使用constexpr-if时出错:在'constexpr'之前应为'(' [英] Errors when using constexpr-if: expected '(' before 'constexpr'

问题描述

我正在尝试使用if-constexpr进行检查，但是遇到诸如此类的错误

I am trying to use if-constexpr to check something, but I encounter errors like

期望'('在'constexpr'之前

expected '(' before 'constexpr'

'else'，而前面没有'if'

'else' without a previous 'if' "

到目前为止，我检查我的代码是否有问题

So far i check there is nothing wrong with my codes

我的编译标志是g ++ -std = c ++ 17 main.cpp

My compiling flag is g++ -std=c++17 main.cpp

#include <iostream>
template<typename T, typename Comp = std::less<T> >
struct Facility
{
template<T ... list>
struct List
{
    static void print()
    {
        std::cout<<"\""<<"Empty List"<<"\""<<"\n";
    }
};
template<T head,T ... list>
struct List<head,list...>
{
    static void print()
    {
        std::cout<<"\"" << head;
        ((std::cout << " " << list), ...);
        std::cout<<"\""<<"\n";
    }
};
template<unsigned N,typename AA>
struct RemoveFirst{};

template<unsigned N,T head,T ... Rest>
struct RemoveFirst<N,List<head,Rest...>>
{
    struct result
    {
        static void print()
        {   
            if constexpr (N == head)
            {
                std::cout<<"";
            }
            else
            {
                std::cout<<"\""<<head;
                ((std::cout << " " << Rest), ...);
                std::cout<<"\""<<"\n";
            }
        }
    };
 };
};
template<int ... intlist>
using IntList = typename Facility<int>::List<intlist...>;

int main()
{
 using IntFacility = Facility<int>;
 using List = IntList<2, 8, 2, 3, 5, 10, 8, 5>;
}

推荐答案

不支持C ++ 17最终版本的较旧版本的GCC(最高6.x)将给出该错误，因为它们可以识别作为关键字，但不了解constexpr-if构造.确保您的GCC是版本7或更高版本.

Older versions of GCC (up to 6.x) which do not support the final version of C++17 will give that error, because they recognize constexpr as a keyword but do not understand the constexpr-if construct. Make sure your GCC is version 7 or later.

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