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?
#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
forevent==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 innerstd::visit
. If i remove it and theauto&&
from the lambda signatures, the code compiles and runs. But is this what you want ? Remember thatstate_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 !
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