templates - 模板函数重载推导错误

Question

我很难把这件事做好。我想创建重载模板函数 hell ，这将使此类调用成为可能且正确(GMock):

ASSERT_EQ(min(1, 2), 1);
ASSERT_EQ(min(std::less<>(),3,2), 2);

auto abs_comp = [](auto el1, auto el2){
  return std::abs(el1) < std::abs(el2);
};
ASSERT_EQ(min(abs_comp, -1, -5), -1);
ASSERT_EQ(min(4, 3, 2, 1), 1);

除了这个断言之外，一切都很好:

ASSERT_EQ(min(std::less<>(), 3,2,1), 2);

当我提取函数本身以获得有意义的错误时:

min(std::less<>(), 3,2,1)

我明白了:

In file included from /home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/container_minimum_test.cpp:4:
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h: In instantiation of ‘First cppchallenge::lang::min(First, Args ...) [with First = int; Args = {}; <template-parameter-1-3> = std::enable_if<true, void>]’:
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:17:30:   required from ‘First cppchallenge::lang::min(First, Args ...) [with First = std::less<void>; Args = {int}; <template-parameter-1-3> = std::enable_if<false, void>]’
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:17:19:   required from ‘First cppchallenge::lang::min(First, Args ...) [with First = std::less<void>; Args = {int, int, int}; <template-parameter-1-3> = std::enable_if<false, void>]’
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/container_minimum_test.cpp:40:33:   required from here
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:17:30: error: no matching function for call to ‘min()’
         return min(first, min(args...));
                           ~~~^~~~~~~~~
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:8:7: note: candidate: ‘template<class T> T cppchallenge::lang::min(T, T)’
     T min(T first, T second) {
       ^~~
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:8:7: note:   template argument deduction/substitution failed:
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:17:30: note:   candidate expects 2 arguments, 0 provided
         return min(first, min(args...));
                           ~~~^~~~~~~~~
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:16:11: note: candidate: ‘template<class First, class ... Args, class> First cppchallenge::lang::min(First, Args ...)’
     First min(First first, Args... args) {
           ^~~
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:16:11: note:   template argument deduction/substitution failed:
/home/rumcajs/CLionProjects/ModernCppChallenge/tst/lang/../../src/lang/container_minimum.h:17:30: note:   candidate expects at least 1 argument, 0 provided
         return min(first, min(args...));
                           ~~~^~~~~~~~~
gmake[3]: *** [CMakeFiles/ModernCppChallengeLang.dir/build.make:102: CMakeFiles/ModernCppChallengeLang.dir/tst/lang/container_minimum_test.cpp.o] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:116: CMakeFiles/ModernCppChallengeLang.dir/all] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:128: CMakeFiles/ModernCppChallengeLang.dir/rule] Error 2
gmake: *** [Makefile:177: ModernCppChallengeLang] Error 2

模板函数如下:

namespace cppchallenge::lang {
    //#1
    template<typename T>
    T min(T first, T second) {
        return first < second ? first : second;
    }

    template<typename First, typename... Args>
    using are_same = std::conjunction<std::is_same<First, Args>...>;

    //#2
    template<typename First, typename... Args, typename = std::enable_if<are_same<First, Args...>::value, void>>
    First min(First first, Args... args) {
        return min(first, min(args...));
    }

    //#3
    template<typename Comparator, typename T>
    T min(Comparator comp, T first, T second) {
        return comp(first, second) ? first : second;
    }

    //#4
    template<typename Comparator, typename First, typename... Args,
    typename = std::enable_if<are_same<First, Args...>::value, void>,
    typename std::enable_if<std::is_convertible<Comparator, std::function<bool(First,First)>>::value>::type>
    First min(Comparator comp, First first, Args... args) {
        return min(comp, first, min(comp, args...));
    }
}

错误指向函数#2，尽管它应该使用#4。

Answer 1

我认为错误出现在以下模板函数中:您必须在最后一个 ::type 之后添加一些内容，如 * = nullptr

template<typename Comparator, typename First, typename... Args,
typename = std::enable_if<are_same<First, Args...>::value, void>,
typename std::enable_if<std::is_convertible<Comparator, std::function<bool(First,First)>>::value>::type * = nullptr> // add * = nullptr
First min(Comparator comp, First first, Args... args) {
    return comp(comp, first, min(comp, args...));
}

或者在最后一个typename std::enable_if之前添加一个typename =。

否则，如果一切顺利(如果Firts和所有Args...相等并且Comparable可以转换为所需的st::function，模板签名变为

template <typename Comparator, typename First, typename ... Args,
          typename = std::enable_if<are_same<First, Args...>::value,
          void>

最后一个void单独来说是没有意义的(并且前面的std::enable_if没有多大用处；但这是另一个问题；参见以下“奖励建议”)

你应该进行类似的转换

template <typename Comparator, typename First, typename ... Args,
          typename = std::enable_if<are_same<First, Args...>::value, void>
          void * = nullptr>
//............^^^^^^^^^^^^

或者也

template <typename Comparator, typename First, typename ... Args,
          typename = std::enable_if<are_same<First, Args...>::value, void>,
          typename = void>
//........^^^^^^^^^^^

额外建议:前面的 SFINAE 测试应该是

typename = std::enable_if_t<are_same<First, Args...>::value, void>
// ......................^^

或者也(最后一个std::enable_if_t参数默认为void)

typename = std::enable_if_t<are_same<First, Args...>::value>

否则测试永远不会工作并且该功能将永远启用(从 First 和 Args... 类型的角度来看)。

SFINAE 测试中的类似问题

First min(First first, Args... args)