c++ - 示例void_t示例无法使用intel编译器19.0.4进行编译

Question

我正在尝试使用intel编译器编译void_t的cppreference示例代码:

#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';
}

我有以下错误:

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)

我使用以下行进行编译:

icpc -std=c++17 main.cpp

而我的icpc版本是:

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

这是否意味着icpc的2019版本不会编译void_t？这似乎很奇怪，因为它可以在Godbolt上运行: https://godbolt.org/z/xoT8vr

Answer 1

该问题与编译器无关，而与与其一起使用的C++标准库的实现有关。例如，在Linux系统上，英特尔icpc通常使用系统安装的libstdc++。注意support for std::void_t was added into libstdc++ in version 6.1。
在 icpc on Godbold uses libstdc++ version 8上，您的系统可能安装了一些较旧的版本，而没有对std::void_t的支持。