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pacmaninbw
Asked: 2024-11-26 01:13:05 +0800 CST

使类似的结构可以在 C++20 或 C++23 中转换

  • 772

我正在尝试获取一个接受范围的构造函数，该范围可以是向量或用户定义结构的数组。

我正在使用 std::convertible_to<>

编译器错误消息是我调用构造函数的地方。

`GenericDictionary<TestFunctionalityEnumDict, std::string> dictionary(testPostivePathVec);`

./SO_Minimize_Code/unitTests/ptp_vector.cpp: In function ‘int main()’:
./SO_Minimize_Code/unitTests/ptp_vector.cpp:27:92: error: no matching function for call to ‘GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char>

我怎样才能TestDataPair转换为DictType？

通用词典.h

#ifndef GENERICDICTIONARY_H_
#define GENERICDICTIONARY_H_

#include <concepts>
#include <ranges>
#include <string>
#include <vector>

template <typename DictID, typename DictName>
class GenericDictionary
{
public:
    struct DictType
    {
        DictID id;
        DictName names;
    };

    template<std::ranges::input_range R>
    requires std::convertible_to<std::ranges::range_reference_t<R>, DictType>
    GenericDictionary(R&& definitions)
    : userInputList{std::ranges::begin(definitions), std::ranges::end(definitions)}
    {
    }
    virtual ~GenericDictionary() = default;

private:

    std::vector<DictType> userInputList;
};

#endif // GENERICDICTIONARY_H_

#include "GenericDictionary.h"
#include <string>
#include <vector>

enum class TestFunctionalityEnumDict
{
    FunctionalTest_0,
    FunctionalTest_1,
    FunctionalTest_2
};

struct TestDataPair
{
    TestFunctionalityEnumDict id;
    std::string names;
};

std::vector<TestDataPair> testPostivePathVec = 
{
    {TestFunctionalityEnumDict::FunctionalTest_0, "Functional Test Str 0"},
    {TestFunctionalityEnumDict::FunctionalTest_1, "Functional Test Str 1"},
    {TestFunctionalityEnumDict::FunctionalTest_2, "Functional Test Str 2"}
};

int main()
{
    GenericDictionary<TestFunctionalityEnumDict, std::string> dictionary(testPostivePathVec);
    
    return EXIT_SUCCESS;
}

完整的错误消息

[1/2] Building CXX object unitTests/CMakeFiles/ptp_vector.dir/ptp_vector.cpp.o
FAILED: unitTests/CMakeFiles/ptp_vector.dir/ptp_vector.cpp.o 
/usr/bin/g++-12  -I./SO_Minimize_Code/. -I./SO_Minimize_Code/./unitTests -Wall -Wextra -pedantic -fprofile-arcs -ftest-coverage -g -std=gnu++23 -Werror -MD -MT unitTests/CMakeFiles/ptp_vector.dir/ptp_vector.cpp.o -MF unitTests/CMakeFiles/ptp_vector.dir/ptp_vector.cpp.o.d -o unitTests/CMakeFiles/ptp_vector.dir/ptp_vector.cpp.o -c ./SO_Minimize_Code/unitTests/ptp_vector.cpp
./SO_Minimize_Code/unitTests/ptp_vector.cpp: In function ‘int main()’:
./SO_Minimize_Code/unitTests/ptp_vector.cpp:27:92: error: no matching function for call to ‘GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char> >::GenericDictionary(std::vector<TestDataPair>&)’
   27 |     GenericDictionary<TestFunctionalityEnumDict, std::string> dictionary(testPostivePathVec);
      |                                                                                            ^
In file included from ./SO_Minimize_Code/unitTests/ptp_vector.cpp:1:
./SO_Minimize_Code/./GenericDictionary.h:21:5: note: candidate: ‘template<class R>  requires (input_range<R>) && (convertible_to<typename std::__detail::__iter_traits_impl<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type, std::indirectly_readable_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type> >::__iter_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type, std::indirectly_readable_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type> >::value_type, GenericDictionary<DictID, DictName>::DictType>) GenericDictionary<DictID, DictName>::GenericDictionary(R&&) [with DictID = TestFunctionalityEnumDict; DictName = std::__cxx11::basic_string<char>]’
   21 |     GenericDictionary(R&& definitions)
      |     ^~~~~~~~~~~~~~~~~
./SO_Minimize_Code/./GenericDictionary.h:21:5: note:   template argument deduction/substitution failed:
./SO_Minimize_Code/./GenericDictionary.h:21:5: note: constraints not satisfied
In file included from ./SO_Minimize_Code/./GenericDictionary.h:4:
/usr/include/c++/12/concepts: In substitution of ‘template<class R>  requires (input_range<R>) && (convertible_to<typename std::__detail::__iter_traits_impl<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type, std::indirectly_readable_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type> >::__iter_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type, std::indirectly_readable_traits<typename std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type> >::value_type, GenericDictionary<DictID, DictName>::DictType>) GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char> >::GenericDictionary(R&&) [with R = TestFunctionalityEnumDict]’:
./SO_Minimize_Code/unitTests/ptp_vector.cpp:27:92:   required from here
/usr/include/c++/12/concepts:72:13:   required for the satisfaction of ‘convertible_to<std::ranges::range_value_t<_Range>, GenericDictionary<DictID, DictName>::DictType>’ [with R = std::vector<TestDataPair, std::allocator<TestDataPair> >&; DictID = TestFunctionalityEnumDict; DictName = std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >]
/usr/include/c++/12/concepts:72:30: note: the expression ‘is_convertible_v<_From, _To> [with _From = TestDataPair; _To = GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::DictType]’ evaluated to ‘false’
   72 |     concept convertible_to = is_convertible_v<_From, _To>
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
./SO_Minimize_Code/./GenericDictionary.h:10:7: note: candidate: ‘constexpr GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char> >::GenericDictionary(const GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char> >&)’
   10 | class GenericDictionary
      |       ^~~~~~~~~~~~~~~~~
./SO_Minimize_Code/./GenericDictionary.h:10:7: note:   no known conversion for argument 1 from ‘std::vector<TestDataPair>’ to ‘const GenericDictionary<TestFunctionalityEnumDict, std::__cxx11::basic_string<char> >&’
ninja: build stopped: subcommand failed.
c++

1 个回答

  1. Best Answer

    由于TestDataPair和GenericDictionary<TestFunctionalityEnumDict, std::string>::DictType相同，因此它们没有理由是不同类型。

    因此，您可以简单地替换

    struct TestDataPair
{
    TestFunctionalityEnumDict id;
    std::string names;
};

    和

    using TestDataPair = GenericDictionary<TestFunctionalityEnumDict, std::string>::DictType;

    为了完整性：

    使用用户定义的转换函数

    struct TestDataPair
{
    TestFunctionalityEnumDict id;
    std::string names;

    operator GenericDictionary<TestFunctionalityEnumDict, std::string>::DictType() const
    {
        return { .id = id, .names = names };
    }
};

    使用 Boost.PFR（过度）：

    #include <boost/pfr/core.hpp>

template <typename T, typename U>
concept PFRConvertibleTo = requires(const T& t, const U& u) {
    requires std::convertible_to<decltype(boost::pfr::structure_tie(t)),
                                 decltype(boost::pfr::structure_tie(u))>;
};

template <typename To>
constexpr auto convert_from_similar =
    [](const PFRConvertibleTo<To> auto& from) {
        To result;
        boost::pfr::structure_tie(result) = boost::pfr::structure_tie(from);
        return result;
    };

template <typename DictID, typename DictName>
class GenericDictionary {
// [...]

    template <std::ranges::input_range R>
        requires std::convertible_to<std::ranges::range_reference_t<R>,
                                     DictType>
    GenericDictionary(R&& definitions)
        : userInputList{std::ranges::begin(definitions),
                        std::ranges::end(definitions)} {}

    template <std::ranges::input_range R>
        requires(
            !std::convertible_to<std::ranges::range_reference_t<R>, DictType> &&
            PFRConvertibleTo<std::ranges::range_reference_t<R>, DictType>)
    GenericDictionary(R&& definitions)
        : GenericDictionary(definitions | std::views::transform(
                                              convert_from_similar<DictType>)) {
    }

// [...]
};

    使用结构化绑定：

    // Will only work for structs with 2 members
template <typename T, typename Other>
static auto make_from_other_2(const Other& other) -> T {
    const auto& [first, second] = other;
    return {first, second};
}

template <typename From, typename To>
concept MemberwiseConvertibleTo2 =
    requires(const From& from) { make_from_other_2<To, From>(from); };

template <typename DictID, typename DictName>
class GenericDictionary {
   public:
    struct DictType {
        DictType() = default;
        DictType(const DictID& id, const DictName& names)
            : id(id), names(names) {}

        template <MemberwiseConvertibleTo2<DictType> Other>
        DictType(const Other& other)
            : DictType(make_from_other_2<DictType, Other>(other)) {}

        DictID id;
        DictName names;
    };

// [...]
};

    一旦P1061可用，我们就可以使我们的辅助函数对任意数量的成员有效：

    template <typename T, typename Other>
static auto make_from_other(const Other& other) -> T {
    const auto& [...elements] = other;
    return {elements...};
}

template <typename From, typename To>
concept MemberwiseConvertibleTo =
    requires(const From& from) { make_from_other<To, From>(from); };

    当然，这也可以用来替代前面例子中的Boost.PFR。

    • 4

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