The C++ detection idiom is not working as expected in Visual Studio 2015 Update 2 CTP

问题

I'm playing with the proposal of standard library support for the C++ detection idiom and compiled the following code with the Microsoft C/C++ Optimizing Compiler Version 19.00.23725 for x64:

#include <iostream>


template<class...>
using void_t = void;

template<class, template<class> class, class = void_t<>>
struct detect : std::false_type { };

template<class T, template<class> class Operation>
struct detect<T, Operation, void_t<Operation<T>>> : std::true_type { };


template<class T>
using bar_t = decltype(std::declval<T>().bar());

template<class T>
using bar_int_t = decltype(std::declval<T>().bar(0));

template<class T>
using bar_string_t = decltype(std::declval<T>().bar(""));


struct foo
{
    int bar() { return 0; }
    int bar(int) { return 0; }
};


int main()
{
    std::cout << detect<foo, bar_t>{} << std::endl;
    std::cout << detect<foo, bar_int_t>{} << std::endl;
    std::cout << detect<foo, bar_string_t>{} << std::endl;

    return 0;
}

The expected output would be

1
1
0

but it is

1
1
1

What is going wrong? I've made a live demo (with exactly the same code) where the output is as expected.

回答1:

Visual C++ has a limited support for expression SFINAE, however, I think it should work with a function return type. So, instead you can try the below implementation:

#include <type_traits>

template <typename...>
using void_t = void;

template <typename, template <typename> class>
auto detect_impl(char) -> std::false_type;

template <typename T, template <typename> class Operation>
auto detect_impl(int) -> decltype(void_t<Operation<T>>(), std::true_type{});

template <typename T, template <typename> class Operation>
using detect = decltype(detect_impl<T, Operation>(0));

(Tested on http://webcompiler.cloudapp.net/)

来源：https://stackoverflow.com/questions/35669239/the-c-detection-idiom-is-not-working-as-expected-in-visual-studio-2015-update

标签

c++

templates

c++14

c++17