示例 void_t 示例无法使用英特尔编译器 19.0.4 进行编译

Question

I'm trying to compile the cppreference example code of void_t with intel compiler :

#include <iostream>
#include <type_traits>
#include <vector>
#include <map>
 
class A {};
 
template <typename T, typename = void>
struct is_iterable : std::false_type {};
template <typename T>
struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
                                  decltype(std::declval<T>().end())>>
    : std::true_type {};
 
// An iterator trait which value_type is always the value_type of the 
// iterated container, even with back_insert_iterator which value_type is void
 
template <typename T, typename = void>
struct iterator_trait 
: std::iterator_traits<T> {};
template <typename T>
struct iterator_trait<T, std::void_t<typename T::container_type>> 
: std::iterator_traits<typename T::container_type::iterator> {};
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << is_iterable<std::vector<double>>::value << '\n';
    std::cout << is_iterable<std::map<int, double>>::value << '\n';
    std::cout << is_iterable<double>::value << '\n';
    std::cout << is_iterable<A>::value << '\n';
 
 
    std::vector<int> v;
 
    std::cout << std::is_same<iterator_trait<decltype(std::back_inserter(v))>::value_type
    , iterator_trait<decltype(v.cbegin())>::value_type >::value << '\n';
}

I have the following error :

main.cpp(11): error: namespace "std" has no member "void_t"
  struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
                             ^

main.cpp(11): error: expected a ">"
  struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
                                   ^

main.cpp(12): error: expected a ";"
                                    decltype(std::declval<T>().end())>>
                                                                     ^

main.cpp(22): error: namespace "std" has no member "void_t"
  struct iterator_trait<T, std::void_t<typename T::container_type>>
                                ^

main.cpp(22): error: expected a ">"
  struct iterator_trait<T, std::void_t<typename T::container_type>>
                                      ^

main.cpp(22): error: expected a ";"
  struct iterator_trait<T, std::void_t<typename T::container_type>>
                                                                 ^

compilation aborted for main.cpp (code 2)

I use the following line for compilation :

icpc -std=c++17 main.cpp

and my icpc version is :

icpc --version
icpc (ICC) 19.0.4.243 20190416
Copyright (C) 1985-2019 Intel Corporation.  All rights reserved.

Is that mean that the 2019 version of icpc does not compile void_t ? That seems strange because it works on godbolt : https://godbolt.org/z/xoT8vr

Answer 1

The problem is not related to a compiler but to the implementation of the C++ Standard Library it works with. For instance, on Linux systems, Intel icpc typically uses system-installed libstdc++. Note that the support for std::void_t was added into libstdc++ in version 6.1 .

While icpc on Godbold uses libstdc++ version 8 , your system likely have some older version installed without the support for std::void_t .