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What should I use instead of void as one of the alternative types in an variant?


What should I use instead of void as one of the alternative types in an variant?

By : DisplayName=
Date : November 27 2020, 01:01 AM
Any of those help What you really want is a type among the alternatives which has a single possible value - not void, which has no possible values (and is problematic in other ways). In other words: A unit type instead of a bottom type.
The standard library has defined, as part of , a "unit type" for this use case: std::monostate (and yes, it's essentially an empty struct). Use it.
code :
#include <variant>

using Foo = int;
using Bar = double;

int main() {
    std::variant<std::monostate, Foo, Bar> v; 
    v = Foo{}; 
}


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Why isn't std::variant allowed to equal compare with one of its alternative types?

Why isn't std::variant allowed to equal compare with one of its alternative types?


By : Dan
Date : March 29 2020, 07:55 AM
help you fix your problem It is an arbitrary decision by the standards committee.
Ok, not quite arbitrary. The point is you have a scale* of strictness of comparison, with points such as:
Does std::variant provide functionality similar to boost::variant<>::types?

Does std::variant provide functionality similar to boost::variant<>::types?


By : user3210184
Date : March 29 2020, 07:55 AM
should help you out I'm not 100% familiar with Boost.MPL, but this should do what you're looking for:
code :
template <class Variant>
struct mpl_types_impl;

template <class... Ts>
struct mpl_types_impl<std::variant<Ts...>> {
    using type = boost::mpl::vector<Ts...>;
};

template <class Variant>
using mpl_types = typename mpl_types_impl<Variant>::type;
incompatible pointer types passing 'void (void *) to parameter of type 'void *

incompatible pointer types passing 'void (void *) to parameter of type 'void *


By : user3224400
Date : March 29 2020, 07:55 AM
like below fixes the issue pthread_create() expects a pointer to a function that takes a void* as input and returns a void* as output, but your function returns a void instead. You just need to add a * to the return type, and add a return statement, eg:
code :
void* function1(void *s) {
    start = *(int *)s;
    return NULL; // <-- or whatever you want
}
C++ std::variant - Type traits to verify that the contained variant types meet some assumptions

C++ std::variant - Type traits to verify that the contained variant types meet some assumptions


By : INO
Date : March 29 2020, 07:55 AM
hope this fix your issue Let's say I have some set of types that all have a common parent: , You can use partial specialization:
code :
template<class>
struct checker : std::false_type {};

template<class... Ts> 
struct checker<std::variant<Ts...>> : 
    std::bool_constant<(has_some_tag<Ts>::value && ...)> {};
template<typename V>
struct some_other_type {
    static_assert(checker<V>::value);
};
template<class... Ts> 
struct checker<std::variant<Ts...>> : std::conjunction<has_some_tag<Ts>...> {};
How to use alternative types of std::variant polymorphically

How to use alternative types of std::variant polymorphically


By : Ajay Tv
Date : September 23 2020, 02:00 PM
I wish this help you Assuming every type in the variant has some field std::string_view token; (whether this is written out in each one or comes from a common, not necessarily polymorphic, base class is up to you), you could write accessors like the following:
code :
std::string_view getToken(const Node& node)
{
  return std::visit([](const auto& n) { return n.token; }, node);
}
std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 0ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&): # @"std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 0ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)"
        mov     rax, qword ptr [rsi + 8]
        mov     rdx, qword ptr [rsi + 16]
        ret
std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 1ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&): # @"std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 1ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)"
        mov     rax, qword ptr [rsi + 16]
        mov     rdx, qword ptr [rsi + 24]
        ret
std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 2ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&): # @"std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 2ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)"
        mov     rax, qword ptr [rsi + 8]
        mov     rdx, qword ptr [rsi + 16]
        ret
std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 3ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&): # @"std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 3ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)"
        mov     rax, qword ptr [rsi + 16]
        mov     rdx, qword ptr [rsi + 24]
        ret

std::__detail::__variant::__gen_vtable<true, std::basic_string_view<char, std::char_traits<char> >, getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&>::_S_vtable:
        .quad   std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 0ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)
        .quad   std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 1ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)
        .quad   std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 2ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)
        .quad   std::__detail::__variant::__gen_vtable_impl<true, std::__detail::__variant::_Multi_array<std::basic_string_view<char, std::char_traits<char> > (*)(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)>, std::tuple<std::variant<Add, Sub, Mul, Div> const&>, std::integer_sequence<unsigned long, 3ul> >::__visit_invoke(getToken(std::variant<Add, Sub, Mul, Div> const&)::$_0&&, std::variant<Add, Sub, Mul, Div> const&)
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