如何将 void 模板类型用于编译时条件代码和 static_assert？

Question

我有一个遍历函数，它为每个元素调用回调，并希望在编译时使用非默认参数实现它的逻辑扩展，如下所示：

#include <type_traits>
#include <iostream>
using namespace std;

template<
    typename Odds = void,
    typename Evens = void,
    typename Skip = void
>
void iterate(
    const auto & on_element,
    const auto & skip 
) {
    for ( int i = 0; i < 6; ++ i ) {
        if constexpr ( ! is_same_v< Skip, void > ) {
            if ( skip.find( i ) != skip.end() )
                continue;
        }

        if ( i % 2 == 1 ) {
            if constexpr ( is_same_v< Odds, void > )
                on_element( i );
        }
        else {
            if constexpr ( is_same_v< Evens, void > )
                on_element( i );
        }

        static_assert(
            is_same_v< Odds, void > || is_same_v< Evens, void >,
            "Either odds or evens are required to get traversed"
        );
    }
}

int main() {
    const auto & on_element = []( const int & i ) {
        cout << "    " << i << endl;
    };

    cout << "Whole range:" << endl;
    iterate( on_element );

    cout << "Should skip specified elements:" << endl;
    iterate( on_element, { 1, 2, 3 } );

    cout << "Should traverse only evens:" << endl;
    iterate< bool >( on_element );

    cout << "Should traverse only odds:" << endl;
    iterate< void, bool >( on_element );

    //should NOT compile if uncomment:
    //iterate< bool, bool >( on_element );
}

但它不能用g++ -std=gnu++2a -fconcepts main.cpp -o main编译：

main.cpp: In function ‘int main()’:
main.cpp:42:25: error: no matching function for call to ‘iterate(const main()::<lambda(const int&)>&)’
   42 |     iterate( on_element );
      |                         ^
main.cpp:10:6: note: candidate: ‘template<class Odds, class Evens, class Skip, class auto:11, class auto:12> void iterate(const auto:11&, const auto:12&)’
   10 | void iterate(
      |      ^~~~~~~
main.cpp:10:6: note:   template argument deduction/substitution failed:
main.cpp:42:25: note:   candidate expects 2 arguments, 1 provided
   42 |     iterate( on_element );
      |                         ^
main.cpp:45:38: error: no matching function for call to ‘iterate(const main()::<lambda(const int&)>&, <brace-enclosed initializer list>)’
   45 |     iterate( on_element, { 1, 2, 3 } );
      |                                      ^
main.cpp:10:6: note: candidate: ‘template<class Odds, class Evens, class Skip, class auto:11, class auto:12> void iterate(const auto:11&, const auto:12&)’
   10 | void iterate(
      |      ^~~~~~~
main.cpp:10:6: note:   template argument deduction/substitution failed:
main.cpp:45:38: note:   couldn’t deduce template parameter ‘auto:12’
   45 |     iterate( on_element, { 1, 2, 3 } );
      |                                      ^
main.cpp:48:33: error: no matching function for call to ‘iterate<bool>(const main()::<lambda(const int&)>&)’
   48 |     iterate< bool >( on_element );
      |                                 ^
main.cpp:10:6: note: candidate: ‘template<class Odds, class Evens, class Skip, class auto:11, class auto:12> void iterate(const auto:11&, const auto:12&)’
   10 | void iterate(
      |      ^~~~~~~
main.cpp:10:6: note:   template argument deduction/substitution failed:
main.cpp:48:33: note:   candidate expects 2 arguments, 1 provided
   48 |     iterate< bool >( on_element );
      |                                 ^
main.cpp:51:39: error: no matching function for call to ‘iterate<void, bool>(const main()::<lambda(const int&)>&)’
   51 |     iterate< void, bool >( on_element );
      |                                       ^
main.cpp:10:6: note: candidate: ‘template<class Odds, class Evens, class Skip, class auto:11, class auto:12> void iterate(const auto:11&, const auto:12&)’
   10 | void iterate(
      |      ^~~~~~~
main.cpp:10:6: note:   template argument deduction/substitution failed:
main.cpp:51:39: note:   candidate expects 2 arguments, 1 provided
   51 |     iterate< void, bool >( on_element );
      |                                       ^

有没有办法在 C++ 中实现它？ 也许有更好的编译时技术来实现所需的行为？

Answer 1

template<
  bool Odds = false,
  bool Evens = false,
  bool Skip = false // or really, true: why pass skip if we won't use it?
>
void iterate(
  const auto & on_element,
  const auto & skip
)

并编写另一个重载：

template<
  bool Odds = false,
  bool Evens = false
> // no bool Skip; lack of arg is enough
void iterate(
  const auto & on_element
)
{
  iterate<Odds, Evens, false>(on_element);
}

然后将! is_same_v< Skip, void > 替换为Skip 和类似的赔率/偶数。

还有

static_assert( !Skip||!std::is_same_v<decltype(skip), const int&>, "You must pass a 2nd argument" );

对生活质量有好处。

Answer 2

在我看来，在这种情况下，您应该避免使用 auto 类型参数。

我提出以下代码

#include <type_traits>
#include <iostream>
#include <set>

using namespace std;

template<
    bool Odds = true,
    bool Evens = true,
    typename Func,
    typename Skip = bool
>
void iterate(
    Func const & on_element,
    Skip const & skip = false
) {
    for ( int i = 0; i < 6; ++ i ) {
        if constexpr ( ! is_same_v< Skip, bool > ) {
            if ( skip.find( i ) != skip.end() )
                continue;
        }

        if ( i % 2 == 1 ) {
            if constexpr ( Odds )
                on_element( i );
        }
        else {
            if constexpr ( Evens )
                on_element( i );
        }

        static_assert(
            Odds || Evens,
            "Either odds or evens are required to get traversed"
        );
    }
}

int main() {
    const auto & on_element = []( const int & i ) {
        cout << "    " << i << endl;
    };

    cout << "Whole range:" << endl;
    iterate( on_element );

    cout << "Should skip specified elements:" << endl;
    iterate( on_element, std::set{ 1, 2, 3 } );

    cout << "Should traverse only evens:" << endl;
    iterate< false >( on_element );

    cout << "Should traverse only odds:" << endl;
    iterate< true, false >( on_element );

    // should NOT compile if uncomment:
    // iterate< false, false >( on_element );
}

Answer 3

也许简化

#include <type_traits>
#include <iostream>
#include <algorithm>
#include <array>

auto Odds = [](auto&& i) { return i % 2 == 0; };
auto Evens = [](auto&& i) { return i % 2 == 1; };
auto None = [](auto&&) { return false; };

template<class...Ts>
struct These
{
    constexpr These(Ts... args)
    : args_ { args... }
    {}

    template<class Arg>
    bool operator()(Arg&& i) const
    {
        return std::any_of(
            std::begin(args_), 
            std::end(args_), 
            [i](auto x) { return x == i; }
        );
    }

    std::array<std::common_type_t<Ts...>, sizeof...(Ts)> args_;
};

template<
    typename Skip = decltype(None)
>
void iterate(
    const auto & on_element,
    Skip&& skip = Skip()
) {
    for ( int i = 0; i < 6; ++ i ) {
        if (skip(i))
            continue;
        std::cout << i << ", ";
    }
    std::cout << std::endl;
}

int main() {
    const auto & on_element = []( const int & i ) {
        std::cout << "    " << i << std::endl;
    };

    std::cout << "Whole range:" << std::endl;
    iterate( on_element );

    std::cout << "Should skip specified elements:" << std::endl;
    iterate( on_element, These( 1, 2, 3 ));

    std::cout << "Should traverse only evens:" << std::endl;
    iterate( on_element , Evens);

    std::cout << "Should traverse only odds:" << std::endl;
    iterate( on_element, Odds );
}

预期输出：

Whole range:

0, 1, 2, 3, 4, 5, 

Should skip specified elements:

0, 4, 5, 

Should traverse only evens:

0, 2, 4, 

Should traverse only odds:

1, 3, 5, 

Answer 4

如果您需要iterate 的版本来获取一个参数，则必须实现一个。