std::variant 模板化失败 class

Question

我正在摆弄 std::variant 但是我似乎无法让它工作。当我使用 c++ 的原始类型时它工作正常，但是如果我使用模板化的 class 我会得到以下编译器错误。有很多我无法理解的错误。这是构建输出。不知道这个有用多少

1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1521,32): error C2672: 'invoke': no matching overloaded function found
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1531): message : see reference to class template instantiation 'std::_Variant_single_visit_result<_Callable,std::integer_sequence<_Ty,5>,std::variant<int,char,bool,std::string,A<int>> &>' being compiled
1>        with
1>        [
1>            _Callable=EventVisitor,
1>            _Ty=size_t
1>        ]
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1654): message : see reference to class template instantiation 'std::_Variant_all_visit_results_same<_Callable,_ListOfIndexVectors,std::variant<int,char,bool,std::string,A<int>> &>' being compiled
1>        with
1>        [
1>            _Callable=EventVisitor
1>        ]
1>C:\Users\jaden\source\repos\Project8\Project8\Main.cpp(55): message : see reference to function template instantiation 'void std::visit<EventVisitor,std::variant<int,char,bool,std::string,A<int>>&,void>(_Callable &&,std::variant<int,char,bool,std::string,A<int>> &)' being compiled
1>        with
1>        [
1>            _Callable=EventVisitor
1>        ]
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): error C2893: Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\type_traits(1588): message : see declaration of 'std::invoke'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): message : With the following template arguments:
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): message : '_Callable=_Callable'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): message : '_Ty1=_First &'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): message : '_Types2={}'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1522,1): error C2780: 'unknown-type std::invoke(_Callable &&) noexcept(<expr>)': expects 1 arguments - 2 provided
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\type_traits(1582): message : see declaration of 'std::invoke'
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(1531,36): error C2955: 'std::_All_same': use of class template requires template argument list
1>C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.28.29333\include\variant(40): message : see declaration of 'std::_All_same'
1>Done building project "Project8.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

这是代码。

#include <iostream>
#include <vector>
#include <variant>

template<class T>
class A
{
public:
    T t;
};

struct EventVisitor
{
    void operator()(std::string s)
    {
        std::cout << s << std::endl;
    }
    void operator()(int i)
    {
        std::cout << i << std::endl;
    }
    void operator()(char c)
    {
        std::cout << c << std::endl;
    }
    void operator()(bool b)
    {
        if (b)
        {
            std::cout << "True" << std::endl;
        }
        else
        {
            std::cout << "False" << std::endl;
        }
    }
};

int main()
{
    // std::variant<int, char, bool, std::string> myVariant; works but if I add A<int> it breaks
    std::variant<int, char, bool, std::string, A<int>> myVariant;

    myVariant = true;
    std::visit(EventVisitor{}, myVariant);
}

Answer 1

编译 std::visit 失败，因为您的访问者没有 A<int> 的处理程序。您必须添加它：

void operator()(A<int> t) {}

或者为您的访问者添加一个模板函数来处理您可能想要使用的所有类型。

std::variant 模板化失败 class

std::variant failing with templated class

c++

variant