“No matching function call to std::visit() using overload pattern.”

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英文:

No matching function call to std::visit() using overload pattern

问题

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英文:

I'm trying to dispatch a variant inside a variant with the visitor overload pattern. However I can't seem to fit the parameters to what is expected. I get this (truncated, for more output see compiler explorer link):

error: no matching function for call to 'visit ...'

Here's my code. What am I overlooking?

Demo

#include <variant>
#include <cstdio>

template <typename... Ts>
struct overload : Ts...
{
    using Ts::operator() ...;
};

int main() {

    struct StateA{};
    struct StateB{};
    struct StateC{};

    using state_type = std::variant<StateA, StateB, StateC>;
    auto state = state_type{StateA{}};

    struct EventA{};
    struct EventB{};

    struct WifiConnectedNote{};
    struct ErrorNote{};

    using QueueVariantType = std::variant<std::monostate, WifiConnectedNote, ErrorNote>;
    
    using event_type = std::variant<std::monostate, QueueVariantType, EventA, EventB>;
    auto event = event_type{EventA{}};

    for (size_t i=0; i < 10; i++) {
        state = std::visit(overload{
            [&](QueueVariantType&& variant, auto&& state_tmp) -> state_type {
                std::visit(overload{
                    [&](WifiConnectedNote&&, auto&&){
                        printf("WifiConnectedNote A\n");
                    },
                    [&](ErrorNote&&, auto&&){
                        printf("ErrorNote A\n");
                    },
                    [&](auto&&, auto&&){
                        printf("Other in Queue variant\n");
                    },
                }, std::forward<decltype(variant)>(variant), std::forward<decltype(state_tmp)>(state_tmp));
                return StateC{};
            },
            [&](auto&&, const StateA&) -> state_type {
                printf("State A\n");
                return StateC{};
            },
            [&](auto&&, const StateB&) -> state_type {
                printf("State B\n");
                return StateA{};
            },
            [&](auto&&, const StateC&) -> state_type {
                printf("State C");
                return StateB{};
            },
        }, std::move(event), state);
    }
}

答案1

得分: 1

  • 你在外部的 std::visit 对于 event==std::monostate 情况缺少一个重载情况。要么提供一个,要么从变体类型中移除状态。
[&](std::monostate, auto&& ) -> state_type {
    ; // 在这里处理
}
  • 内部的 std::visit 中未使用 state_tmp。如果我移除它和 lambda 签名中的 auto&&,代码会编译并运行。但这是否符合您的要求?请记住,state_tmp 并不是变体类型本身,而是外部 std::visit 解析的底层类型。这就是内部访问失败的原因。

希望这可以帮助您继续!

英文:
  • You are missing an overload case for the outer std::visit for event==std::monostate. Either provide one or remove the state from the variant type.

      [&](std::monostate, auto&& ) -> state_type {
          ; // Do stuff here
      }
    
  • state_tmp is not used in the inner std::visit. If i remove it and the auto&& from the lambda signatures, the code compiles and runs. But is this what you want ? Remember that state_tmp is not the variant type itself, but the underlying type resolved by the outer std::visit. That's why the inner visit fails.

Hope you can take from here !

huangapple
  • 本文由 发表于 2023年5月17日 14:56:54
  • 转载请务必保留本文链接:https://go.coder-hub.com/76269296.html
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