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//#pragma once
#include <array>
#include <initializer_list>
#include <algorithm>
template<class T,std::size_t N>
class ConstexprVector {
	std::array<T, N> Buff{};
	std::size_t C = 0;
public:
 
	typedef T ValueType;
	typedef typename std::array<T, N>::iterator Iterator;
	typedef typename std::array<T, N>::const_iterator ConstIterator;
	constexpr ConstexprVector(){};
	explicit ConstexprVector(const std::initializer_list<T>& L){
		std::size_t V = 0;
		auto it1 = L.begin();
		auto it2 = Buff.begin();
		for (it1 ,it2 ; it1 != L.end() && it2 != Buff.end(); it1++, it2++) {
			*it2 = *it1;
			V++;
		}
		C = V;
	}
	constexpr std::size_t Size() const{
		return C;
	}
	constexpr std::size_t MaxSize() const {
		return N;
	}
	constexpr bool PushBack(const T& In) {
		if (C >= Buff.size()) return false;
		Buff[C] = In;
		C++;
		return true;
	}
	/** /
	template<class... A>
	constexpr bool EmplaceBack(const A& ...In) {
		if (C >= Buff.size()) return false;
		Buff[C] = T(In...);
		C++;
		return true;
	}
	/**/
	constexpr bool PopBack() {
		if (C == 0)return false;
		//Buff[C]~T();
		C--;
		return true;
	}
 
	constexpr bool Clear() {
		while (PopBack());
		return true;
	}
 
	constexpr Iterator begin() {
		return Buff.begin();
	}
 
	constexpr ConstIterator begin() const {
		return Buff.begin();
	}
	constexpr Iterator end() {
		return Buff.begin()+C;
	}
	constexpr ConstIterator end() const {
		return Buff.begin()+C;
	}
};
 
#include <iostream>
#include <cstdint>
 
//#include "ConstexprVector.h"
 
typedef ConstexprVector<std::uint64_t, 128> DType;
 
constexpr std::uint64_t Calc(const DType& D) {
	std::uint64_t C = 0;
	for (const DType::ValueType& o : D) {
		C += o;
	}
 
	return C;
}
 
int main() {
	//constexpr std::initializer_list<DType::ValueType> L{ 1,2,3,4,5,6,7,8,9 };
	constexpr DType D{ };
	constexpr std::uint64_t R = Calc(D);
 
	std::cout << R << std::endl;
 
	return true;
}

prog.cpp: In function ‘int main()’:
prog.cpp:92:34:   in constexpr expansion of ‘Calc(D)’
prog.cpp:82:35: error: ‘constexpr ConstexprVector<T, N>::ConstIterator ConstexprVector<T, N>::begin() const [with T = long unsigned int; long unsigned int N = 128ul; ConstexprVector<T, N>::ConstIterator = const long unsigned int*]’ called in a constant expression
  for (const DType::ValueType& o : D) {
                                   ^
prog.cpp:62:26: note: ‘constexpr ConstexprVector<T, N>::ConstIterator ConstexprVector<T, N>::begin() const [with T = long unsigned int; long unsigned int N = 128ul; ConstexprVector<T, N>::ConstIterator = const long unsigned int*]’ is not usable as a constexpr function because:
  constexpr ConstIterator begin() const {
                          ^~~~~
prog.cpp:63:21: error: call to non-constexpr function ‘const value_type* std::array<_Tp, _Nm>::begin() const [with _Tp = long unsigned int; long unsigned int _Nm = 128ul; std::array<_Tp, _Nm>::const_iterator = const long unsigned int*; std::array<_Tp, _Nm>::value_type = long unsigned int]’
   return Buff.begin();
                     ^

Standard output is empty