我需要使用什么编译器选项/库来使用detect_or_t类型特征？ [英] What compiler option/library do I need to use detect_or_t type trait?

问题描述

我试图从< experimental / type_traits> 使用 std :: experimental :: detect_or_t 。

我需要编译以下示例的编译器，选项，版本或库来从 http://en.cppreference.com/w/cpp/experimental/is_detected

#include< experiments / type_traits>
#include< cstddef>

模板< class T>
使用diff_t = typename T :: difference_type;

模板< class Ptr>
using difference_type = std :: experimental :: detected_or_t< std :: ptrdiff_t，diff_t，Ptr>;

struct Meow {using difference_type = int; };
struct Purr {};

int main（）
{
static_assert（std :: is_same< difference_type< Meow> ;, int> :: value，Meow的difference_type应该是int！
static_assert（std :: is_same< difference_type< Purr>，std :: ptrdiff_t> :: value，Purr的difference_type应该是ptrdiff_t！）;



我试过使用 clang ++ -std = c ++ 14 和 g ++ -std = c ++ 14 。还有 -std = c ++ 1y 和 -std = c ++ 17 。我总是得到这个：

main.cpp：8：44：error：'detected_or_t'在命名空间'std :: experimental'中没有命名模板类型

解决方案

这些特征首先添加到GCC 6.1.0中的libstdc ++中，在 GCC 6发行说明中：

• 对第二版图书馆基础TS中大多数功能的实验性支持。

手册中的执行状态表，位于
https://gcc.gnu.org/onlinedocs /gcc-6.1.0/libstdc++/manual/manual/status.html#table.cxx1z_ts_status

我对libc ++不太确定，不支持铿锵3.9.1中的版本，但受当前主干支持，所以我认为它们首次出现在Clang 4.0.0中。

I am trying to use std::experimental::detect_or_t from <experimental/type_traits>.

What compiler, option, version or library do I need to compile the following example from http://en.cppreference.com/w/cpp/experimental/is_detected ?

#include <experimental/type_traits>
#include <cstddef>

template<class T>
using diff_t = typename T::difference_type;

template <class Ptr>
using difference_type = std::experimental::detected_or_t<std::ptrdiff_t, diff_t, Ptr>;

struct Meow { using difference_type = int; };
struct Purr {};

int main()
{
    static_assert(std::is_same<difference_type<Meow>, int>::value, "Meow's difference_type should be int!");
    static_assert(std::is_same<difference_type<Purr>, std::ptrdiff_t>::value, "Purr's difference_type should be ptrdiff_t!");
}

I tried using clang++ -std=c++14 and g++ -std=c++14. Also with -std=c++1y and -std=c++17. I always get this:

main.cpp:8:44: error: 'detected_or_t' in namespace 'std::experimental' does not name a template type

解决方案

Those traits were first added to libstdc++ in GCC 6.1.0, as documented in the GCC 6 release notes:

• Experimental support for most features of the second version of the Library Fundamentals TS.

And the implementation status tables in the manual, at https://gcc.gnu.org/onlinedocs/gcc-6.1.0/libstdc++/manual/manual/status.html#table.cxx1z_ts_status

I'm less sure about libc++, but they're not supported by the version in Clang 3.9.1 but are supported by the current trunk, so I think they first appeared in Clang 4.0.0

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