		using propagate_on_container_swap = std::false_type;
+
+		template<class U>
+		struct rebind{
+			using other = allocator<U>;
+		};
+
+	private:
+		constexpr bool has_overflow(size_type n)const{
+			return n > max_size();
+		}
+
+	public:
+		constexpr allocator(void) = default;
+		constexpr allocator(const allocator&) = default;
+		constexpr allocator(allocator&&) = default;
+		template<class U>
+		constexpr allocator(const allocator<U>&)noexcept{}
+		constexpr ~allocator(void) = default;
+
+		//'::operator new' is never a constexpr call as of C++23. However, 'std::allocator::allocate' *is* transiently constexpr as of C++20,
+		//even though it directly calls '::operator new' as is stated in the standard. Therefore, when evaluating this call with support for
+		//'if consteval', we can use the 'std::allocator::allocate' constexpr-ness when this is in a constant evaluation context.
+		constexpr pointer allocate(size_type n){
+			size_type bytes = has_overflow(n) ? std::numeric_limits<size_type>::max() : n*sizeof(T);
+
+			REXY_if_consteval{
+					std::allocator<value_type> a;
+					const size_type num_items = n / sizeof(value_type);
+					return a.allocate(num_items);
+				}else{
+					if constexpr(alignof(T) <= __STDCPP_DEFAULT_NEW_ALIGNMENT__){
+						return reinterpret_cast<pointer>(::operator new(bytes));
+					}else{
+						return reinterpret_cast<pointer>(::operator new(bytes, static_cast<std::align_val_t>(alignof(T))));
+					}
+				} //if consteval
+		}
+
+
+		constexpr void deallocate(pointer p, size_type n){
+			REXY_if_consteval{
+					std::allocator<value_type> a;
+					const size_type num_items = n / sizeof(value_type);
+					return a.deallocate(p, num_items);
+				}else{
+
+				#if !defined(__cpp_sized_deallocation)
+					//clang does not enable ::operator delete(void* ptr, std::size_t sz) by default for some reason
+					if constexpr(alignof(T) <= __STDCPP_DEFAULT_NEW_ALIGNMENT__){
+						::operator delete(p);
+					}else{
+						::operator delete(p, static_cast<std::align_val_t>(alignof(T)));
+					}
+				#else //__cpp_sized_deallocation
+					size_type bytes = has_overflow(n) ? std::numeric_limits<size_type>::max() : n*sizeof(T);
+					if constexpr(alignof(T) <= __STDCPP_DEFAULT_NEW_ALIGNMENT__){
+						::operator delete(p, bytes);
+					}else{
+						::operator delete(p, bytes, static_cast<std::align_val_t>(alignof(T)));
+					}
+				#endif //__cpp_sized_deallocation
+
+			} //if consteval
+
+		}
+		constexpr size_type max_size(void)const{
+			return std::numeric_limits<size_type>::max()/sizeof(T);
+		}
+	};
+
+}
+
+#endif